Geography Reference
In-Depth Information
We'll look at DEMs in more depth in Chapter 10 , Geoprocessing, on
page 149 , where we'll see how to use them in analyzing, creating con-
tours, and generating shaded reliefs—also known as hillshades.
The Grid of Silver
Let's see how we can use a grid to do some qualitative analysis. Harri-
son's interests are diverse—this time he's off to do some prospecting for
silver. In terms of the methodology, he could just as easily be examining
the concentration of hazardous materials or four-leaf clovers.
Harrison has found the results of a soil sampling survey that was done
over a regular grid. Each sample was analyzed for a number of ele-
ments, but he is interested only in silver. Fortunately for Harrison,
along with the results is a raster grid in Arc/Info Binary Grid format.
Harrison fires up QGIS and finds he can easily load the grid by choos-
ing the Add a Raster Layer tool from the toolbar and changing the file type
to “AIG GRASS and all other files (*).” The grid consists of a directory
containing several files. The one we (and Harrison) are interested in is
the . adf file. This contains the grid data, and QGIS knows how to dis-
play this format. In Figure 4.6 , on the following page, you can see what
the grid looks like loaded into QGIS and underlain by the DRG for the
region. In order to see the underlying DRG, we've made the grid par-
tially transparent using the transparency slider on the raster properties
dialog box.
Notice that the grid is “tilted.” That's because it has been transformed
from the original coordinate system, a simple X-Y grid, to one that
places it where it belongs in the real world. From looking at it, we
can conclude that whoever laid out the grid didn't do it using cardinal
directions of north-south and east-west. Transforming it to the proper
coordinate system also gives it a ragged appearance along the edges
since it's not possible to approximate a straight line given the cell size
in this grid.
We can use the Identify tool just as we did when working with our vec-
tor data (Section 3.5 , Identifying Features, on page 57 ) . Unlike vector
data where we might have several attributes for a feature, what we're
identifying here is the value for a single cell.
 
 
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