Cartographic projection

The foundation of PROJ is the large number of projections available in the library. This section is devoted to the generic parameters that can be used on any projection in the PROJ library.

Below is a list of PROJ parameters which can be applied to most coordinate system definitions. This table does not attempt to describe the parameters particular to particular projection types. These can be found on the pages documenting the individual projections.

Parameter Description
+a Semimajor radius of the ellipsoid axis
+axis Axis orientation
+b Semiminor radius of the ellipsoid axis
+ellps Ellipsoid name (see proj -le)
+k Scaling factor (deprecated)
+k_0 Scaling factor
+lat_0 Latitude of origin
+lon_0 Central meridian
+lon_wrap Center longitude to use for wrapping (see below)
+over Allow longitude output outside -180 to 180 range, disables wrapping (see below)
+pm Alternate prime meridian (typically a city name, see below)
+proj Projection name (see proj -l)
+units meters, US survey feet, etc.
+vunits vertical units.
+x_0 False easting
+y_0 False northing

In the sections below most of the parameters are explained in details.

Units

Horizontal units can be specified using the +units keyword with a symbolic name for a unit (ie. us-ft). Alternatively the translation to meters can be specified with the +to_meter keyword (ie. 0.304800609601219 for US feet). The -lu argument to cs2cs or proj can be used to list symbolic unit names. The default unit for projected coordinates is the meter. A few special projections deviate from this behaviour, most notably the latlong pseudo-projection that returns degrees.

Vertical (Z) units can be specified using the +vunits keyword with a symbolic name for a unit (ie. us-ft). Alternatively the translation to meters can be specified with the +vto_meter keyword (ie. 0.304800609601219 for US feet). The -lu argument to cs2cs or proj can be used to list symbolic unit names. If no vertical units are specified, the vertical units will default to be the same as the horizontal coordinates.

Note

proj do not handle vertical units at all and hence the +vto_meter argument will be ignored.

Scaling of output units can be done by applying the +k_0 argument. The returned coordinates are scaled by the value assigned with the +k_0 parameter.

False Easting/Northing

Virtually all coordinate systems allow for the presence of a false easting (+x_0) and northing (+y_0). Note that these values are always expressed in meters even if the coordinate system is some other units. Some coordinate systems (such as UTM) have implicit false easting and northing values.

Longitude Wrapping

By default PROJ wraps output longitudes in the range -180 to 180. The +over switch can be used to disable the default wrapping which is done at a low level in pj_inv(). This is particularly useful with projections like the equidistant cylindrical where it would be desirable for X values past -20000000 (roughly) to continue past -180 instead of wrapping to +180.

The +lon_wrap option can be used to provide an alternative means of doing longitude wrapping within pj_transform(). The argument to this option is a center longitude. So +lon_wrap=180 means wrap longitudes in the range 0 to 360. Note that +over does not disable +lon_wrap.

Prime Meridian

A prime meridian may be declared indicating the offset between the prime meridian of the declared coordinate system and that of greenwich. A prime meridian is declared using the “pm” parameter, and may be assigned a symbolic name, or the longitude of the alternative prime meridian relative to greenwich.

Currently prime meridian declarations are only utilized by the pj_transform() API call, not the pj_inv() and pj_fwd() calls. Consequently the user utility cs2cs does honour prime meridians but the proj user utility ignores them.

The following predeclared prime meridian names are supported. These can be listed using with cs2cs -lm.

Meridian Longitude
greenwich 0dE
lisbon 9d07‘54.862”W
paris 2d20‘14.025”E
bogota 74d04‘51.3”E
madrid 3d41‘16.48”W
rome 12d27‘8.4”E
bern 7d26‘22.5”E
jakarta 106d48‘27.79”E
ferro 17d40’W
brussels 4d22‘4.71”E
stockholm 18d3‘29.8”E
athens 23d42‘58.815”E
oslo 10d43‘22.5”E

Example of use. The location long=0, lat=0 in the greenwich based lat/long coordinates is translated to lat/long coordinates with Madrid as the prime meridian.

cs2cs +proj=latlong +datum=WGS84 +to +proj=latlong +datum=WGS84 +pm=madrid
0 0
3d41'16.48"E    0dN 0.000

Axis orientation

Starting in PROJ 4.8.0, the +axis argument can be used to control the axis orientation of the coordinate system. The default orientation is “easting, northing, up” but directions can be flipped, or axes flipped using combinations of the axes in the +axis switch. The values are:

  • “e” - Easting
  • “w” - Westing
  • “n” - Northing
  • “s” - Southing
  • “u” - Up
  • “d” - Down

They can be combined in +axis in forms like:

  • +axis=enu - the default easting, northing, elevation.
  • +axis=neu - northing, easting, up - useful for “lat/long” geographic coordinates, or south orientated transverse mercator.
  • +axis=wnu - westing, northing, up - some planetary coordinate systems have “west positive” coordinate systems

Note

The +axis argument does not work with the proj command line utility.