A further complication in naming projections results from the recent introduction of the North American Datum of 1983 (NAD83) as a replacement for the long-established North American Datum of 1927 (NAD27). NAD83 is based on the GRS80 spheroid, derived from satellite geodesy, whereas NAD27 uses the Clarke 1866 spheroid. A point having a given latitude and longitude in NAD27 may be displaced on the order of tens of meters from another point having the identical latitude and longitude in NAD83. In some cases, "n27", "n83", "27", or "83" is appended to the projection key to indicate which datum has been used; this has not been done in a consistent manner.
Most GIS and image processing software packages provide support for converting between projections.
The standard projections currently defined are listed below; details of each projection are available by clicking on its name.
Some additional projections are used only for data output by, or intended for input into, local hydrologic and atmospheric models.
Arc/Info provides the PROJECT command to transform a dataset to a new projection.
The LAS REMAP program may be used to create a "mapping grid" which defines a piecewise bilinear approximation to the transformation from one projection or coordinate system to another. The mapping grid is then used by program GEOM or REGRID to actaully transform the image. Program TRANCOORD is also available for transforming the coordiantes of a set of points. Both REMAP and TRANCOORD use projection definitions stored in a master file; for the projections used in this database, this file is HREF="http://dbwww.essc.psu.edu/dbtop/map_projections/las.proj"> /eidb/map_projections/las.proj.
A free standalone program for converting coordinate pairs between projections, PROJ.4 , is installed on the ESSC network. It may be invoked using the command "proj"; the man page for proj provides additional information. The PROJ.4 package also includes a set of C-language coordinate transformation functions for incorporation into other programs.
The USGS and other agencies are in the process of moving from the Clarke 1866 spheroid to a new spheroid derived from satellite geodesy (GRS80, whose axes match those of the Defense Mapping Agency's WGS84 spheroid to within a few millimeters), with a corresponding change in North American Datum from NAD27 to NAD83. As a result, the latitude and longitude of a point within the conterminous 48 states may change by an amount equivalent to as much as 100 meters when going from NAD27 to NAD83. For UTM, the value of the northing, which is computed as the distance from the equator along the surface of the approximating spheroid, is increased by an additonal 150 to 250 meters because the GRS80 spheroid is somewhat larger than the Clarke 1866 spheroid. The Arc/Info PROJECT command and the LAS REMAP and TRANCOORD modules now support conversions between NAD27 and NAD83. It is not clear how adquately other image processing and GIS software packages correct for these differences. BE PREPARED FOR PROBLEMS!Note that different software packages interpret the upper-left and lower-right coordinates of an image or GIS cover differently. In Arc/Info, the upper left corner of a gridded data set is interpreted to mean the upper left corner of the upper left grid cell, and the lower right corner of the data set is the lower right corner of the lower right grid cell; this is consistent with the definition for a vector GIS coverage. LAS and ERDAS, on the other hand, associate the value assigned to a pixel or grid cell with the point at the center of the cell; accordingly, the upper left and lower right corners of the image or GIS layer are the coordinates of the centers of the upper left and lower right pixels, respectively. When using Arc/Info GRIDIMAGE and IMAGEGRID (which are also used by LAS routines ARC2LAS and LAS2ARC), the proper conversion is made between the Arc/Info corner coordinates and the LAS/ERDAS corner coordinates.
Projection units: Degrees with decimal fraction, measured from latitude = 0, longitude = 0; west longitudes and south latitudes have negative values. Spheroid: Clarke 1866. Note: latitude and longitude values are dependent upon the spheroid chosen.
Projection units: Degrees with decimal fraction, measured from latitude = 0, longitude = 0; west longitudes and south latitudes have negative values. Spheroid: GRS80/WGS84. Note: latitude and longitude values are dependent upon the spheroid chosen.
Projection units: Seconds of arc, measured from latitude = 0, longitude = 0; west longitudes and south latitudes have negative values. Spheroid: Clarke 1866. Note: latitude and longitude values are dependent upon the spheroid chosen.
