M_Map: User's Guide v1.4

1. Getting started

First, get all the files, either as a zip archive or a gzipped tar-file and unpack them. If you are unpacking the zip file MAKE SURE YOU ALSO UNPACK SUBDIRECTORIES! Now, start up Matlab (version 5 or higher). Make sure that the toolbox is in your path. This can be done simply by cd'ing to the correct directory.

Alternatively, if you have unpacked them into directory /users/rich/m_map (and /users/rich/m_map/private), then you can add this to your search path:

path(path,'/users/rich/m_map');

addpath /users/rich/m_map

To follow along with this document, you would then use a Web-browser to open file:/users/rich/m_map/map.html, that is, this HTML document.

Note: you may want to install M_Map as a toolbox accessible to all users. To do this, unpack the files into $MATLAB/toolbox/m_map, add that directory to the list defined in $MATLAB/toolbox/local/pathdef.m, and update the cache file using

rehash toolboxcache

Instructions for installing the (optional) high-resolution bathymetry database are given in Section 9 , and instructions for installing the (optional) high-resolution GSHHS coastline database is given in Section 10 . However, we should first check that the basic setup is OK.

To see an example map, try this:

m_proj('oblique mercator');
m_coast;
m_grid;

This is a line map of the Oregon/British Columbia coast, using an Oblique Mercator projection (A few more complex maps can be generated by running the demo function m_demo).

The first line initializes the projection. Defaults are set for the different projection, so you can easily see what a specific projection looks like, but all projections have a number of optional parameters as well. To get the same map without using the defaults, you would use

m_proj('oblique mercator','longitudes',[-132 -125], ...
       'latitudes',[56 40],'direction','vertical','aspect',.5);

The exact meanings of the various options is given in Section 2 . However, notice that longitudes are specified using a signed notation - East longitudes are positive, whereas West longitudes are negative (Also note that a decimal degree notation is used, so that a longitude of 120 30'W is specified as -120.5).

The second line draws a coastline, using the 1/4 degree database. Coastlines with greater resolution can be drawn, using your own database (see also Section 7 ). m_coast can be called with various line parameters. For example,

m_coast('linewidth',2,'color','r');

draws a thicker red coastline. Filled coastlines can also be drawn, using the 'patch' option (followed by any of the usual PATCH property/value pairs).

m_coast('patch',[.7 .7 .7],'edgecolor','none');

draws a coastline with a gray fill and no border.

The third line superimposes a grid. Although there are many possible options that can be used to customize the appearance of the grid, defaults can always be used (as in the example). These options are discussed in Section 4 . You can get a list of the options using the GET syntax:

m_grid get

which acts somewhat like the get(gca) syntax for regular plots.

Finally, suppose you want to show and label the location of, say, a mooring at 129W, 48 30'N.

[X,Y]=m_ll2xy(-129,48.5);
line(X,Y,'marker','square','markersize',4,'color','r');
text(X,Y,' M5','vertical','top');

m_ll2xy (and its inverse m_xy2ll) convert from longitude/latitude coordinates to those of the projection. Various clipping options can also be specified in converting to projection coordinates. If you are willing to accept default clipping setting, you can use the built-in functions m_line and m_text :

m_line(-129,48.5,'marker','square','markersize',4,'color','r');
m_text(-129,48.5,' M5','vertical','top');

Finally (!), we may want to alter the grid details slightly. Note that, a given map must only be initialized once.

clf
m_coast('patch',[.7 .7 .7],'edgecolor','none');
m_grid('xlabeldir','end','fontsize',10);
m_line(-129,48.5,'marker','square','markersize',4,'color','r');
m_text(-129,48.5,' M5','vertical','top');

2. Specifying projections

In order to get a list of the current projections,

m_proj get

m_proj('set');

Which currently return the following list:

Available projections are:
     Stereographic
     Orthographic 
     Azimuthal Equal-area
     Azimuthal Equidistant
     Gnomonic
     Satellite
     Albers Equal-Area Conic
     Lambert Conformal Conic
     Mercator
     Miller Cylindrical
     Equidistant Cylindrical
     Oblique Mercator
     Transverse Mercator
     Sinusoidal
     Gall-Peters
     Hammer-Aitoff
     Mollweide
   Robinson
  UTM

If you want details about the possible options for any of these projections, add its name to the above command, e.g.

m_proj('set','stereographic');
     'Stereographic'                                   
     <,'lon<gitude>',center_long>                      
     <,'lat<itude>', center_lat>                       
     <,'rad<ius>', ( degrees | [longitude latitude] )>
     <,'rec<tbox>', ( 'on' | 'off' )>

You can also get details about the current projection. For example, in order to see what the default parameters are for the sinusoidal projection, we first initialize it, and then use the 'set' option:

m_proj('sinusoidal');
m_proj get
Current mapping parameters -
 Projection: Sinusoidal  (function: mp_tmerc)
 longitudes: -90  30 (centered at -30)       
 latitudes: -65  65                          
 Rectangular border: off

In order to initialize a projection, you usually specify some location parameters that define the geometry of the projection (longitudinal limits, central parallel, etc.), as well as parameters that define the extent of the map (whether it is in a rectangular axis, what the border points are, etc.). These vary slightly from projection to projection.

Two useful properties for projections are (1) the ability the preserve angles for differentially small regions, and (2) the ability to preserve area. Projections satisfying the first condition are called conformal , those satisfying the second are called equal-area . No projection can be both. Many projections (especially global projections) are neither, instead an attempt has been made to aesthetically balance the errors in both conditions.

