Projections used for datasets

Most datasets in this database are in one of a relatively small number of projections. We have attempted to use a consistent set of abbreviated names for the various projections; these abbreviated names are often incorporated into the names of datasets and directories, and are also used in the projection definition file used by the LAS image processing software. Since the same projection type (e.g., Albers equal area) can be applied to different geographic regions by using different parameters, it is necessary to include region information in these short names (e.g., us_alb and pa_alb).

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.

geodeg: Latitude and longitude in degrees with decimal fraction; west longitudes and south latitudes are negative.
geodeg83: Latitude and longitude in degrees with decimal fraction, NAD83 datum; west longitudes and south latitudes are negative.
geosec: Latitude and longitude in seconds of arc; west longitudes and south latitudes have negative values.
geosec83: Latitude and longitude in seconds of arc, NAD83 datum; west longitudes and south latitudes have negative values.
mi_crstm: Transverse Mercator for Michigan, extensively used by the Center for Remote Sensing at Michigan State University.
na_lamcc: Lambert conformal conic covering North America.
na_lamaz: Lambert azimuthal equal area covering North America -- this is projection used by EDC for DEM and landcover data. EDC also uses us_lamaz for some North American data.
pa_alb: Albers equal area centered on Pennsylvania, NAD 83 datum. The upper left corner of a 286km x 496km rectangle containing PA is at (365000,-216000); the standard 390 km x 370 km rectangle used to bound the Susquehanna River Basin has its upper left corner at (450000, -80000). NOTE: This is a new version of the pa_alb projection, defined using the GRS80/WGS84 spheroid (NAD83 datum). Relative to the original pa_alb projection, which uses the NAD27 datum, points within Pennsylvania have an increase in easting between 22 and 26 meters, and in northing, between 10 and 16 meters.
pa_alb27: Albers equal area centered on Pennsylvania, NAD 27 datum. The upper left corner of a 286km x 496km rectangle containing PA is at (365000,-216000); the standard 390 km x 370 km rectangle used to bound the Susquehanna River Basin has its upper left corner at (450000, -80000). NOTE: This projection is defined using the Clarke 1866 spheroid (NAD27 datum). If the GRS80 spheroid (NAD83 datum), derived from satellite geodesy, were used instead, the Albers coordinates of a given point would be slightly greater than for NAD27. For points within Pennsylvania, the increase in easting is between 22 and 26 meters, and in northing, between 10 and 16 meters.
us_alb (also us_alb27): Albers equal area centered on conterminous 48 states, NAD27 datum.
us_alb83: Albers equal area centered on conterminous 48 states, NAD83 datum.
us_lamaz: Lambert azimuthal equal area covering 48 conterminous U.S. states -- this is projection used by EDC for DEM and landcover data.
usne_alb: Albers equal area optimized for northeastern U.S. extending from Ohio and Virginia to Maine, NAD83 datum. The upper left corner of a 1300 km E-W by 1550 km N-S rectangle containing this regions, with a 30 to 40 km buffer around the outside edges of these states, is located at -800000 meters easting, 1320000 meters northing.
utmZZ: UTM zone ZZ, NAD83 datum, where the zone number, ZZ, is between 10 and 19. The central longitude is 183 - 6*ZZ west of Greenwich; the zone extends 3 degrees either side of the central longitude.
utm17n27: UTM zone 17, NAD27 datum (western half of PA).
utm18n27: UTM zone 18, NAD27 datum (eastern half of PA).

Some additional projections are used only for data output by, or intended for input into, local hydrologic and atmospheric models.

hydro_na

A Lambert conformal conic projection used for low resolution hydrologic modeling of the entire North American continent with false easting and northing values chosen to place coordinates (0, 0) at the the southeast corner of the model grid.

msprcpps

A polar stereographic projection used by NOAA/NCEP for multi-sensor precipitation grids. The NCEP spherical radius has been reduced to 6371007 m; grid spacings and corner coordinates must be adjusted accordingly.

rtmalcc

A lambert conformal conic projection used for the National Weather Service Real Time Mesoscale Analysis (RTMA) products; the same projection is used for the 5-km National Digital Forecast Database (NDFD) grid. This grid uses a spherical earth of radius 6171.200 km, which has the same surface area as the International 1909 spheroid.

ruclcc80

A lambert conformal conic projection approximating the projection used by NOAA for Rapic Update Cycle (RUC) model outputs on the AWIPS-252 grid. For compatibility with other data, the AWIPS spherical earth has been replaced by the GRS80 spheroid; grid spacings and corner coordinates must be adjusted accordingly -- but corner longitude/latitide values can be matched only approximately.

The MM5 mesoscale atmospheric model uses a non-standard Lambert conformal conic projection, centered at the center of the outer domain used for a particular model run. The projection names incorporate the central latitude and longitude of the domain. Currently defined are

mml_3984

Lambert conformal conic for Mesoscale Model, centered at 39 deg. N, 84.5 deg. W, using spherical earth of radius 6370 km.

mml_4095

Lambert conformal conic for Mesoscale Model, centered at 40 deg. N, 95 deg. W, using spherical earth of radius 6370 km.

mml_4193

Lambert conformal conic for Mesoscale Model, centered at 41 deg. N, 93 deg. W, using spherical earth of radius 6370 km.

mml_maha

Lambert conformal conic for Mesoscale Model, centered on Mahantango Creek (40.7 deg. N, 76.625 deg W), using spherical earth of radius 6370 km.

mml_us48

Lambert conformal conic for Mesoscale Model, centered at 38.7 deg. N, 97 deg. W (the approximate center of the 48 conterminous states), using spherical earth of radius 6370 km.

Converting Between Projections

The projection definition and geolocation files for LAS (<image-name>.ddr) and Arc/Info (prj.adf and bnd.adf) record full descriptions of the map projection for each image or GIS layer. This is also true of many other software packages, although some (such as ERDAS 7.5) do not record all necessary parameters to support converting between projections.

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.

Parameters for Standard Projections

geodeg

	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.

geodeg83

	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.

geosec

	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.

geosec83

	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.

na_lamcc

	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

mi_crstm

	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

na_lamaz

	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

pa_alb

	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

pa_alb27

	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

us_alb (also us_alb27)

	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

us_alb83

	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

us_lamaz

	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

usne_alb

	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

utmZZ

	Projection units:	meters
	Spheroid:		GRS80/WGS84
	Central longitude:	6*ZZ - 183 degrees
	False easting:		500,000 meters
	False northing:		0

utm17n27

	Projection units:	meters
	Spheroid:		Clarke 1866
	Central longitude:	-81 deg (W)
	False easting:		500,000 meters
	False northing:		0

utm18n27

	Projection units:	meters
	Spheroid:		Clarke 1866
	Central longitude:	-75 deg (W)
	False easting:		500,000 meters
	False northing:		0

Parameters for North American Hydrologic Model

hydro_na

	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

Parameters for Multisensor Precipitation Grids

msprcpps

	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

Parameters for RTMA/NDFD Grids

rtmalcc

	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

Parameters for RUC grid approximation

ruclcc80

	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

Parameters for Mesoscale Model Projections

mml_3984

	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

mml_4095

	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

mml_4193

	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

mml_maha

	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

mml_us48

	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

Last change: 2004 Nov. 15, R. A. White / raw@essc.psu.edu