Geography Reference
In-Depth Information
Determining the Projection
There are a number of ways to determine the projection for a dataset.
In some cases it's pretty easy—in others it can be quite difficult if the
person creating the data failed to include the information from the out-
set. For example, a shapefile is usually (or should be) accompanied by a
.
prj
file containing the projection information. This is just a text file con-
taining the projection parameters in what is known as Well-Known Text
(WKT) format, defined by the OGC OpenGIS Simple Features Implemen-
tation Specification for SQL. If you open a .
prj
file in your favorite text
editor, you'll see something similar to this:
GEOGCS["WGS 84",DATUM
["WGS_1984",SPHEROID["WGS 84",6378137,298.257223563,AUTHORITY["EPSG","7030"]],
AUTHORITY["EPSG","6326"]],
PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],
UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],
AUTHORITY["EPSG","4326"]
]
From looking at the WKT, we can determine that this layer is in geo-
graphic coordinates (GEOGCS), meaning it's not projected. We also see
that it is based on the WGS-84 datum and the units are degrees. There
is also a bunch of authority information that indicates the EPSG codes
for each section. See the sidebar on page
143
for additional information
on EPSG.
This gives us enough information to determine whether the layer can be
used with the rest of our data or whether we need to do some conversion
to make things line up properly. Let's look at the WKT for a projected
coordinate system:
PROJCS["Albers Equal Area",
GEOGCS["clark66",DATUM["D_North_American_1927",
SPHEROID["clark66",6378206.4,294.9786982]],
PRIMEM["Greenwich",0],
UNIT["Degree",0.017453292519943295]],
PROJECTION["Albers"],
PARAMETER["standard_parallel_1",55],
PARAMETER["standard_parallel_2",65],
PARAMETER["latitude_of_origin",50],
PARAMETER["central_meridian",-154],
PARAMETER["false_easting",0],
PARAMETER["false_northing",0],
UNIT["Meter",1]
]
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