Note: Most projections are currently spherical rather than ellipsoidal. UTM is an ellipsoidal projection, and both the lambert conformal conic and albers equal-area conic can be specified with ellipses if desired. This is sometime useful when you have data (e.g. from a GIS package) at scales of Canadian provinces or US states, which are often mapped using one of these projections. It is unlikely that this will be a problem in normal usage.

`<,'zon<e>', value 1-60>`

`<,'hem<isphere>',value 0=N,1=S>`

These are computed automatically if not specified. The ellipsoid defaults to 'normal', a spherical earth of radius 1 unit, but other options can also be chosen using the following property:

`<,'ell<ipsoid>', ellipsoid>`

For a list of available ellipsoids try m_proj('set','utm').

The big difference between UTM and all the other projections is that for ellipsoids other than 'normal' the projection coordinates are in meters of easting and northing. To take full advantage of this it is often useful to call m_proj with 'rectbox' set to 'on' and not to use the long/lat grid generated by m_grid (since the regular matlab grid will be in units of meters).

Sinusoidal

This projection is usually called "pseudo-cylindrical" since parallels of latitude appear as straight lines, similar to their appearance in cylindrical projections tangent to the equator. However, meridians curve together in this projection in a sinusoidal way (hence the name), making this map equal-area.

Gall-Peters

Parallels of latitude and meridians both appear as straight lines, but the vertical scale is distorted so that area is preserved. This is useful for tropical areas, but the distortion in polar areas is extreme.

Conic Projections

Conic projections result from projecting onto a cone wrapped around the sphere. The vertex of the cone lies on the rotational axis of the sphere. The cone is either tangent at a single latitude, or can intersect the sphere at two separated latitudes. It is a useful projection for mid-latitude areas of large east-west extent. The following properties affect these projections:

`<,'lon<gitude>',[min max]> <,'lat<itude>',[min max]>`

These specify the limits of the map.

`<,'clo<ngitude>',value>`

The central longitude appears as a vertical on the page. The default value is the mean longitude, although it may be set to any value (even one outside the limits).

`<,'par<allels>',[lat1 lat2]>`

The standard parallels can be specified. Either one or two parallels can be given, the default is a single parallel at the mean latitude

`<,'rec<tbox>', ( 'on' | 'off' )>`

The map limits can either be based on latitude/longitude (the default), or the map boundaries can form an exact rectangle which contain the given limits. Unless the region being mapped is small, it is best to leave this 'off' .

The default is to use a spherical earth model for the mapping transformations. However, ellipsoidal coordinates can also be specified. This tends to be useful only for doing coordinate transformations (e.g., if a particular gridded database in in this kind of projection, and you want to find lat/long data), since the difference would be impossible to see by eye on a plot. The particular ellipsoid used can be chosen using the following property:

`<,'ell<ipsoid>', ellipsoid>`

For a list of available ellipsoids try m_proj('set','albers').

Albers Equal-Area Conic

This projection is equal-area, but not conformal

Lambert Conformal Conic

This projection is conformal, but not equal-area.

Miscellaneous global projections

There are a number of projections which don't really fit into any of the above categories. Mostly these are global projections (i.e. they show the whole world), and they have been designed to be "pleasing to the eye". I'm not sure what use they are in general, but they make nice logos!

Hammer-Aitoff

An equal-area projection with curved meridians and parallels.

Mollweide

Also called the Elliptical or Homolographic Equal-Area Projection. Parallels are straight (and parallel) in this projection. Note that

example 4

m_proj('mollweide');
m_coast('patch','r');
m_grid('xaxislocation','middle');

Robinson

Not equal-area OR conformal, but supposedly "pleasing to the eye".

Yeah, but which projection should I use?

Well, it depends really on how large an area you are mapping. Usually, maps of the whole world are Mercator, although often the Miller Cylindrical projection looks better because it doesn't emphasize the polar areas as much. Another choice is the Hammer-Aitoff or Mollweide (which has meridians curving together near the poles). Both are equal-area. It's probably not a good idea to use these projections for maps that don't have the equator somewhere near the middle. The Robinson projection is not equal-area or conformal, but was the choice of National Geographic (for a while, anyway), and also appears in the IPCC reports.

If you are plotting something with a large north/south extent, but not very wide (say, North and South America, or the North and South Atlantic), then the Sinusoidal or Mollweide projections will look pretty good. Another choice is the Transverse Mercator, although that is usually used only for very large-scale maps.

For smaller areas within one hemisphere or other (say, Australia, the United States, the Mediterranean, the North Atlantic) you might pick a conic projection. The differences between the two available conic projections are subtle, and if you don't know much about projections it probably won't make much difference which one you use.

If you get smaller than that, it doesn't matter a whole lot which projection you use. One projection I find useful in many cases is the Oblique Mercator, since you can align it along a long (but narrow) coastal area. If map limits along lines of longitude/latitude are OK, use a Transverse Mercator or Conic Projection. The UTM projection is also useful.

Polar areas are traditionally mapped using a Stereographic projection, since for some reason it looks nice to have a "bullseye" pattern of latitude lines.

If you want to get a quick idea of what any projection looks like, default parameters for all functions are set for a "typical" usage, i.e. to get a quick idea of what any projection looks like, you can do so without having to figure out a lot of numerical values:

m_proj('stereographic');  % Example for stereographic projection
m_coast;
m_grid;

Map scales

M_Map usually scales the map so that it fits exactly within the current axes. If you just want a nice picture (which is mostly the case) then this is exactly what you need. On the other hand, sometimes you want to print things out at some exact scale (i.e. if you really much prefer sitting at your desk with a ruler and a piece of paper trying to figure out how far apart Bangkok and Tokyo are). Use the m_scale primitive for this - for a 1:250000 map, call

m_scale(250000);

after you have drawn everything (Be careful - a 1:250000 map of the world is a lot bigger than 8.5"x11" sheet of paper).

