How do I create a proj4 string for a project grid?
The table below gives a list of the parameters that are used by to construct a proj4 string (see The PROJ projections page for full details). In most cases, you will only require a subset of these.
| Parameter | Description |
|---|---|
| +a | Semimajor radius of the ellipsoid axis |
| +axis | Axis orientation |
| +b | Semiminor radius of the ellipsoid axis |
| +ellps | Ellipsoid name |
| +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 |
| +pm | Alternate prime meridian (typically a city name, see below) |
| +proj | Projection name |
| +units | meters, US survey feet, etc. |
| +vunits | vertical units. |
| +x_0 | False easting |
| +y_0 | False northing |
See this page for a full list of proj projection names
An Example (PCG94)
As an example, let's create a proj4 string for the Landgate Perth Coastal Grid (PCG94) coordinate system (see https://www0.landgate.wa.gov.au/business-and-government/specialist-services/geodetic/project-grids for details). In this case, the data we have is
| Perth | GDA94 (PCG94) |
|---|---|
| Central meridian | 115°49'00" |
| Central scale factor | 0.99999906 |
| False easting | 50 000.0 |
| False northing | 3 800 000.0 |
| Extent NW corner | S31°20'00" |
| E115°26'30" | |
| Extent SE corner | S33°25'00" |
| E116°05'00" | |
| Height limit | 165m |
Using these values, and noting that these project grids are based on a Transverse Mercator projection, the proj4 string is
+proj=tmerc +lat_0=0 +lon_0=115.8166666666667 +k=0.99999906 +x_0=50000 +y_0=3800000 +ellps=GRS80 +units=m