The foundation of PROJ is the large number of projections<../operations/projections/index>
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<../operations/projections/index>
.
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.
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.
Input units for parameters that can be understood to be either decimal degrees or radians are interpreted to be decimal degrees by convention.
Explict specification of input units can be accomplished by adding the appropriate suffix to input values.
+----------------+---------------------+ | Suffix | Unit | +================+=====================+ | d | Decimal degrees | +----------------+ + | D | | +----------------+ + | ° | | +----------------+---------------------+ | r | Radians | +----------------+ + | R | | +----------------+---------------------+
Example of use. The longitude of the central meridian +lon_0=90
, can also be expressed more explictly with units of decimal degrees as +lon_0=90d
or in radian units as +lon_0=1.570796r
(approximately).
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.
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 eqc
or merc
where it could be desirable for X values past -20000000 (roughly) to continue past -180 instead of wrapping to +180.
Note however that for most projections where the 180 meridian does not project to a straight line, +over
will have no effect or will not lead to expected results.
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
.
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
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.