This option is usually only useful for large-scale maps, i.e. maps of very small areas).

If you wish to know the current scale, calling m_scale without any parameters will calculate and return that value.

To return to the default scaling call m_scale('auto').

(PS - If you do want to find distances from Bangkok to anywhere, plot an azimuthal equidistant projection of the world centered on Bangkok (13 44'N, 100 30'E), and choose a fairly small scale, like 1:200,000,000). Another option would be to use range rings, see example 11.

Map coordinate systems - geographic and geomagnetic.

Latitude/Longitude is the usual coordinate system for maps. In some cases UTM coords are also used, but these are really just a simple transformation based on the location of the equator and certain lines of longitude. On the other hand, there are occasions when a coordinate system based on some other set of axes is useful. For example, in space physics data is often projected in coordinates based on the magnetic poles. M_Map has a limited capabality to deal with data in these other coordinate systems. m_coord allows you to chnage the coordinate system from geographic to geomagnetic. The following code gives you the idea:

lat=[25*ones(1,100) 50*ones(1,100) 25]; lon=[-99:0 0:-1:-99 -99]; clf subplot(121); m_coord('IGRF2000-geomagnetic'); % Treat all lat/longs as geomagnetic m_proj('stereographic'); m_coast; m_grid; m_line(lon,lat,'color','r'); % "lat/ln" assumed geomagnetic on the geomagnetic map m_coord('geographic'); % Switch to assuming geographic m_line(lon,lat,'color','c'); % Now they are treated as geographic subplot(122); m_coord('geographic'); % Define all in geographic m_proj('stereographic'); m_coast; m_grid; m_line(lon,lat,'color','c'); m_coord('IGRF2000-geomagnetic'); % Now assume that values are in geomagnetic m_line(lon,lat,'color','r');

Note that this option is not used very much, hence is not fully supported. In particular, filled coastlines may not work properly.

3. Coastlines and Bathymetry

M_Map includes two fairly simple databases for coastlines and global elevation data. Highly-detailed databases are not included in this release because they are a) extremely large and b) extremely time-consuming to process (loops are inherently involved). If more detailed maps are required, section 9 and section 10 give instructions on how to add some freely-available high-resolution datasets. Read section 7 and section 8 if you want to add your own coastline/bathymetry data.

Coastline options

M_Map includes a 1/4 degree resolution coastline database. This is suitable for maps covering large portions of the globe, but is noticeably coarse for many large-scale applications. Users not satisfied with their regional map are directed to section 7 and/or section 10 for more information on creating and using high-resolution coastlines. The M_Map database is accessed using the m_coast function. Coastlines can be drawn as simple lines, using

m_coast('line', ...optional line arguments );

m_coast( optional line arguments );

m_coast('patch', ...optional patch arguments );

m_coast('patch',[.7 .7 .7],'edgecolor','g');

Many older (ocean) maps are created with speckled land boundaries, which looks very nice in black and white. You can get a speckled boundary with

m_coast('speckle', ....optional m_hatch arguments);

m_hatch

Example 13

Note that line coastlines are usually drawn rather rapidly. Filled coastlines take considerably more time to generate (because map limits are not necessarily rectangular, clipping must be accomplished in m-files).

Topography/Bathymetry options

M_Map can access a 1-degree resolution global elevation database (actually, this database is is included in the Matlab distribution, used by of $MATLAB/toolbox/matlab/demos/earthmap.m). A contour map of elevations at default levels can be drawn using

m_elev;

m_elev('contour',LEVELS, optional contour arguments);

m_elev('contour',LEVELS,'edgecolor','b');

m_elev('contourf',LEVELS, optional contourf arguments);

[Z,LONG,LAT]=m_elev([LONG_MIN LONG_MAX LAT_MIN LAT_MAX]);

4. Customizing axes

Grid lines and labels

In order to get the perfect grid, you may want to experiment with different grid options. Two functions are useful here, M_GRID which draws a grid, and M_UNGRID which erases the current grid (but leaves all coastlines and user-specified data alone). Try

m_proj('Lambert');
m_coast;
m_grid;

to get a Lambert conic projection of North America. Now try

m_ungrid

The coastline is still there, but the grid has disappeared and the axes shows raw X/Y projection coordinates. Now, try this:

m_grid('xtick',10,'tickdir','out','yaxislocation','right','fontsize',7);

The various options that can be changed are:

`'box',( 'on' | 'off' | 'fancy' )`

This specifies whether or not an outline box is drawn. Three types of outline boxes are available: 'on', the default, is a a simple line. Two types of fancy outline boxes are available. If 'tickdir' is 'in', then alternating black and white patches are made (see

example 2

'tickdir'

'out'

example 6

`'xtick',( num | [value1 value2 ...])`

'ytick'

`'xticklabels',[label1;label2 ...]`

'yticklabels'

`'xlabeldir', ( 'middle' | 'end' )`


'ylabeldir'

`'ticklen',value`

`'tickdir',( 'in' | 'out' )`

'box'

'fancy'

`'tickstyle',( 'dd' | 'dm' )`

`'color',colorspec 'linewidth', value 'linestyle', ( linespec | 'none' ) 'fontsize',value 'fontname',name`

`'XaxisLocation',( 'bottom' | 'middle' | 'top' )`

`'YaxisLocation',( 'left' | 'middle' | 'right' )`

Titles and x/ylabels

Titles and x/ylabels can be added to maps using the title and x/ylabel functions in the usual way (this is a change from v1.0 in which the 'visible' property had to be explicitly set to 'on'; this is now done within m_grid).

