195 lines
7.3 KiB
Markdown
195 lines
7.3 KiB
Markdown
# PROJ4JS [](https://travis-ci.org/proj4js/proj4js)
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Proj4js is a JavaScript library to transform point coordinates from one coordinate system to another, including datum transformations.
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Originally a port of [PROJ](https://proj.org/) ([then known as PROJ.4](https://proj.org/faq.html#what-happened-to-proj-4)) and GCTCP C ([Archive](https://web.archive.org/web/20130523091752/http://edcftp.cr.usgs.gov/pub/software/gctpc/)) it is
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a part of the [MetaCRS](https://trac.osgeo.org/metacrs/wiki) group of projects.
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## Installing
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Depending on your preferences
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```bash
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npm install proj4
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bower install proj4
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component install proj4js/proj4js
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```
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or just manually grab the file `proj4.js` from the [latest release](https://github.com/proj4js/proj4js/releases)'s `dist/` folder.
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If you do not want to download anything, Proj4js is also hosted on [cdnjs](https://www.cdnjs.com/libraries/proj4js) for direct use in your browser applications.
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## Using
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The basic signature is:
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```javascript
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proj4([fromProjection, ]toProjection[, coordinates])
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```
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Projections can be proj or wkt strings.
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Coordinates may be an object of the form `{x:x,y:y}` or an array of the form `[x,y]`.
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When all 3 arguments are given, the result is that the coordinates are transformed from projection1 to projection 2. And returned in the same format that they were given in.
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```javascript
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var firstProjection = 'PROJCS["NAD83 / Massachusetts Mainland",GEOGCS["NAD83",DATUM["North_American_Datum_1983",SPHEROID["GRS 1980",6378137,298.257222101,AUTHORITY["EPSG","7019"]],AUTHORITY["EPSG","6269"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.01745329251994328,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4269"]],UNIT["metre",1,AUTHORITY["EPSG","9001"]],PROJECTION["Lambert_Conformal_Conic_2SP"],PARAMETER["standard_parallel_1",42.68333333333333],PARAMETER["standard_parallel_2",41.71666666666667],PARAMETER["latitude_of_origin",41],PARAMETER["central_meridian",-71.5],PARAMETER["false_easting",200000],PARAMETER["false_northing",750000],AUTHORITY["EPSG","26986"],AXIS["X",EAST],AXIS["Y",NORTH]]';
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var secondProjection = "+proj=gnom +lat_0=90 +lon_0=0 +x_0=6300000 +y_0=6300000 +ellps=WGS84 +datum=WGS84 +units=m +no_defs";
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//I'm not going to redefine those two in latter examples.
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proj4(firstProjection,secondProjection,[-122.305887, 58.9465872]);
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// [-2690575.447893817, 36622916.8071244564]
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```
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The library can also parse coordinates provided with an elevation and measure, again as an object of the form `{x:x,y:y,z:z,m:m}` or an array of the form `[x,y,z,m]`.
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```javascript
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proj4(firstProjection,secondProjection,[-122.305887, 58.9465872,10]);
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// [-2690575.447893817, 36622916.8071244564, 10]
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```
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If only 1 projection is given then it is assumed that it is being projected *from* WGS84 (fromProjection is WGS84).
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```javascript
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proj4(firstProjection,[-71,41]);
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// [242075.00535055372, 750123.32090043]
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```
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If no coordinates are given an object with two methods is returned, its methods are `forward` which projects from the first projection to the second and `inverse` which projects from the second to the first.
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```javascript
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proj4(firstProjection,secondProjection).forward([-122.305887, 58.9465872]);
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// [-2690575.447893817, 36622916.8071244564]
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proj4(secondProjection,firstProjection).inverse([-122.305887, 58.9465872]);
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// [-2690575.447893817, 36622916.8071244564]
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```
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And as above if only one projection is given, it's assumed to be coming from wgs84:
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```javascript
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proj4(firstProjection).forward([-71,41]);
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// [242075.00535055372, 750123.32090043]
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proj4(firstProjection).inverse([242075.00535055372, 750123.32090043]);
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// [-71, 40.99999999999986]
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```
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Note: The generation of the floating point value `40.99999999999986` in this example represents the fact that some variance in precision is involved in any conversion between one coordinate reference system and another.
