Mapping Hacks

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Chapter 6, Mapping on Your Desktop

#76 Explore the Effects of Global Warming

HACK

The first thing we notice is that the Netherlands almost disappear com-

pletely! Additionally, major cities like London, Copenhagen, and Venice fare

rather poorly. This visualization practically begs our other question: if the

sea level were to rise by 10 meters, how many people might be displaced? To

address this question, we’ll have to redo our analysis to specifically identify

which cells in our DEM raster will be submerged.

Method 2: Applying Raster Algebra

Our second approach will use raster algebra to identify which parts of our

elevation model lie below 10 meters, and hence are at risk of being sub-

merged. Raster algebra is simply any process that derives each cell of a new

output layer by applying a user-specified equation to each corresponding

cell in a set of input layers. The r.mapcalc tool performs this function in

GRASS:

GRASS 5.7.0:~/climate > r.mapcalc 'submerged = if(dem <= 10, 1, null( ))'

100%

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

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

Data range is 1 to 1

> 1 blue

> end

Color table for [submerged] set to rules

Our call to r.mapcalc creates a new layer called submerged, which contains a

value of

1 everywhere dem contains a value less than or equal to 10, and a

null value everywhere else. This gives us a raster layer that consists solely ...