Mapping Hacks

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Chapter 6, Mapping on Your Desktop

#77 Become a GRASS Ninja

HACK

completely within a single country—such as the British Isle—shows up, but

any landmass spanning more than one country—like, say, mainland North

America—has simply disappeared. However,

d.vect borders type=boundary

shows that the vectors are all there. What gives?

It turns out that, had the ninja trainee paid close attention, as every ninja

must, he would have noted the following warning emitted by each execu-

tion of v.patch:

Patch complete. 1 files patched.

Intersections at borders will have to be snapped.

Lines common between files will have to be edited.

The header information also may have to be edited.

A message like this from GRASS is a sure sign that the vector layer in ques-

tion needs a run through v.clean to fix what ails it and reassure GRASS that

the polygons contained therein really are polygons. The following does the

trick:

GRASS:~/hacks/world > v.clean in=borders out=borders2 tool=rmdupl,snap,bpol

v.clean needs a few minutes to work its magic (this is 13 megs of vector data,

after all) and it won’t overwrite the original layer, so we have to create a new

layer called

borders2. A quick call to d.vect reveals, as shown in Figure 6-53,

that the

borders2 layer does indeed contain a vector map of the world’s

international boundaries.

Having verified for ourselves that this is indeed the case, we can engage in a

bit of cleanup ...