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## Named Projections
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If you prefer to define a projection as a string and reference it that way, you may use the proj4.defs method which can be called 2 ways, with a name and projection:
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```js
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proj4.defs('WGS84', "+title=WGS 84 (long/lat) +proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees");
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```
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or with an array
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```js
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proj4.defs([
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[
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'EPSG:4326',
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'+title=WGS 84 (long/lat) +proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees'],
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[
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'EPSG:4269',
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'+title=NAD83 (long/lat) +proj=longlat +a=6378137.0 +b=6356752.31414036 +ellps=GRS80 +datum=NAD83 +units=degrees'
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]
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]);
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```
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you can then do
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```js
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proj4('EPSG:4326');
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```
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instead of writing out the whole proj definition, by default proj4 has the following projections predefined:
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- 'EPSG:4326', which has the following alias
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- 'WGS84'
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- 'EPSG:4269'
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- 'EPSG:3857', which has the following aliases
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- 'EPSG:3785'
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- 'GOOGLE'
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- 'EPSG:900913'
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- 'EPSG:102113'
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Defined projections can also be accessed through the proj4.defs function (`proj4.defs('EPSG:4326')`).
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proj4.defs can also be used to define a named alias:
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```javascript
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proj4.defs('urn:x-ogc:def:crs:EPSG:4326', proj4.defs('EPSG:4326'));
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```
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## Axis order
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By default, proj4 uses `[x,y]` axis order for projected (cartesian) coordinate systems and `[x=longitude,y=latitude]` for geographic coordinates. To enforce the axis order of the provided proj or wkt string, use the
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```javascript
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proj4(fromProjection, toProjection).forward(coordinate, enforceAxis);
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proj4(fromProjection, toProjection).inverse(coordinate, enforceAxis);
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```
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signatures with `enforceAxis` set to `true`:
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```javascript
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proj4('+proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees +axis=neu', firstProjection).forward([41, -71], true);
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// [242075.00535055372, 750123.32090043]
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proj4('+proj=longlat +ellps=WGS84 +datum=WGS84 +units=degrees +axis=neu', firstProjection).inverse([242075.00535055372, 750123.32090043], true);
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//[40.99999999999986, -71]
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//the floating points to answer your question
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```
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## Grid Based Datum Adjustments
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To use `+nadgrids=` in a proj definition, first read your NTv2 `.gsb` file (e.g. from https://github.com/OSGeo/proj-datumgrid) into an ArrayBuffer, then pass it to `proj4.nadgrid`. E.g:
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```javascript
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const buffer = fs.readFileSync('ntv2.gsb').buffer
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proj4.nadgrid('key', buffer);
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```
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then use the given key in your definition, e.g. `+nadgrids=@key,null`. See [Grid Based Datum Adjustments](https://proj.org/usage/transformation.html?highlight=nadgrids#grid-based-datum-adjustments).
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## TypeScript
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TypeScript implementation was added to the [DefinitelyTyped repository](https://github.com/DefinitelyTyped/DefinitelyTyped).
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```bash
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$ npm install --save @types/proj4
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```
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## Developing
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To set up build tools make sure you have node and grunt-cli installed and then run `npm install`.
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To do the complete build and browser tests run
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```bash
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node_modules/.bin/grunt
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```
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To run node tests run
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```bash
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npm test
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```
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To run node tests with coverage run
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```bash
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npm test --coverage
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```
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To create a build with only default projections (latlon and Mercator) run
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```bash
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node_modules/.bin/grunt build
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```
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To create a build with only custom projections include a comma separated list of projections codes (the file name in 'lib/projections' without the '.js') after a colon, e.g.
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```bash
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node_modules/.bin/grunt build:tmerc
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#includes transverse Mercator
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node_modules/.bin/grunt build:lcc
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#includes lambert conformal conic
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node_modules/.bin/grunt build:omerc,moll
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#includes oblique Mercator and Mollweide
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```
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