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Coordinate Reference Systems

A coordinate reference system (CRS) defines how coordinates relate to locations on the Earth. It combines a datum (which models the Earth's shape using an ellipsoid), a coordinate system (which defines the axes), and often a projection (which maps the curved surface onto a flat plane).

There are two main types:

  • Geographic CRS — positions on the ellipsoid expressed as latitude and longitude (degrees)
  • Projected CRS — positions on a flat map expressed as easting and northing (meters, feet, etc.)

For background on coordinate systems, datums, and projections:

Creating a CRS

To create a coordinate reference system, you can use CRS codes, Well-Known Text (WKT) strings, or old-style Proj strings (which are deprecated):

crs1 = Proj::Crs.new('EPSG:4326')

crs2 = Proj::Crs.new('urn:ogc:def:crs:EPSG::4326')

crs3 = Proj::Crs.new('+proj=longlat +datum=WGS84 +no_defs +type=crs')

crs4 = Proj::Crs.new(<<~EOS)
  GEOGCRS["WGS 84",
  DATUM["World Geodetic System 1984",
        ELLIPSOID["WGS 84",6378137,298.257223563,
                  LENGTHUNIT["metre",1]]],
  PRIMEM["Greenwich",0,
         ANGLEUNIT["degree",0.0174532925199433]],
  CS[ellipsoidal,2],
  AXIS["geodetic latitude (Lat)",north,
       ORDER[1],
       ANGLEUNIT["degree",0.0174532925199433]],
  AXIS["geodetic longitude (Lon)",east,
       ORDER[2],
       ANGLEUNIT["degree",0.0174532925199433]],
  USAGE[
      SCOPE["unknown"],
          AREA["World"],
      BBOX[-90,-180,90,180]],
  ID["EPSG",4326]]
EOS

Notice when using the old-style Proj4 string, the addition of the +type=crs value.

Axis Order

By default transformations accept coordinates in the units and axis order of the source CRS and return transformed coordinates in the units and axis order of the target CRS.

For most geographic CRS definitions, the units will be in degrees. For geographic CRS definitions from EPSG, the order is latitude then longitude.

For projected CRS definitions, the units vary (metre, us-foot, etc.). For projected CRS definitions from EPSG with EAST/NORTH directions, the order may be east then north, or north then east.

If you prefer a uniform axis order regardless of the source and target CRS objects, call normalize_for_visualization:

normalized = transform.normalize_for_visualization

The normalized transformation will return longitude/latitude order for geographic CRS objects and easting/northing for most projected CRS objects.

For more information see proj.org/faq.