Legend Boxes

A legend box can be added to the map using m_legend. Only some of the functionality of legend is currently included. The legend box can be dragged and dropped using the mouse button.

Scale Bars

A scale bar can be added to the map using m_ruler. The bar is drawn horizontally or vertically, and will create a 'nice' number of ticks (although this can be changed with another calling parameter). The location is specified in normalized coordinates (i.e. between 0 and 1) so you can adjust placement on the map. It is probably best to call this AFTER calling m_grid since m_grid resets the normalization.

WARNING - the scalebar is probably not useful for any global (i.e. whole-world) or even a significant-part-of-the-globe map, but I won't stop you using it. Caveat user!

5. Adding your own data

The purpose of this package is to allow you to map your own data! Once a suitable grid and (possibly) a coastline have been chosen, you can add your own lines, text, or contour plots using built-in M_Map drawing functions which handle the conversion from longitude/latitude coordinates to projection coordinates. These drawing functions are very similar to the standard Matlab plotting functions, and are described in the next section.

Sometimes you may want to convert between longitude/latitude and projection coordinates without immediately plotting the data. This might happen if you want to interactively select points using ginput, or if you want to draw labels tied to a specific point on the screen rather than a particular longitude/latitude. Projection conversion routines are described in sections 5.2 and 5.3. Once raw longitude/latitude coordinates are converted into projection coordinates, standard Matlab plotting functions can be used.

Maps are drawn to fit within the boundaries of the plot axes. Thus their scale is somewhat arbitrary. If you are interested in making a map to a given scale, e.g. 1:200000 or something like that, you can do so by using the m_scale primitive, see section 2.6 . The data units are the projection coordinates, which are distances expressed as a fraction of earth radii. To get a map "distance" between two points, use the Cartesian distance between the points in the projection coordinate system and multiply by your favourite value for the earth's radius, usually around 6370 km (exception - the UTM projection uses coordinates of northing and easting in meters, so no conversion is necessary).

Caution: One problem that sometimes occurs is that data does not appear on the plot due to ambiguities in longitude values. For example, if plot longitude limits are [-180 180], a point with a longitude of, say, 200, may not appear in cylindrical and conic projections. This is not a bug. Handling the clipping in "wrapped around" curves requires adding points (rather than just moving them) and is therefore incompatible with various other requirements (such as keeping input and output matrices the same size in the conversion routines described below).

Drawing lines, text, arrows, patches, hatches, speckles and contours

For most purposes you do not need to know what the projection coordinates actually are - you just want to plot something at a specified longitude/latitude. Most of the time you when you want to plot something on a map you want to do so by specifying longitude/latitude coordinates, instead of the usual X/Y locations. To do so in M_Map, replace calls to

 plot, line, text, quiver,
patch, contour, and contourf

with M_Map equivalents that recognize lontgitude/latitude coordinates by prepending "m_" to the function name. For example,


  m_plot(LONG,LAT,...line properties)   % draw a line on a map (erase current plot) 
  m_line(LONG,LAT,...line properties)   % draw a line on a map 
  m_quiver(LONG,LAT,U,V,S)              % A quiver plot 
  m_text(LONG,LAT,'string')             % Text 
  m_patch(LONG,LAT,..patch properties)  % Patches.

Each of these functions will handle the coordinate conversion internally, and will return a vector of handles to the graphic objects if desired. The only difference between these functions and the standard Matlab functions is that the first two arguments MUST be longitude and latitude.

One caveat applies to m_patch. For compatibility reasons this uses the same code that applies to coastline filling. Coastlines come either as either "islands" or "lakes", and M_Map keeps track of the difference by assuming curves are oriented so that the filled area ("land") is always on the right as we go around the curve. This is slightly different than the convention used in patch which always fills the inside. Keeping track of this difference is relatively straightforward in a Cartesian system, but not so easy in spherical coordinates. In the absence of other information m_patch tries to do the right thing, but (especially when the patch intersects a map boundary) it can get confused. If a patch isn't filling correctly, try reversing the order of points using flipud or fliplr.

Data gridded in longitude and latitude can also be contoured:

m_contour(LONG,LAT,VALUES)
m_contourf(LONG,LAT,VALUES)

Again, these functions will return handles to graphics objects, allowing (for example) the drawing of labelled contours:

[cs,h]=m_contour(LONG,LAT,VALUES)
clabel(cs,h,'fontsize',6);

Fancy arrows (i.e. with width, head shape, and colour specifications) can be generated using m_vec.m . See the on-line help for more details about the use of m_vec.

You can also get hatched areas by calling m_hatch:

  m_hatch('single',LONG,LAT,...hatch properties)    % Interior Single Hatches. 
  m_hatch('cross',LONG,LAT,...hatch properties)     % Interior Crossed Hatches.

m_line

  
  m_hatch('speckle',LONG,LAT,...speckle properties)  % Speckled edges.

Example 13

m_hatch

Drawing images and p_color

m_pcolor is a drop-in replacement for p_color, but you must be careful with its use near map boundaries. Ideally one would want data to extend up to (but not across) a map boundary (i.e. polygons are clipped). However, due to the way in which matlab handles surfaces this is not easily done. Instead - unless you are using a simple cylindrical or conic projection - you will probably get a ragged edge for the coloured surface.

