Projection Definition File

Name	Projection Definition File
Acronym	PRJ
File Extension	*.prj

The prj-file is a plain text file describing the the coordinate system and projection information using text format.

A prj-file can be used to define a custom Orbit Coordinate system, which can be used to define the dataset crs : Coordinate Reference Systems in Orbit
The prj-file cannot be used as direct dataset coordinate definition. A dataset coordinate definition in Orbit is saved in the Orbit Resource Description file.

The name of the file should be a numeric code, like “1031370.prj” for example. This numeric code should be larger than 1000000 (one million) to avoid overlap with existing EPSG codes. An optional description can be added to the filename if an underscore character follows the numeric code, like “1031370_Belge 1972 - Belgian Lambert 72.prj” for example.

Information in PRJ file

Some typical information contained in the .prj file is:

Name of projection
Geographic coordinate system
Datum
Spheroid
Prime meridian
Map projection
- Latitude of origin
- Scale factor
- Central meridian
- False northing
- False easting
- Standard parallels
Units used

PRJ Features

Format

The text is created in the WKT-format: Support of WKT is under development. Not all features are fully supported (like vertical coordinate systems or special units). Feel free to send an email to support@orbitgis.com if your WKT file is not working as expected.

Do not forget to add the “TOWGS84” element (with seven parameters) to the “DATUM” element. See section “7.3.18 TOWGS84” of the WKT specification.

the units for the seven parameters are as follows: (meter,meter,meter, arc-sec,arc-sec,arc-sec, parts-per-million). The signs of the angles should be as the EPSG “Position Vector 7-parameter transformation” method (code 9606), not the “Coordinate Frame Rotation” method (code 9607). See “EPSG Guidance Note 7, Part 2” at: http://info.ogp.org.uk/geodesy/guides/docs/G7-2.pdf

Example

PROJCS["NAD83 / Austin", \
  GEOGCS["NAD83", \
    DATUM["North_American_Datum_1983", \
      SPHEROID["GRS 1980", 6378137.0, 298.257222101], \
      TOWGS84[0.0, 0.0, 0.0, 0.0, 0.0, 0.0, 0.0]], \
    PRIMEM["Greenwich", 0.0], \
    UNIT["degree", 0.017453292519943295], \
    AXIS["Lon", EAST], \
    AXIS["Lat", NORTH]], \
  PROJECTION["Lambert_Conformal_Conic_2SP"], \
  PARAMETER["central_meridian", -100.333333333333], \
  PARAMETER["latitude_of_origin", 29.6666666666667], \
  PARAMETER["standard_parallel_1", 31.883333333333297], \
  PARAMETER["false_easting", 2296583.333333], \
  PARAMETER["false_northing", 9842500.0], \
  PARAMETER["standard_parallel_2", 30.1166666666667], \
  UNIT["m", 1.0], \
  AXIS["x", EAST], \
  AXIS["y", NORTH], \
  AUTHORITY["EPSG","100002"]]

The text is inserted on a single line or with backslash characters

Supported Projection Methods

EPSG	Projection Name
9812	Hotine_Oblique_Mercator
9819	Krovak
9801	Lambert_Conformal_Conic_1SP
9802	Lambert_Conformal_Conic_2SP
9803	Lambert_Conformal_Conic_2SP_Belgium
9802	Lambert_Conformal_Conic
9804	Mercator_1SP
9815	Oblique_Mercator
9809	Oblique_Stereographic
9807	Transverse_Mercator

Why use PRJ-files

A lot of pre-defined Coordinate Reference Systems and Coordinate Transformations are accessible through the Orbit Software by using the EPSG Geodetic Parameter Dataset. However, the EPSG database does not record all possible geodetic parameters in use around the world. In addition to the pre-defined CRS in the EPSG database, OrbitGIS accepts custom CRS definitions by creating seperate PRJ-files.

Last modified:: 2014/10/16 21:23