GMT is an open
source collection of ~60 tools for
manipulating geographic and Cartesian data sets (including filtering,
fitting, gridding, projecting, etc.) and producing Encapsulated
File (EPS) illustrations ranging from simple x-y plots via contour
maps to artificially illuminated surfaces and 3-D perspective views.
~30 map projections and transformations and comes
with support data such as coastlines, rivers, and political boundaries.
is developed and maintained by
H. F. Smith
with help from
global set of volunteers,
and is supported by the
It is released under the
General Public License.
There are two well-known public-domain data sets used by GMT. One is known as the World Data Bank II or CIA Data Bank (WDB) and contains coastlines, lakes, political boundaries, and rivers. The other, the World Vector Shoreline (WVS) only contains shorelines between saltwater and land (i.e., no lakes). It turns out that the WVS data is far superior to the WDB data as far as data quality goes, but as noted it lacks lakes, not to mention rivers and borders. GMT uses the WVS whenever possible and supplements it with WDB data. These data are available on CD-ROM from the National Geophysical Data Center in Boulder, Colorado.
How do you map a round world onto a
flat piece of paper or computer screen?
You may have noticed that when you peel an orange, you can't flatten the orange peel without stretching or tearing it. The same is true of a map of the earth. The curved earth's surface can't be put onto a flat map without stretching or tearing it. The process of mapping a curved earth onto a flat surface is called "projection."
The rectangular map was made using a
projection which flattens the globe by stretching the upper latitudes
-- this results in Canada looking a lot larger than it should.
The second map was made by projecting the globe onto a cone.
While not a perfect fit, it better preserves the actual sizes and
shapes of geographic features near the poles.
GMT implements 25 different map projections grouped into four categories depending on the nature of the projection. The groups are
- Conic map projections
- Azimuthal map projections
- Cylindrical map projections
- Miscellaneous projections
For more information about the projections GMT uses, see the Technical Reference and Cookbook in the GMT DOCS link. For general information about map projections, see Map Projection Overview or search for "map projections" on the internet.