Geography Reference
In-Depth Information
One note of caution before we move on: some databases, while they do support the
geometry_columns
and
spatial_sys_ref
metadata tables, don't create them by default. MS
SQL 2008 is noted for this; it uses its own methods for storing spatial metadata. You may
find that in some cases you will be required to create some of these tables manually before
you can use your database. Additionally, you may also find that some databases create the
tables but use a slightly different naming convention, especially for the
geometry_columns
table. For this reason, it's always better to use the official OGC-compliant spatial SQL
command set (which can be downloaded from
http://www.opengeospatial.org/standards/sfs
)
to manipulate the data in these tables, rather than trying to manipulate the entries directly.
Coordinate and Spatial Location Systems
Before we can get onto the technical fun stuff and start to play, we have to cover a little more
theory. You must understand why all these different SRIDs and coordinate systems exist.
I'd like to send you merrily on your way into your first GIS adventure right now and say this
stuff really doesn't matter; however, the truth is I can't and it does matter. In fact, it matters a
great deal.
If you don't comprehend this coordinate stuff correctly, it's possible to map an automobile's
track as being in the middle of the Atlantic Ocean. While this may not matter for the
application you're working on—you may be looking at a general overview of customer
dispersal, for example—you should still try to make sure your application is as accurate as it
can possibly be.
So the answer to the million-dollar question, "Why do we have to deal with all this coordinate
stuff?" boils down to one thing, and one thing only:
The Earth is not flat.
There, I said it. And all naysayers out there who still believe it is need to build themselves a
top-notch GIS and check it out.
Jokes aside though, it's the fact that our planet is a sphere that causes all these coordinate
system headaches. To make matters even worse, our humble home is not even a perfectly
round sphere. It's slightly elongated around its axis, a little like a rugby ball, but not quite as
pronounced. This causes further complications because the math we need to use as we look
at positions closer to the poles must compensate for the differences in the Earth's curvature.
Degrees, Minutes, and GPS
Okay, so how exactly do we deal with this curvature? There MUST be one measurement
that makes sense throughout the whole globe, right? If not, then how on Earth do airplanes
and ships navigate from country to country without getting lost or having to keep track all of
these different SRIDs?
You'll be pleased to know there is, but it's not as straightforward as just mapping an
x
position and a
y
position at a certain place on the globe.
