+
+ +
+

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.

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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 (i.e. us-ft). Alternatively the translation to meters can be +specified with the +to_meter keyword (i.e. 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 behavior, 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 (i.e. us-ft). Alternatively the translation to +meters can be specified with the +vto_meter keyword (i.e. 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 does 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.

+
+
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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.

+
+
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+