Parent Topic: PROJ
PROJECTIONS
The following are a list of the supported projections and their
required input. Projections are usually categorized by the type
of the surface the earth is projected onto:
- cylindrical, where the projection is a cylinder wrapped around
the globe (e.g. Mercator projection).
- conic, where the projection is a cone placed over the globe
(e.g. Lambert Conformal Conic projection).
- azimuthal, where the globe is projected onto a plane (e.g.
Stereographic projection).
Other projections may be formed by modifying the basic types
(e.g. pseudo-cylindrical). The projection type is listed below
as the "General class" for the projection.
In projecting a sphere (the earth) to a surface (the image), the
geometry of the sphere is changed. A given projection usually
retains some characteristics of the sphere, usually at the expense
of other characteristics. The following terms are listed below as
"Characteristics" for the projections where:
- Equal-area implies a feature on one part of the map would cover
the same area, if it also occurred on any other part of the map.
- Conformal implies "correct form or shape". Actually it infers
the relative local angles about every point of the map as being
shown correctly. This only applies to smaller, local areas.
All projections distort shape for larger areas.
- Equidistant projections show true scale between one point (or
two points) and every other point on the map.
- Rectangular coordinates mean that the grid of the projection
can be used to approximate distances, directions and areas
of the earth surface. Rectangular coordinates are also
referred to as plane coordinates.
The earth model for the projection may be defined in the MAPUNITS
string or in the ELLIPS parameter. (See the EARTH_MODELS section.)
Some projections can process the earth as an ellipsoid or a sphere,
while some only process the earth as a sphere. This is noted for
the projection as the "Earth model:" using an "ellipsoid or sphere"
or "sphere". This is important if the earth model is not supported
and is defined as values in the ELLIPS parameter:
- For projections that use an "ellipsoid or sphere", ELLIPS may be
a. the semi-major axis (or equatorial radius or A) for the earth
b. the semi-minor axis (or polar radius or B) or the eccentricity
squared.
If the unsupported earth model is a sphere, this could be
defined in either of the following ways:
- do not specify a second value (i.e. only one radius is defined).
- specify the equatorial radius again (i.e. the polar radius
equals the equatorial radius).
- specify the second value as zero (i.e. the eccentricity
squared is zero).
In general, non-azimuthal projections (i.e. cylindrical or
conic) usually use an "ellipsoid or sphere". If no earth
model is defined for a projection that uses an "ellipsoid or
sphere", the earth model ellipsoid defaults to E000 (i.e.
Clarke 1866 or NAD 1927).
Projections that use a "sphere", will use the first value in
ELLIPS as the radius of the earth. If there is a second value
defined, it is ignored.
Azimuthal projections usually use a "sphere", but not an ellipsoid.
If no earth model is defined for projections that use a "sphere",
the default will be E019 (i.e. Normal Sphere of 6370997 metres).
The fields required for each projection are listed. Any parameters
not mentioned are not used for the projection.
Possible errors are listed for each projection. In general any
of the projections will exit with E333 ("Projection was not
initialized"), if the required parameters for the projection were
not specified.
Parent Topic: PROJ
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