Mapping Hacks

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Chapter 6, Mapping on Your Desktop

#76 Explore the Effects of Global Warming

HACK

and see, if you like. d.what.rast can be used to query individual raster cells

with your mouse.)

To address this, we’ll employ r.null to set unpopulated areas in our popula-

tion data set to “No Value.” For good measure, we will replace the original

population layer with the new one and assign it a custom color map, since

none of the built-in color maps look very good with it:

GRASS 5.7.0:~/gis/europe > r.null population setnull=0

Writing new data for [population]... 100%

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

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

Data range is 0 to 626281

> 0 green

> 1000 yellow

> 10000 red

> 700000 purple

> end

Color table for [population] set to rules

GRASS 5.7.0:~/climate > d.erase

GRASS 5.7.0:~/climate > d.rast population bg=blue

100%

Once the custom color map is applied, we get a very nice looking popula-

tion map of Europe, as shown in Figure 6-51.

Estimating Population Displacement

Now, back to our second question, which was, “How many people in West-

ern Europe will be displaced from their homes by a 10-meter rise in sea

level?” We are finally at the point where we can estimate this number, by

tallying up the values of all the cells in our

population layer that match the

non-null cells in our

submerged layer. Fortunately, GRASS provides a func-

tion for summing all