Geography Reference
In-Depth Information
You can see once you have your locations set up in GRASS, projecting to
a new coordinate system is fairly simple. Using the same methodology,
you can also project rasters using
r.proj
, which we'll do as part of our
next geoprocessing task. Let's move on to some topics that are more
along the lines of “classic” geoprocessing.
10.2
Line-of-Sight Analysis
Line-of-sight analysis (LOS) is interesting from a curiosity standpoint as
well as for hard analysis. Suppose you want to know what you can see
from the top of the local mountain (assuming you don't live in Kansas).
With the right data to work with, LOS analysis can show you all the
areas that are visible from a given point on the map. Some practical
applications are determining the visibility of features in site planning.
Can the new garbage dump be seen from the local park? How many
people will be able to see the new 75-foot-tall monster transmission
tower (I have a new one in my backyard)? When doing LOS analysis, we
can specify not only the location to view from but also the height of the
observer (that would be us). Let's take a look at a simple LOS example.
In our example, we will use the GRASS
r.los
command to create a view-
shed (area we can see) from a given point. To do the analysis, we need
a raster dataset that has elevation information. A couple of examples
are the USGS Digital Elevation Model (DEM) product and the National
Elevation Dataset (NED).
2
We will use a 1:63,360 DEM (that's 1 inch = 1 mile) in our LOS analysis.
The steps to get from a raw DEM to our LOS viewshed are as follows:
1. Download the DEM.
2. Import the DEM into our world latitude-longitude mapset.
3. Project the DEM into the Albers coordinate system.
4. Use
r.los
to do the LOS analysis.
5. Use
r.mapcalc
to set unwanted values to null.
6. Display the results.
map interface to select your area of interest. You can find links to a good number of the
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