Maps are flat, but the surfaces they represent are curved. Transforming the three-dimensional space onto a two-dimensional map is called "projection" Many examples exist to help explain the projection process, but one of the best desribes the result of trying to flatten an orange peel. Take an orange and remove the peel, as much as possible, in one piece. When you try to flatten the peel, the edges crack, pieces break off, and parts of the peel remain raised and distorted.
Projections
make it possible to create maps of areas of the earth with as little
distortion as possible. The projection process affects four properties:
area, shape, distance, and direction. There
is no projection that maintains the integrity of all four properties
at the same time. A particular projection should be chosen based on
the importance to your project of one of the affected properties. For
example, if you want to analyze land use with respect to the percentage
of area used for different purposes (i.e. agricultural versus residential)
the reliability of your results rest with an accurate estimate of area.
Therefore, you would choose an "equal area" projection such
as Albers Conical Equal Area. An "equal area" projection is
one that reports accurate area measurements while incurring some distortion
of the remaining three properties-shape, distance, and direction. If
you want to optimize for shape, you would choose a "conformal projection"
such as Lambert Conformal Conic. A conformal projection maintains shapes
such as rectangles (buildings) at the expense of area, distance, and
direction. The Universal Transverse Mercator (UTM) projection tries
to maintain a happy medium among all four properties. This projection
is commonly used for smaller areas such as USGS 7.5' quadrangles. A
Geodetic datum is a reference point that describes the position, orientation
and scale relationships of a reference ellipsoid to the Earth. The North
American Datum of 1927 (NAD 27) uses Clarke spheroid of 1866 to represent
the shape of the earth. The North American Datum of 1983 (NAD 83) is
based upon both Earth and satellite observations, using the GRS80 spheroid.
Referencing geodetic coordinates to the wrong datum can result in position
errors of hundreds of meters. Be sure to check the datum under Spatial
Reference Information in PASDA metadata to be sure the data you are
using is in the same datum.
EXAMPLES:
Figure 1. Pennsylvania counties shown in the Albers Equal Area projection.
Albers Conical Equal Area
Within PASDA metadata you will find an area called "Spatial Reference
Information". This information lets you know what projection, datum,
and ellipsoid were used in creating the data. In addition, it also provides
you with the parameters for the projection. In the example below, the
standard parallels, longitude of central meridian, and latitude of projection
origin are listed. These are lines (as in latitude/longitude lines)
of reference that define the parameters of the projection.
Spatial Reference Information:
Horizontal Coordinate System Definition:
Planar:
Map Projection:
Map
Projection Name: Albers Conical Equal Area
Albers Conical Equal Area:
Standard Parallel: 40.0
Standard Parallel: 42.0
Longitude of Central Meridian: -78.0
Latitude of Projection Origin: 39.0
False Easting: 0.0
False Northing: 0.0
Planar Coordinate Information:
Planar
Coordinate Encoding Method: coordinate pair
Planar
Distance Units: meters
Geodetic Model:
Horizontal Datum Name: North
American Datum of 1927
Ellipsoid Name: Clarke 1866
Lambert Conformal
Conic Projection
Another common projection for PASDA data is Lambert Conformal Conic.
You
can see in the Spatial Reference Information below that the standard
parallels,
longitude of central meridian, and latitude of projection origin are
different from the
Albers Conical Equal Area projection parameters.
Spatial Reference
Information:
Horizontal Coordinate System Definition:
Planar:
Map
Projection:
Map Projection Name: Lambert Conformal Conic
Albers Conical Equal Area:
Standard Parallel: 33.0
Standard Parallel: 45.0
Longitude of Central Meridian: 96.0
Latitude of Projection Origin: 39.0
False Easting: 0.0
False Northing: 0.0
Planar Coordinate Information:
Planar
Coordinate Encoding Method: coordinate pair
Coordinate
representation:
Abscissa Resolution:
Ordinate Resolution:
Planar
Distance Units: meters
Geodetic Model:
Horizontal
Datum Name: North American Datum of 1927
Ellipsoid
Name: Clarke 1866
Figure 2. Pennsylvania counties shown in the Lambert Conformal Conic
projection.
Putting It All
Together
Figure 3. Pennsylvania counties shown in both Albers and Lambert
projections.
In order to effectively use PASDA data, you need to be sure that you
have
downloaded data that is in the same projection and datum. As you can
see in
Figure 3 below, data sets with different projections will not fit together
or "overlay."
The two county boundary data sets-one in Albers, the other in Lambert,
were
brought up in the same view in ArcGIS. It is clear that these two data
sets could
not be used together. Therefore, it is important to check the metadata
for each data
set before you download it. Checking the metadata saves time and effort.
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