Projection units: Seconds of arc, measured from latitude = 0, longitude = 0; west longitudes and south latitudes have negative values. Spheroid: GRS80/WGS84. Note: latitude and longitude values are dependent upon the spheroid chosen.
Projection units: meters Spheroid: GRS80/WGS84 1st standard parallel: 30 deg N 2nd standard parallel: 60 deg N Central meridian: -100 deg (W) Latitude of origin: 45 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: Clarke 1866 Central scale factor: .9996 Central meridian: -86 deg (W) Latitude of origin: 44 deg N False easting: 359987.0 False Northing: 344917.0
Projection units: meters Spheroid: sphere of radius 6,370,997 meters Central longitude: -100 deg (W) Central latitude: 50 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: GRS80/WGS84 1st standard parallel: 40 deg N 2nd standard parallel: 42 deg N Central meridian: -78 deg (W) Latitude of origin: 39 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: Clarke 1866 1st standard parallel: 40 deg N 2nd standard parallel: 42 deg N Central meridian: -78 deg (W) Latitude of origin: 39 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: Clarke 1866 1st standard parallel: 29 deg 30 min N 2nd standard parallel: 45 deg 30 min N Central meridian: -96 deg (W) Latitude of origin: 23 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: GRS80/WGS84 1st standard parallel: 29 deg 30 min N 2nd standard parallel: 45 deg 30 min N Central meridian: -96 deg (W) Latitude of origin: 23 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: sphere of radius 6,370,997 meters Central longitude: -100 deg (W) Central latitude: 45 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: GRS80/WGS84 1st standard parallel: 38 deg N 2nd standard parallel: 44 deg N Central meridian: -76 deg (W) Latitude of origin: 36 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: GRS80/WGS84 Central longitude: 6*ZZ - 183 degrees False easting: 500,000 meters False northing: 0
Projection units: meters Spheroid: Clarke 1866 Central longitude: -81 deg (W) False easting: 500,000 meters False northing: 0
Projection units: meters Spheroid: Clarke 1866 Central longitude: -75 deg (W) False easting: 500,000 meters False northing: 0
Projection units: meters Spheroid: GRS80/WGS84 1st standard parallel: 30 deg N 2nd standard parallel: 60 deg N Central meridian: -100 deg (W) Latitude of origin: 45 deg N False easting: 3700000 False northing: 4100000
Projection units: meters Spheroid: sphere of radius 6,371,007 m Central meridian: -105 deg (W) Latitude of true scale: 60 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: sphere of radius 6,371,200 m 1st standard parallel: 25 deg N 2nd standard parallel: 25 deg N Central meridian: -95 deg (W) Latitude of origin: 0 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: GRS80/WGS84 1st standard parallel: 25 deg N 2nd standard parallel: 25 deg N Central meridian: -95 deg (W) Latitude of origin: 39 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: sphere of radius 6,370,000 meters 1st standard parallel: 30 deg N 2nd standard parallel: 60 deg N Central meridian: -84 deg 30 min (W) Latitude of origin: 41 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: sphere of radius 6,370,000 meters 1st standard parallel: 30 deg N 2nd standard parallel: 60 deg N Central meridian: -95 deg (W) Latitude of origin: 40 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: sphere of radius 6,370,000 meters 1st standard parallel: 30 deg N 2nd standard parallel: 60 deg N Central meridian: -93 deg (W) Latitude of origin: 41 deg N False easting: 0 False northing: 0
Projection units: meters Spheroid: sphere of radius 6,370,000 meters 1st standard parallel: 30 deg N 2nd standard parallel: 60 deg N Central meridian: -76 deg 37 min 30 sec (W) Latitude of origin: 40 deg 42 min N False easting: 0 False northing: 0
Projection units: meters Spheroid: sphere of radius 6,370,000 meters 1st standard parallel: 30 deg N 2nd standard parallel: 60 deg N Central meridian: -97 deg (W) Latitude of origin: 38 deg 42 min N False easting: 0 False northing: 0