Mapping Hacks

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Chapter 6, Mapping on Your Desktop

#77 Become a GRASS Ninja

HACK

So far, so good. However, a single glance at Figure 6-52 immediately reveals

that it depicts rather a few more boundaries than one is accustomed to see-

ing on a map of Europe. The international boundaries are there, to be sure,

but so is every other first-level administrative boundary in the world, mak-

ing individual countries a bit hard to pick out. This is well and good, but it

also means we’re not done yet.

As it turns out, ESRI distributes a cntry98.shp data set that

avoids some of these data-creation issues. However, we’ve

never been able to find a copy of this data set available for

download on the Web.

Objective #2: Reunite the World’s Nations

The would-be GRASS ninja is not daunted at this juncture. Surely, the ninja

trainee must concur, GRASS must have some facility for merging polygons.

We simply tell GRASS to merge all of the adjacent shapes belonging to the

same country in a new vector layer, and we are done.

If a ninja-in-training may be permitted one flaw, perhaps that flaw is over-

confidence. After a bit of poking in the GRASS documentation, we were

forced to conclude that GRASS had no such tool, per se. GRASS has a tool,

v.extract—which extracts vector features into a new layer, possibly based on

an SQL

WHERE query against the attribute table—and even has an option to

“dissolve” the borders of adjacent polygons in ...