There is no m_image. the image() function plots data in rectangular pixels only, and in general projected data will NOT appear as rectangular pixels. If you want to display a large pixel image on your map, there are several options:

If your georeferenced image is in lat/long coordinates (i.e. each data row is along a line of constant latitude, each column a line of equal longitude), then you can use m_pcolor with shading flat. This is reasonably satisfactory (although it can be slow for large images), but you MUST remember to offset your coordinates by one-half of the pixel spacing. This is because of the different behaviors of p_color and image when given the same data.

image will center the drawn (i,j) pixel on the (i,j)th entry of the X/Y matrices.
p_color with shading flat will draw a panel between the (i,j),(i+1,j),(i+1,j+1),(i,j+1) coordinates of the X/Y matrices with a color corresponding to the data value at (i,j). Thus everything will appear shifted by one half a pixel spacing.

Satellite data from mid-latitudes is sometime amenable to this solution. See the examples of satellite data manipulation.

If your figure has already been placed in some projection, and if you know the exact parameters of that projection, you can probably use a straight image call and then overplot an M_Map map. For example, polar satellite images are often in a polar stereographic projection. In this case you should use m_ll2xy to get the screen coordinates of the image corners, then use those points in an image() call before overplotting your data. See in particular This example .
HINT - check to see that coastlines overplot to make sure this is working correctly.

Drawing tracklines

It is sometimes useful to annotate lines representing the time-varying location of a ship, aircraft, or satellite with time and date annotations. This can be done using m_track.

m_proj('UTM','long',[-72 -68],'lat',[40 44]);
m_gshhs_i('color','k');
m_grid('box','fancy','tickdir','out');

% fake up a trackline
lons=[-71:.1:-67];
lats=60*cos((lons+115)*pi/180);
dates=datenum(1997,10,23,15,1:41,zeros(1,41));

m_track(lons,lats,dates,'ticks',0,'times',4,'dates',8,...
        'clip','off','color','r','orient','upright');

See the on-line help for more details about the use of m_track, and the different options for setting fontsize, tick spacing, date formats, etc.

While fiddling with the various parameters, it is often handy to be able to erase the plotted tracks without erasing the coastline and grid. This can be done using

m_ungrid track

m_ungrid('track')

Drawing range rings and geodesics

One nifty thing that is sometimes useful is the ability to draw circles at a given range or ranges from a specific location. This can be done using m_range_ring, which has 3 required calling parameters: LONG, LAT, RANGE, followed by any number of (optional) line specification property/value pairs.

Example 11

m_range_ring

If you want to plot circular geodesics (i.e. curves which are perpedicular to the range rings at all ranges), m_lldist can find both distances and points along the geodesics between points. Example 13 illustrates how to use m_lldist.

If you care about the difference between great circle and ellipsoidal geodesics (a very very small proportion of users I would bet) then m_fdist (which computes the position at a given range/bearing from another), m_idist (distance and bearings between points), and m_geodesic (points along the geodesic) can be used with a variety of (user-specified) ellipses. The calling sequence for these is different than for m_lldist for historical reasons.

Converting longitude/latitude to projection coordinates

If you want to use projection coordinates (perhaps you want to compute map areas, or distances, or you want to make a legend in the upper left corner), the following command converts longitude/latitude coordinates to projection coordinates.

[X,Y]=m_ll2xy(LONG,LAT, ...optional clipping arguments )

where LONG, LAT, X, and Y are matrices of the same size. Projection coordinates are equal to true distances near the center of the map, and are expressed as fractions of an earth radius. To get a distance, multiply by the radius of the earth (about 6370km). The exception is the UTM projection which provides coordinates of northing and easting in meters.

The possible clipping arguments are

`'clip','on'`

This is the default. Columns of LONG and LAT are assumed to form lines, and these are clipped to the map limits. The first point outside the map is therefore moved to the map edge, and all other points are converted the NaN.

`'clip','off'`

No clipping is performed. This is sometimes useful for debugging purposes.

`'clip','point'`

Points are tested against the map limits. Those outside the limits are converted to NaN, those inside are converted to projection coordinates. No points are moved. This option is useful for point data (such as station locations).

`'clip','patch'`

Points are tested against the map limits. Those outside the limits changed into a point exactly on the limits. Those inside are converted to projection coordinates. This option may be useful when trying to draw patches, however it probably won't work well.

Converting projection coordinates to longitude/latitude

Conversion from projection coordinates to longitude/latitude is straightforward:

[LONG,LAT]=m_xy2ll(X,Y)

There are no options.

Computing distances between points

Geodesic (great circle) distances on a spherical earth can be computed between pairs of either geographic (long/lat) or map (X/Y) coordinates using the functions m_lldist and m_xydist. For example,

DIST=m_lldist([20 30],[44 45])

computes the distance from 20E, 44N to 30E, 45N. Alternatively, if you want to compute the distance between two points selected by the mouse:

[X,Y]=ginput(2);
DIST=m_xydist(X,Y)

will return that distance. Because of the inaccuracies implicit in a spherical earth approximation the true geodesic distances may differ by 1% or so from the computed distances.

If you want greater accuracy, then you must calculate geodesics on an ellipsoidal earth. There is a very accurate numerical algorithm for doing so ( Vincenty's algorithm), which is implemented in the functions m_idist, m_fdist, and m_geodesic. For example,

 [distance,a12,a21] = m_idist(lon1,lat1,lon2,lat2,spheroid)

distance

(lon1,lat1)

(lon2,lat2)

'wgs84'

m_proj('get','utm')

a12

a21

m_fdist is used to get the location of a point at a given bearing and distance from a specified point.

Finally, if you want to plot a geodesic on a map, then m_geodesic can be used to generate a vector of points along the elliptical geodesic between two specified points. If you ever find yourself needing this, I'd be interested in knowing about it!

