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Projection Basics

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This tutorial introduces the basic classes, methods, and functions of the proj module. It focuses mainly on basic topics such as:

  • Projection construction and representation
  • Coordinate reference system transformations

EPSG

The European Petroleum Survey Group is the body that maintains the EPSG Geodetic Parameter Dataset, better known as the “EPSG database”. This dataset is the primary source of supported coordinate reference systems in GeoTools, and by extension GeoScript.

Projection Objects

The Projection class is the primary class of the proj module. Projection objects can be constructed in one of two ways. The first is by specifying an EPSG code reference. The second is by specifying the Well Known Text (WKT) of the coordinate reference system directly.

>>> from geoscript.proj import Projection

>>> Projection('EPSG:4326')
GEOGCS["WGS 84",
  DATUM["World Geodetic System 1984",
    SPHEROID["WGS 84", 6378137.0, 298.257223563, AUTHORITY["EPSG","7030"]],
    AUTHORITY["EPSG","6326"]],
  PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
  UNIT["degree", 0.017453292519943295],
  AXIS["Geodetic latitude", NORTH],
  AXIS["Geodetic longitude", EAST],
  AUTHORITY["EPSG","4326"]]

>>> Projection('GEOGCS["GCS_WGS_1984",DATUM["WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]')
GEOGCS["GCS_WGS_1984",
  DATUM["WGS_1984",
    SPHEROID["WGS_1984", 6378137.0, 298.257223563]],
  PRIMEM["Greenwich", 0.0],
  UNIT["degree", 0.017453292519943295],
  AXIS["Longitude", EAST],
  AXIS["Latitude", NORTH]]
js> var proj = require("geoscript/proj")

js> var wgs84 = proj.Projection("epsg:4326")
js> wgs84
<Projection EPSG:4326>

js> wgs84.wkt
GEOGCS["WGS 84",
  DATUM["World Geodetic System 1984",
    SPHEROID["WGS 84", 6378137.0, 298.257223563, AUTHORITY["EPSG","7030"]],
    AUTHORITY["EPSG","6326"]],
  PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
  UNIT["degree", 0.017453292519943295],
  AXIS["Geodetic longitude", EAST],
  AXIS["Geodetic latitude", NORTH],
  AUTHORITY["EPSG","4326"]]

  js> var p = proj.Projection('GEOGCS["GCS_WGS_1984",DATUM["WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]')
  js> p
  <Projection EPSG:4326>
groovy:000> import geoscript.proj.Projection

groovy:000> p = new Projection("EPSG:4326")
===> EPSG:4326

groovy:000> p.id
===> EPSG:4326

groovy:000> p.wkt
===> GEOGCS["WGS 84",
  DATUM["World Geodetic System 1984",
    SPHEROID["WGS 84", 6378137.0, 298.257223563, AUTHORITY["EPSG","7030"]],
    AUTHORITY["EPSG","6326"]],
  PRIMEM["Greenwich", 0.0, AUTHORITY["EPSG","8901"]],
  UNIT["degree", 0.017453292519943295],
  AXIS["Geodetic longitude", EAST],
  AXIS["Geodetic latitude", NORTH],
  AUTHORITY["EPSG","4326"]]

groovy:000> p = new Projection('GEOGCS["GCS_WGS_1984",DATUM["WGS_1984",SPHEROID["WGS_1984",6378137,298.257223563]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]]')
groovy:000> p.wkt
===> GEOGCS["GCS_WGS_1984",
  DATUM["WGS_1984",
    SPHEROID["WGS_1984", 6378137.0, 298.257223563]],
  PRIMEM["Greenwich", 0.0],
  UNIT["degree", 0.017453292519943295],
  AXIS["Longitude", EAST],
  AXIS["Latitude", NORTH]]

groovy:000> p.id
===> EPSG:432

Determining EPSG Codes

Often a spatial reference system is encountered that does not have an EPSG code associated with it. This can happen for a few different reasons.

