Projections
Maps are flat, but the surfaces they represent are curved. Transforming the three-dimensional space onto a two-dimensional map is called "projection". There are many examples that can be used to describe the projection process. One of the most common describes 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.
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 | |||||
Figure 1. Pennsylvania counties shown in the Albers Equal Area projection.
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.
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 ArcView. 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.
Figure 3. Pennsylvania counties shown in both Albers and Lambert projections.