6. More complex plots

For ideas on how to make more complex plots, see the Examples. These plots are also included in the function m_demo.

7. Removing data from a plot

Once a given map includes several elements a certain amount of fiddling is usually necessary to satisfy the natural human urge to give the image a certain aesthetic quality. If the image includes complicated coastlines which take a long time to draw (e.g. those discussed below) than clearing the figure and redrawing soon becomes tedious. The m_ungrid command introduced above can be used to selectively remove parts of the figure. For example:

m_proj('lambert','long',[-160 -40],'lat',[30 80]);
m_coast;
m_range_ring(-123,49,[1e3:1e3:10e3],'color','r');

draws range rings at 1000km increments from my office. But I am unsatisfied with this, and want to redraw using only 200km increments. I can remove the effects of m_range_ring and redraw using:

m_ungrid range_ring
m_range_ring(-123,49,[200:200:2000],'color','r');

In general the results of m_ANYTHING can be deleted by calling m_ungrid ANYTHING.

m_ungrid can recognize and delete specific elements by searching the 'tag' property of all plot elements, which is set by the various different M_Map routines.

8. Adding your own coastlines

If you are interested in a particular area and want a higher-resolution coastline than that used by m_coast, the best procedure is to

Get a subset of points from some high-resolution database, and
convert to screen coordinates using m_ll2xy, and plot.

One place where high-resolution coastline data can be obtained is The Coastline Extractor. Follow the instructions there to get coastline data in a matlab-readable file, and download to your computer. If the file is saved as "coast.dat", you can plot it (as lines) using the following:

load coast.dat
m_line(coast(:,1),coast(:,2));

Filled coastlines will require more work. You should first read the instructions there on joining the coastline data into continuous segments. If you are lucky, (i.e. no lakes or anything else), you may achieve success with

load coast.dat
[X,Y]=m_ll2xy(coast(:,1),coast(:,2),'clip','patch');
k=[find(isnan(X(:,1)))];
for i=1:length(k)-1,
    x=coast([k(i)+1:(k(i+1)-1) k(i)+1],1);
    y=coast([k(i)+1:(k(i+1)-1) k(i)+1],2);
    patch(x,y,'r');
end;

If this does not work, read the comments in private/mu_coast, orient the curves in the desired fashion, and use m_usercoast to load your own data.

DCW political boundaries

As of 2011 the DCW web site has been decommissioned. The following information is retained for historical reasons only. New users see the next section on shapefiles.

Files containing political boundaries for various countries and US states can be downloaded from http://www.maproom.psu.edu/dcw/. Select an area and choose the "download points" option (rather than "download data"). Once downloaded to your machine use m_plotbndry to access and plot the desired boundary. For example, if you downloaded various US states into a subdirectory "states:,

m_plotbndry('states/arizona','color,'r')

ESRI Shapefiles and Natural Earth Political Boundaries

A de facto standard for the interchange of vector data are ESRI shapefiles. A dataset comes in (at minimum) 3 files, each with the same root name but with .dbf, .shp, and .shx extensions. Files can contain point, line or polygon information, as well as other fields in a self-describing way. For more information see this description .

Many (all?) shapefiles can be read in using m_shaperead, which returns a data structure containing the information in the files. However, figuring out what to do with the contents requires you to examine the contents of the data structure. It may also be useful to examine projection information in the .prj text file, which contains information about map projections, especially if you are overlaying data from different sources.

You can usually at least create a simple plot of the data stored in files datafile.shp, datafile.shx and datafile.dbf using

M=m_shaperead('datafile'); 
clf; 
for k=1:length(M.ncst), 
  line(M.ncst{k}(:,1),M.ncst{k}(:,2)); 
end;

Political Boundary info is available in shapefile format from Natural Earth.

9. Adding your own topography/bathymetry

A number of global and regional topography databases are available at NCAR . Several are available for free from their ftp site.

As long as the data is stored in a mat-file as a rectangular matrix in longitude/latitude, then m_contour or m_contourf can be used to plot that data.

Sandwell and Smith Bathymetry

A recent new bathymetry with approximately 1km resolution in lower latitude areas is being used by many people. This dataset is described at http://topex.ucsd.edu/marine_topo/ and is available as a 134Mb binary file at ftp://topex.ucsd.edu/pub/global_topo_2min/ (get the file topo_X.Y.img where X.Y is the version number). The authors have included an m-file (mygrid_sand.m) which can extract portions of the data (you will have to modiy path names within the code). Once this database (and the m-file) is installed on your computer, you can use it in M_Map very easily. A typical usage is as follows:

% Extract data
[elevations,lat,lon]=mygrid_sand([lat_south lat_north long_west long_east]);

% Use in M_Map command
m_contour(lon,lat,elevations);

mygrid_sand.m

10. Using TerrainBase 5-minute or ETOPO 2- or 1-minute global bathymetry/topography

For many purposes the elevation database accessed by M_Map provides adequate resolution. However, there are also many cases when more detail is desired. I have not included a higher-resolution database because it would greatly increase the size of the package. However, v1.2 includes m-files to access and plot a popular global 5-minute bathymetry/topography database, after a few minutes of work.

This section provides instructions on how to download TerrainBase, and convert it from a 56Mb ASCII file to a 18Mb binary file using m_tba2b.m. It is then straightforward to access and plot bathymetry from this file using m_tbase.m, which is in every way functionally identical to m_elev (see Section 3.2).

TerrainBase is also available on CDrom, and is also commonly stored in netcdf (or other) binary format somewhere on many academic networks. If you modify m_tbase.m to access data from one of these sources, let me know!

