Geography Reference
In-Depth Information
• ArcSDE Raster
• USGS SDTS DEM (
*CATD.DDF
)
• USGS ASCII DEM (.
dem
)
The list of formats may seem a bit daunting—don't worry if you don't
recognize all of them; the point is to illustrate the wide range available.
At last count, GDAL included support for seventy-three different raster
formats.
GDAL Utilities
Let's take a brief look at the command-line utilities that are part of
GDAL. We won't cover them all, just the ones that you're likely to find
useful from the outset. For a complete list and documentation, see the
GDAL website.
gdalinfo
This handy utility reports information about a raster, including,
if applicable, the coordinate system, color palette, extents, and
probably more than you want to know about your raster. This is
a quick way to examine a raster and get some information on it
without having to load it into a desktop application.
gdal_translate
This command allows you to copy a raster file and convert it to
another format. You can also add coordinate system information
to the output or create a subset of the image by specifying a sub-
window in pixel coordinates. Another neat trick is setting a certain
value in the output to
nodata,
, making it transparent (depending
of course on the software you use to view it).
gdaladdo
This utility adds overviews (commonly called pyramids) to a raster
to improve the display speed at smaller scales. On an image with
pyramids, as you zoom in, more detail will appear. One note of
caution, when using
gdaladdo
, your original image may be mod-
ified. For a GeoTIFF the pyramids are stored right in the original
image. It's a good idea to create a backup before running
gdaladdo
.
gdalwarp
With
gdalwarp
, you can “warp,” in other words, transform, a raster
from one coordinate system to another. This comes in handy if you
want to convert a raster into a local projection to match your other
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