Geography Reference
In-Depth Information
Joe Asks. . .
What's a Datum?
Although we could go into a complicated definition, a datum
is just a model for determining the coordinates of points on the
earth. You are likely to encounter the North American Datum
of 1927 (NAD 27), the North American Datum of 1983 (NAD 83),
and the World Geodetic System of 1984 (WGS 84).
It's important to make sure that either your data is in the same
datum or you are using software that can convert between
datums on the fly. What happens if you mix datums? Your data
won't line up as it should. All projections are based on a datum,
so make sure to understand your data before you start trying to
put it all together.
and longitude in a given datum. You'll find a lot of data in Geographic—
just make sure that the datum matches your intended use.
9.2
Working with Projec
tio
ns
Let's look at what we need to know to work with projections. When
using a chunk of data in our OSGIS software, we should determine the
following:
• Projection
• Units of measure
• Datum
In practice, most people don't care too much about these things until
they have a problem and the data doesn't overlay properly. Or worse
yet, it looks fine, and they think it's fine, but it's not. This can lead to
making decisions based on bad information. So, it's best to check your
projection parameters to make sure that everything is displayed where
it belongs.
Harrison just acquired a new megasized bird layer and is excited to use
it. Let's look at the ways he can discover the projection information.
Search WWH ::
Custom Search