Geoprocessing - Desktop GIS: Mapping the Planet with Open Source Tools - page 153

Upper Left ( 249831.465, 1318446.415)
Lower Left ( 249831.465, 1289076.572)
Upper Right ( 271695.535, 1318446.415)
Lower Right ( 271695.535, 1289076.572)
Center ( 260763.500, 1303761.494)
Band 1 Block=410x6 Type=Byte, ColorInterp=Red
Band 2 Block=410x6 Type=Byte, ColorInterp=Green
Band 3 Block=410x6 Type=Byte, ColorInterp=Blue

We can see from the gdalinfo output that our new TIFF is a three-
band image and compressed with packbits compression. That's good,
because it's what we specified when we created the image. Notice what's
missing—there is no coordinate system defined for the image. When you
export an image using r.out.tiff , it doesn't encode the coordinate system
information into the TIFF. We could add this using gdal_translate . For
more information on gdal_translate and friends, see Section 11.2 , Using
GDAL and OGR, on page 186 .

When you go to share your georeferenced hillshade maps with the rest
of the world, make sure to include the world file. Otherwise, it may fall
off the face of the earth when your friends attempt to display it with the
rest of their data.

10.5 Merging Digital Elevation Models

In this section we'll look at how to merge DEMs to create a single map
layer in GRASS, and there is a very good reason to do so. As you look
around the Internet, you'll find that a lot of the available data (DEMs
included) is tiled. This means you have to download more than one file
to get the complete dataset. Sometimes just having one tile is fine, as
long as it covers the area you need. Other times, you might find you
need several adjacent tiles to get the coverage you want. To illustrate
the process, we'll merge several GTOPO30 5

DEMs into a single layer.

To begin, we fetched all the DEMs for the Americas and stashed them
in a directory. The DEM files are distributed in a tar-gzipped format so
you'll have to unpack them before proceeding. On Linux or OS X you
can just use this:

tar -xzf w100s10.tar.gz

On Windows use a zip file manager that supports tar.gz files such as
7-zip. Before we proceed with the import, we need to edit the . HDR file

5. http://edc.usgs.gov/products/elevation/gtopo30/gtopo30.html