Mapping Hacks

Explore the Effects of Global Warming #76

Chapter 6, Mapping on Your Desktop

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HACK

Since the DEM layer import sets existing ocean cell values to null (i.e., no

value), we use the

bg parameter of d.rast to display these background cells as

blue, instead of white. Figure 6-47 shows the result. Now we can start to

examine what the coastlines will look like if the sea levels rise.

Method 1: Hacking the Color Table

Let us suppose the effect of global warming is to cause sea levels to rise by

10 meters, or about 32 feet. This is hopefully a very pessimistic prediction,

but it will serve to highlight the effects in question. There are two ways of

approaching our visualization of the resulting coastline. The really hackish

approach, which we will look at first, is to leave the data itself alone but

hack the DEM layer’s color table, so that the submerged bits appear to be

underwater.

First, let’s assign a custom color map to the DEM, using the

rules option of

r.colors, which lets us specify a set of values (in this case, elevation in

meters) and a color to assign to each one. GRASS will then shade each cell in

the DEM relative to the values we specify in our color map:

GRASS 5.7.0:~/climate > r.colors dem color=rules

Enter rules, "end" when done, "help" if you need it.

Figure 6-47. A colorized elevation model of Western Europe

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Chapter 6, Mapping on Your Desktop

#76 Explore the Effects of Global Warming ...