Projections and Coordinate Systems - Desktop GIS: Mapping the Planet with Open Source Tools - page 131

Here we see that the layer is projected (PROJCS); it's based on the
Clarke 1866 spheroid, NAD 27 datum; the projection is Albers; and
the units are meters. The WKT also contains the parameters (standard
parallels, origin, and central merdian) for the Alaska Albers Equal Area
Conic projection.
The key things to look for are the PROJCS or GEOGCS at the beginning
of the WKT specification, the PROJECTION and DATUM keywords, and
the UNIT keyword. These are enough to tell us whether it's suitable for
use with our other data.
The second way to determine the projection for a layer is using the gdal-
info command for rasters and ogrinfo for vector layers. Recall that these
utilities are part of GDAL/OGR. We took a look at these commands in
Chapter 3 , Working with Vector Data, on page 37 and Chapter 4 , Work-
ing with Raster Data, on page 67 . If you look back to those chapters,
you'll see that both commands display the projection information in the
same WKT format we just looked at.
The final way to determine a dataset's projection is to load it into your
OSGIS application and check the properties for the layer. This works
fine and is handy if you already have the layer loaded, but it's quicker
to look at the WKT for a vector or use one of the GDAL/OGR commands
to get the information. Since gdalinfo and ogrinfo work with nearly every
format you'll encounter, it's worth installing and using them.
Data Problems
If you find that your data isn't lining up like you expect, it's either a
projection problem or you just have lousy data. Seriously, though, most
alignment problems are due to either a projection problem or differing
datums. The first thing to do is use your sleuthing skills to examine the
projection and datum for your layers. If you are seeing a big difference
in alignment, it's likely a projection problem. If it's a small difference
(less than 500 meters, for example), you likely have a datum problem.
To get to the bottom of it, you have a couple of choices. If your OSGIS
application supports on-the-fly projection (and datum shift if required),
enable it and make sure that the projections are recognized by the
application. If this solves the problem, you don't need to do anything
else; just keep in mind that you are transforming data on the fly—you
haven't changed the original data in any way.