The input
map data set must be in map data set format (see SAS/GRAPH Map Data Sets),
and it must contain these variables:
The X and Y variables contain the values that are to
be projected.
In addition, the input map data set also can contain
Other variables in the input map data set do not affect
the GPROJECT procedure.
Note:
Projection is appropriate for map
data sets in which the X and Y variable values represent longitude and latitude.
Some of the map data sets that are supplied with SAS/GRAPH
have already been projected; such data set should not be projected again. ![[cautend]](../common/images/cautend.gif)
The following is a list of all of the Institute-supplied data
sets that contain X and Y variables whose values are unprojected:
See
Using Default Projection Specifications
for an illustration of this type of input map data set and the variables it
contains.
Most
map data sets contain both sets of variables (X, Y and LONG, LAT) for projected
and unprojected maps. In these cases, the X and Y variables will produce
a projected map so you don't need to use the GPROJECT procedure. However,
you may want to use the LONG and LAT variables to reproject the map using
a different projection type. To do this you must first rename the LONG and
LAT variables. It is necessary to rename the LONG and LAT variables because
the GPROJECT procedure looks for variables that are named X and Y by default.
You can create a new map data set using the OUT= option, drop the current
X and Y variables, and rename the LONG and LAT variables. Your new data set
will then contain unprojected values in X and Y. The following statements
illustrate how to do this:
proc gproject data=map.austral
(drop=x y rename=(long=x lat=y))
out=reflib.newaust;
id id;
run;
For additional information on the supplied SAS/GRAPH
map data sets, see SAS/GRAPH Map Data Sets and the METAMAPS data
set in your maps data set directory.
Longitude and Latitude Coordinates
shows the standard coordinate system for map data sets with coordinates in
longitude and latitude. For the longitude and latitude values (below and
to the right of the figure, respectively) the upper value is expressed in
degrees and the lower value is expressed in radians. A radian is approximately
57.3 degrees.
Longitude and Latitude Coordinates
![[IMAGE]](./images/01215363.gif)
By default, the GPROJECT procedure assumes that the units
for the input coordinate values are radians and that values for the horizontal
coordinate increase from east to west across the map. If your map coordinates
are stored as degrees of arc, use the DEGREE option in the PROC GPROJECT statement.
If the horizontal coordinate values in the map increase west-to-east rather
than east-to-west, use the EASTLONG option in the PROC GPROJECT statement.
See Options
for details of DEGREE and EASTLONG.
The unprojected map data sets that are provided with
SAS/GRAPH can be projected if you
use the default procedure characteristics: coordinate units in the data sets
are radians, and horizontal values increase east-to-west.
The GPROJECT procedure
performs three different types of projection:
Albers' equal-area projection with two standard parallels (the default method),
Lambert's conformal projection with two standard parallels, or the gnomonic
projection (an azimuthal equidistant projection).
These sections describe the basic theory of each projection
method. For comparison, Unprojected Map shows an unprojected map of the
northern hemisphere.
Unprojected Map
![[IMAGE]](./images/01215364.gif)
The Albers' projection
is a conic
projection from the surface of the sphere to a cone secant to the sphere,
cutting it at two standard parallels of latitude. The axis of the cone coincides
with an extension of the polar axis of the sphere. Each section of the resulting
map bears a constant ratio to the area of the sphere. In general, distortion
in shape tends to increase toward the poles in latitudes outside of the two
standard parallels.
The Albers' projection is well suited to portray areas
of large and small east-to-west extent and produces satisfactory results in
most cases. However, both standard parallels must lie on the same side of
the equator, so this method may not be suitable for map data sets of large
north-to-south extent that span the equator. For those map data sets, use
the gnomonic projection method.
Albers' Projection
illustrates an Albers' equal-area projection of the northern hemisphere.
Albers' Projection
![[IMAGE]](./images/01215365.gif)
The Lambert's projection is obtained from a secant cone in the same manner as Albers'
projection.
In the Lambert's projection, meridians of longitude are straight lines that
radiate from the apex of the cone, while parallels of latitude are concentric
circles. The Lambert's projection is somewhat better than the Albers' projection
at representing the original shape of projected unit areas, while the Albers'
projection is somewhat better at representing relative sizes of projected
unit areas.
The Lambert's projection is ideal for navigational charts
and maps of relatively small east-to-west extent. However, as in the Albers'
projection, both standard parallels must lie on the same side of the equator,
so this method may not be suitable for map data sets that span the equator.
For those map data sets, use the gnomonic projection method.
Lambert's Projection
illustrates a Lambert's conformal projection of the northern hemisphere.
Lambert's Projection
![[IMAGE]](./images/01215366.gif)
The gnomonic projection is a planar projection
from the surface of the sphere directly onto an imaginary plane tangent to
the sphere at the map projection pole. By default, the projection pole is
placed at the center of the map data set that is to be projected, but you
can specify the projection pole to be anywhere on the surface of the sphere.
(See POLELONG= and POLELAT= options.)
In the gnomonic
projection, distortion increases as
the distance from the map pole increases. Because of this distortion, the
PROC GPROJECT procedure deletes all of the observations that lie more than
85 degrees from the map pole. The gnomonic projection is best suited for
mapping areas of small east-to-west extent.
Gnomonic Projection
illustrates a gnomonic projection of the northern hemisphere.
Gnomonic Projection
![[IMAGE]](./images/01215367.gif)
Copyright © 1999 by SAS Institute Inc., Cary, NC, USA. All rights reserved.
