# 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

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.