Q for all GMT users:
Where does the data come from? I recently helped out a friend of mine, who is a mathematician. He had to auto generate maps, so he used gmt. But he couldn´t tell me, where he got the geodata from. Which data formats can it handle?
Thanks in advance,
A simple answer would be: "Where all data comes from- GMT can use any sort of input."
A better answer would be:
For the shorline/political bounds/water bodies/ect. the GSHHS
is used. This provides a multi-scale linework for most small scale maps produced with GMT.
For the map that I submitted below, a couple datasources were used:
1. the California topography layer was downloaded from the Ca GIS page
. This data had to be inverse-projected back to lat-long, and have some holes filled. The inverse projection was done with GDAL, and the fixing of errors was done with GRASS. This file was essentially a pre-made hillshade, derived from generalized elevation data. GMT has the ability to generate a hillshade directly from raw elevation data as well.
2. The state and county lines were derrived from shapefiles, also from the Ca GIS page. They too were inverse-projected to lat/long and converted to a GMT-friendly text format. The conversion of shapefiles to GMT-style can be done with a freely available <i>shp2gmt</i>.
3.The point data was plotted from a simple text file of lat/long pairs, directly dumped from my GPS.
Summary: The data can come from nearly anywhere, but it usually must first be converted to a GMT-friendly format (i.e. NetCDF or ascii text). Also, the data may come in projected or un-projected coordinates, however all data within a single map should be in the same projection. If the input data are in lat/long, GMT will project it all to the coordinate system defined when plotting the data- as was the case of my example map.