How to install TerrainBase:
1. get and uncompress the tbase.Z file from http://dss.ucar.edu/datasets/ds759.2/ into the m_map directory.
2. Run m_tba2b('PATHNAME') to store the resulting 18Mb binary file as PATHNAME/tbase.int.
3. Delete the original ASCII file tbase.
4. Edit the PATHNAME setting in m_tbase to point to the location of this file.
That's it! Test things out with this map of the western mediterranean:
```
m_proj('lambert','lon',[-10 20],'lat',[33 48]);
m_tbase('contourf');
m_grid('linestyle','none','tickdir','out','linewidth',3);
```
As of Apr 2006, there is a corrected higher-resolution (2 minute) database ETOPO2. Download http://rda.ucar.edu/dsszone/ds759.3/etopo2_2006apr/etopo2_2006apr.raw.gz (a gzipped binary), gunzip it into a 116Mb file, edit the PATHNAME setting in m_etopo2 to point to the location of this file, and then use it in the same way as m_tbase and m_elev. UCAR requires users to register and the second link won't work without you doing this (go to first link and follow instructions).

In 2014, it was pointed out to me that the above is obsolete. First, there is a corrected 2-minute ETOPO database - ETOPO2v2 which you should be using instead. Now, ETOPO2v2 is a little more complicated, because it comes in 4 version - big-endian and little-endian, in both cell-centered and grid-centered versions.
It doesn't particularly matter if you get big- or little-endian since you can modify the fopen line in m_etopo2 to account for this. I recommend getting the grid-centered version, since it works "better" when you are contouring the elevations (it will be more likely to extend all the way up to the map edge without weird little 'gaps').
In any case, download one of the zipped binaries, unzip it, and then edit 4 lines in m_etopo2 to set the PATHNAME, the filename in the fopen line, as well as setting the last option to 'b' or 'l' for big-endian or little-endian formats. Then make sure the grid and resolution parameters are set appropriately. If you forget (or get them wrong), code may run but it won't give the right bathymetry!
If you want even higher resolution bathymetry, you can also use the 1-minute ETOPO1 . This appears to come in two versions: grid or cell-referenced, both little-endian. Again, I recommend the grid-referenced version. Modify the relevant lines in m_etopo2 in the same way as for ETOPO2v2.

11. Using GSHHS high-resolution coastline database

Installing GSHHS

When drawing maps there is always a tradeoff between the execution time of the generating program and the resolution of the resulting map. Included in M_Map is a 1/4 degree coastline database which can be used to generate very fast maps, with adequate resolution for many purposes.

However, it is often desirable to be able to make detailed maps of limited geographic areas. For this purpose a higher-resolution coastline database is necessary. I have not included such a database in M_Map because it would greatly increase the size of the package. However, I have included m-files to access and use a popular high-resolution database called GSHHS

As distributed, GSHHS consists of a hierarchical set of databases at different resolutions. The lowest or "crude" resolution is not as good as the M_Map database, although it contains many more inland lakes. The "high" resolution consists of points about 200m apart. There is also an even finer "full" resolution. You can install part or all of the database (depending on how much disk space you have available). The "full" resolution occupies 90Mb of disk space, and successively coarser resolutions are smaller by about 1/4. Thus "high" resolution occupies 21Mb, "intermediate" uses 6Mb, and "low" uses 1.1Mb (one reason for not always using "high" resolution is that the entire 90Mb database must be read and processed each call, which may take some time).

How to install GSHHS:

Go to http://www.ngdc.noaa.gov/mgg/shorelines/data/gshhs/.
Get and uncompress any or all of the files gshhs_c.b.gz, gshhs_l.b.gz, gshhs_i.b.gz and/or gshhs_h.b.gz in a convenient directory. One useful place is in m_map/private. GSHHS data format has changed between v1.2 and 1.3, and again for v2.0, but m_map should be able to figure this out.
If the database files are not in subdirectory m_map/private, you must edit the FILNAME settings in m_gshhs_c.m, m_gshhs_l.m, m_gshhs_i.m, m_gshhs_h.m and/or m_gshhs_f.m to point to the appropriate files.

Using GSHHS effectively

m_coast

Section 3

m_gshhs_h('patch',[.5 .5 .5]);

m_proj ...  % set up projection parameters

% This command does not draw anything - it merely processes the 
% high-resolution database using the current projection parameters 
% to generate a smaller coastline file called "gumby"

m_gshhs_h('save','gumby');

% Now we can draw a few maps of the same area much more quickly

figure(1);
m_usercoast('gumby','patch','r');
m_grid;

figure(2);
m_usercoast('gumby','linewidth',2,'color','b');
m_grid('tickdir','out','yaxisloc','left');

etc.

12. M_Map toolbox contents and description

Contents.m - toolbox contents
m_demo.m - demonstrates a few maps.

m_proj.m - initializes projection
m_coord - geomagnstic to geographic coords
m_grid.m - draws grids
m_scale.m - forces map to a given scale
m_ruler - draws a scale bar

m_ungrid.m - erases map elements (if you want to change parameters)
m_coast.m - draws a coastline
m_elev.m - draws elevation data
m_tbase.m - draws elevation data from high-resolution database
m_etopo2.m - draws elevation data from (another) high-resolution database
m_gshhs_c.m - draws coastline from GSHHS crude database
m_gshhs_l.m - draws coastline from GSHHS low-resolution database
m_gshhs_i.m - draws coastline from GSHHS intermediate-resolution database
m_gshhs_h.m - draws coastline from GSHHS high-resolution database
m_gshhs_f.m - draws coastline from GSHHS full resolution database
m_plotbndry.m - draws a political boundary from the DCW
m_usercoast.m - draws a coastline using a user-specified subset database.
m_shaperead.m - reads ESRI shapefiles