  • The coordinate reference system does not exist in the EPSG database
  • The WKT representation is not complete enough to map to an EPSG entry
>>> from geoscript.proj import Projection

>>> wkt = 'PROJCS["NAD_1983_HARN_StatePlane_Colorado_Central_FIPS_0502_Feet", GEOGCS["GCS_North_American_1983_HARN", DATUM["Unknown", SPHEROID["GRS_1980", 6378137.0, 298.257222101]], PRIMEM["Greenwich", 0.0], UNIT["degree", 0.017453292519943295], AXIS["Longitude", EAST], AXIS["Latitude", NORTH]], PROJECTION["Lambert_Conformal_Conic_2SP"], PARAMETER["central_meridian", -105.5], PARAMETER["latitude_of_origin", 37.833333333333336], PARAMETER["standard_parallel_1", 39.75], PARAMETER["false_easting", 3000000.000316083], PARAMETER["false_northing", 999999.999996], PARAMETER["scale_factor", 1.0], PARAMETER["standard_parallel_2", 38.45], UNIT["foot_survey_us", 0.3048006096012192], AXIS["X", EAST], AXIS["Y", NORTH]]'
>>> p = Projection(wkt)
>>> p.id
None
js> var proj = require("geoscript/proj")

js> var wkt = 'PROJCS["NAD_1983_HARN_StatePlane_Colorado_Central_FIPS_0502_Feet", GEOGCS["GCS_North_American_1983_HARN", DATUM["Unknown", SPHEROID["GRS_1980", 6378137.0, 298.257222101]], PRIMEM["Greenwich", 0.0], UNIT["degree", 0.017453292519943295], AXIS["Longitude", EAST], AXIS["Latitude", NORTH]], PROJECTION["Lambert_Conformal_Conic_2SP"], PARAMETER["central_meridian", -105.5], PARAMETER["latitude_of_origin", 37.833333333333336], PARAMETER["standard_parallel_1", 39.75], PARAMETER["false_easting", 3000000.000316083], PARAMETER["false_northing", 999999.999996], PARAMETER["scale_factor", 1.0], PARAMETER["standard_parallel_2", 38.45], UNIT["foot_survey_us", 0.3048006096012192], AXIS["X", EAST], AXIS["Y", NORTH]]'
js> var p = proj.Projection(wkt)

js> p
<Projection>

js> p.id === undefined
true
groovy:000> import geoscript.proj.Projection

groovy:000> wkt = 'PROJCS["NAD_1983_HARN_StatePlane_Colorado_Central_FIPS_0502_Feet", GEOGCS["GCS_North_American_1983_HARN", DATUM["Unknown", SPHEROID["GRS_1980", 6378137.0, 298.257222101]], PRIMEM["Greenwich", 0.0], UNIT["degree", 0.017453292519943295], AXIS["Longitude", EAST], AXIS["Latitude", NORTH]], PROJECTION["Lambert_Conformal_Conic_2SP"], PARAMETER["central_meridian", -105.5], PARAMETER["latitude_of_origin", 37.833333333333336], PARAMETER["standard_parallel_1", 39.75], PARAMETER["false_easting", 3000000.000316083], PARAMETER["false_northing", 999999.999996], PARAMETER["scale_factor", 1.0], PARAMETER["standard_parallel_2", 38.45], UNIT["foot_survey_us", 0.3048006096012192], AXIS["X", EAST], AXIS["Y", NORTH]]'

groovy:000> p = new Projection(wkt)
===> PROJCS["NAD_1983_HARN_StatePlane_Colorado_Central_FIPS_0502_Feet",
  GEOGCS["GCS_North_American_1983_HARN",
    DATUM["Unknown",
      SPHEROID["GRS_1980", 6378137.0, 298.257222101]],
    PRIMEM["Greenwich", 0.0],
    UNIT["degree", 0.017453292519943295],
    AXIS["Longitude", EAST],
    AXIS["Latitude", NORTH]],
  PROJECTION["Lambert_Conformal_Conic_2SP"],
  PARAMETER["central_meridian", -105.5],
  PARAMETER["latitude_of_origin", 37.833333333333336],
  PARAMETER["standard_parallel_1", 39.75],
  PARAMETER["false_easting", 3000000.000316083],
  PARAMETER["false_northing", 999999.999996],
  PARAMETER["scale_factor", 1.0],
  PARAMETER["standard_parallel_2", 38.45],
  UNIT["foot_survey_us", 0.3048006096012192],
  AXIS["X", EAST],
  AXIS["Y", NORTH]]

groovy:000> p.id
===> null

In this case some useful online resources are:

http://spatialreference.org/

Catalogue of many spatial reference systems.

http://prj2epsg.org

Service for mapping projection Well Known Text to the EPSG database.