Survey of Desktop Mapping Software - Desktop GIS: Mapping the Planet with Open Source Tools - page 275

data. You can also specify multiple input files to create a mosaic
from a group of rasters.
gdaltindex
If you use MapServer, you'll find this utility handy for creating an
index of the area covered by a group of rasters. Using gdaltindex
you can create a tile index (a shapefile) that can be used with
MapServer. You could also use the tile index as a coverage map to
see where your rasters are located.
gdal_contour
This utility will create contour lines from a Digital Elevation Model
(DEM), which is essentially a raster composed of cells of a given
size. Each cell has one attribute: an elevation. The smaller the
cells, the more accurately the elevation is depicted. With gdal_
contour , you specify the contour interval, and it kindly creates a
shapefile with contour lines. There are other ways to do this, but
gdal_contour is a quick way to make contour lines.
gdal_merge.py
If you have a bunch of adjacent images, gdal_merge.py allows you
to mosaic them together to create a single seamless image. This
can be handy, for example, when piecing together DRGs for a local
area so you can load just the one image along with your other data.
gdal-config
This utility is used to print the configuration options and other
information about the installed version of GDAL, including which
raster formats are supported. If you end up getting crazy and com-
piling your own OSGIS software, gdal-config is almost always used
during the configuration process to set things up for compiling
with the GDAL/OGR libraries.
OGR-Supported Formats
While GDAL is used with rasters, OGR provides tools to manipulate
vector GIS layers. OGR supports a wide variety of formats. In some
cases, the OGR library can create many of the formats it supports while
others are read-only. Many of the formats can also be georeferenced 15
using the library or the OGR utilities. OGR also has an impressive list
15. In this case, georeferencing means the format can contain information about the
coordinate system.