Maps
Where does the data for these maps
come from?
Some maps were
rectangular, but some had curved edges. What's
up with that?
How were these maps made?
GMT is an open
source collection of ~60 tools for
manipulating geographic and Cartesian data sets (including filtering,
trend
fitting, gridding, projecting, etc.) and producing Encapsulated
PostScript
File (EPS) illustrations ranging from simple x-y plots via contour
maps to artificially illuminated surfaces and 3-D perspective views.
GMT supports
~30 map projections and transformations and comes
with support data such as coastlines, rivers, and political boundaries.
GMT
is developed and maintained by
Paul
Wessel
and
Walter
H. F. Smith
with help from
a
global set of volunteers,
and is supported by the
National Science
Foundation.
It is released under the
GNU
General Public License.
Where does the data for these maps come from?
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.
Projections
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.