m_plot.m - draws line data in map coords
m_line.m - draws line data in map coords
m_text.m - adds text data in map coords
m_legend.m - Draw a legend box
m_patch.m - adds patch data in map coords
m_pcolor - draws pcolor surface
m_quiver - draws arrows for vector data
m_contour - draws contour lines for gridded data
m_contourf - draws filled contours
m_track - draws annotated tracklines
m_range_ring - draws range rings

m_ll2xy.m - converts from long/lat to map coordinates
m_xy2ll.m - converts from map coordinates to long/lat
m_geo2mag - converts from magnetic to geographic coords
m_mag2geo - the reverse
m_lldist - distance between long/lat points
m_xydist - distance between map coordinate points

m_fdist - location of point at given range/bearing along ellipsoidal earth
m_idist - range/bearings between points on ellipsoidal earth
m_geodesic - points on geodesics between given points on ellipsoidal earth
m_tba2b.m - used in installing high-resolution elevation database.
m_vec.m - fancy arrows

private/mp_azim.m - azimuthal projections
private/mp_cyl.m - cylindrical projections (equatorial)
private/mp_conic.m - conic projections
private/mp_tmerc.m - transverse cylindrical projections
private/mp_utm.m - elliptical universal transverse cylindrical projections
private/mp_omerc.m - oblique cylindrical projection
private/mu_util.m - various utility routines
private/mu_coast.m - routines to handle coastlines.
private/mc_coords - coordinate conversion.
private/clabel.m - patched version of clabel (matlab v5.1 version does not contain capabilities for different text properties).
private/m_coasts.mat - coastline data

map.html - documentation intro
private/mapug.html - users guide
various .gif - examples.

13. Known Problems and Bugs

Running M_Map with Matlab5.0 (Student versions?) can sometimes produce errors since 5.0 has various bugs and "features" that do not appear in later versions. One in particular has cropped up - the file m_coast.mat is sometimes not found when using m_coast.m. The easiest solution is to put ../m_map/private into your path as well as ../m_map (in later versions matlab can find the mat-file).
Running M_Map on a PC with Matlab5.1 can sometimes produce a lot of
```
> Warning: Divide by zero.
```
messages. This is due to a bug in Matlab (actually due to the compiler TMW used) that results in an incorrect warning flag being set when dividing some numbers by NaN. You can safely ignore these errors and wait for v5.2
If plotted data is coloured white, this will be changed to black in the postscript output. This is due to the workings of the print command. In order to avoid this, set the figure background to white, i.e.
```
set(gcf,'color','white')
```
Generally weird-looking stuff that happens when you use filled contours. For some reason this has been a glory-hole for all kinds of weird bugs in MATLAB. Most of them relate somehow to the way in which the map background interacts with contourf patches, and how the 'renderer' (the internal matlab code that figures out what goes on top of what) works, or doesn't work. Unfortunately I can't think of way that works around the problem in all cases, but if you asee something weird, try:
```
 set(findobj('tag','m_grid_color'),'facecolor','none') 
```
after the
```
m_grid
```
call, or
```
 set(gcf,'renderer','opengl'); 
```
(under Unix you may have to do this one on starting MATLAB)
Things not appearing correctly in tiff output. Matlab uses ghostscript to covert from ps to many other formats. But their version has some problems. It is better to print to postscript and do the conversion (say, to tiff) yourself.

14. OCTAVE Compatibility Issues

GNU Octave

M_Map currently runs under Octave, sort of. In M_Map 1.4f a number of small incompatibilities have been addressed (mostly by spelling out the full names of graphics objects properties, rather than by using the shortened but unique versions allowable in MATLAB) and so it doesn't immediately bomb. However, there are still features that either don't work, or don't work properly.

Note: since "private" subdirectories are not implemented (as of Octave 3.2.3), you must add the m_map/private subdirectory explicitly to the path:

addpath [whatever]/m_map 
addpath [whatever]/m_map/private

Features that don't work, or don't work well, include:

Octave line commands do not accept MATRIX x/y parameters. This means your code might need to be changed when m_line is called.

3D patch functionality is not completely implemented in Octave. This means that fancy box outlines will not work, and so these have been disabled in M_Map if it recognizes that Octave is being used.

2D Patch functionality is still imperfect. Filled patches are not always correctly filled. This means filled coastlines, or filled contours, sometimes just look wrong. Stick with line plots.

Clipping functionality for line objects in Octave is imperfect. Sometimes lines outside the plot boundaries are not drawn even when 'clipping' is set to `'off'. Live with it.

The simple bathymetry called by m_elev comes from a built-in Matlab demo .mat file and this is not available in Octave.

Things 'in front' or 'behind' other things in plots are not always correctly rendered as being in front or behind.

Text label alignment (especially for the lat/long grid) can be a bit odd sometimes. Live with it.

Hatching and speckled outlines require units of 'points' for the axis, and although you can change the units property to 'points' it doesn't seem to change the numbers returned for the position. So avoid hatching or speckles.

15. Changes since last release

Finally including m_pcolor. Actually I could have done this a long time ago, but there are some philosophical reasons not to - it doesn't really do the "right" thing. But on the other hand it does do *something* (and I give examples)
geomagnetic/geographic coordinate systems. Let me know if you use this. It was an interesting thing to add but perhaps a waste of time...
Super-accurate geodesics on the ellipsoidal earth!

Speckled coastlines for the old-fashioned hand-drawn look

A SHAPEFILE reader.

Back to home page

Last changed 4/Dec/2011. Questions and comments to rich@eos.ubc.ca