Geography Reference
In-Depth Information
Getting the DEM
The first task is to get the DEM and get it ready for use in the analy-
sis. The DEM we chose is for the Anchorage C6 quadrangle in Alaska.
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The file (
ancc6.gz
) came gzipped, so before it can be imported, it must be
unzipped. We used
gzip -d
to unzip it and then renamed it to
ancc6.dem
.
If we wanted to, we could view it right now using QGIS because it sup-
ports USGS ASCII DEMs. To import it into GRASS, we need a geo-
graphic location since the DEMs coordinates are in degrees of latitude
and longitude. The datum of the location must match the DEM as well.
In the case of our DEM, that's NAD 27. If you can't remember the gory
To import the DEM into our geographic location, from the GRASS shell
we use the following:
r.in.gdal input=ancc6.dem output=ancc6_dem title="Anchorage C6 DEM"
Now we need to project the DEM to our Albers coordinate system. You
might be asking why we have to project it. Well, the answer is,
r.los
doesn't work with geographic coordinates. If you try it, you'll get a nice
message along the lines of this:
ERROR: Lat/Long support is not (yet) implemented for this module.
To project the DEM, we use the
r.proj
command. But first we need to
have an Alaska Albers location created using the proper parameters.
For this example, we created one and set the default region (part of the
creation process) to just the area of our DEM. If you need to know the
parameters for a location, you can use
g.proj -p
from the GRASS shell to
print the projection information for the current location.
> g.proj -p
-PROJ_INFO-------------------------------------------------
name : Albers Equal Area
proj : aea
datum : nad27
a : 6378206.4
es : 0.006768657997291279
lat_1 : 55
lat_2 : 65
lat_0 : 50
lon_0 : -154
x_0 : 0
y_0 : 0
3.
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