Geography Reference
In-Depth Information
a choice. You are forced to work in a particular projection for one rea-
son or another. When Harrison wanted to display his bird sightings on
the DRG, he needed to make sure they were in the same projection.
Rather than “warp” the raster, he found it easier to convert his sight-
ings from geographic to UTM, the same projection as the DRG. You can
warp your rasters (no, it's not illegal) if you find it more convenient
than transforming the dozens of vector layers in your dataset. For an
There are plenty of books and online resources that delve into the
details of projections and datums. Our goal here is to give you a brief
yet practical introduction to provide what you need to know to work
with your data. At the end of the chapter, you'll find some additional
resources you can use to learn more about the sometimes complex
world of projections and coordinate systems.
9.1
Projection Flavors
Projections come in three main flavors: planar or azimuthal, conic, and
cylindrical. The type indicates how the projection is constructed.
Azimuthal
In an azimuthal projection, the sphere (that's the earth) is pro-
jected onto a flat or planar surface. Examples of azimuthal projec-
tions include Orthographic, Stereographic, Gnomonic, Azimuthal
Equal Distant, and Lambert Azimuthal Equal Area.
Conic
In a conic projection, a spherical surface is projected on to a cone.
Examples of conic projections include Albers Equal Area, Lambert
Conformal, Equidistant Conic, and Polyconic Conic.
Cylindrical
In a cylindrical projection, the sphere is projected on to the walls
of a cylinder. Examples of cylindrical projections include Mercator,
Transverse Mercator, Oblique Mercator, Space Oblique Mercator,
and Miller Cylindrical. There are also a couple of pseudocylindri-
cal projections: Robinson Pseudo-cylindrical and Sinusoidal Equal
Area Pseudo-cylindrical.
Of course, the last projection we need to mention is Geographic, which
really isn't a projection at all. It's just a coordinate system of latitude
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