OpenLayers

Namespace: proj

ol.proj

The ol.proj namespace stores:

  • a list of ol.proj.Projection objects, one for each projection supported by the application
  • a list of transform functions needed to convert coordinates in one projection into another.

The static functions are the methods used to maintain these. Each transform function can handle not only simple coordinate pairs, but also large arrays of coordinates such as vector geometries.

When loaded, the library adds projection objects for EPSG:4326 (WGS84 geographic coordinates) and EPSG:3857 (Web or Spherical Mercator, as used for example by Bing Maps or OpenStreetMap), together with the relevant transform functions.

Additional transforms may be added by using the http://proj4js.org/ library (version 2.2 or later). You can use the full build supplied by Proj4js, or create a custom build to support those projections you need; see the Proj4js website for how to do this. You also need the Proj4js definitions for the required projections. These definitions can be obtained from https://epsg.io/, and are a JS function, so can be loaded in a script tag (as in the examples) or pasted into your application. The first time there is a request for a projection, either with a ol.proj.projectionLike or directly with ol.proj.get, the code will check if the Proj4js library and the necessary definition are loaded; if so, it will register the appropriate ol.proj.Projection object and add transform functions between the new projection and all the existing ones. See examples/wms-image-custom-proj for an example of this. Because the check for presence of the Proj4js library and the definition only takes place on the first request for them, this means they can be loaded dynamically as needed; for example, with user-supplied data where you don't know in advance what projections are needed, you can initially load minimal support and then load whichever are requested.

Note that Proj4js does not support projection extents. If you want to add one for creating default tile grids, you can add it after the Projection object has been created with setExtent, for example, ol.proj.get('EPSG:1234').setExtent(extent).

In addition to Proj4js support, any transform functions can be added with ol.proj.addCoordinateTransforms. To use this, you must first create a ol.proj.Projection object for the new projection and add it with ol.proj.addProjection. You can then add the forward and inverse functions with ol.proj.addCoordinateTransforms. See examples/wms-custom-proj for an example of this.

Note that if no transforms are needed and you only need to define the projection, just add a ol.proj.Projection with ol.proj.addProjection. See examples/wms-no-proj for an example of this.

Classes

Projection

Members

ol.proj.METERS_PER_UNIT{Object.<ol.proj.Units, number>}

Meters per unit lookup table.

Methods

ol.proj.addCoordinateTransforms(source, destination, forward, inverse)

src/ol/proj.js, line 205

Registers coordinate transform functions to convert coordinates between the source projection and the destination projection. The forward and inverse functions convert coordinate pairs; this function converts these into the functions used internally which also handle extents and coordinate arrays.

Name Type Description
source ol.ProjectionLike

Source projection.
destination ol.ProjectionLike

Destination projection.
forward function

The forward transform function (that is, from the source projection to the destination projection) that takes a ol.Coordinate as argument and returns the transformed ol.Coordinate.
inverse function

The inverse transform function (that is, from the destination projection to the source projection) that takes a ol.Coordinate as argument and returns the transformed ol.Coordinate.

ol.proj.addEquivalentProjections(projections)

src/ol/proj.js, line 105

Registers transformation functions that don't alter coordinates. Those allow to transform between projections with equal meaning.

Name Type Description
projections Array.<ol.proj.Projection>

Projections.

ol.proj.addProjection(projection)

src/ol/proj.js, line 147

Add a Projection object to the list of supported projections that can be looked up by their code.

Name Type Description
projection ol.proj.Projection

Projection instance.

ol.proj.equivalent(projection1, projection2){boolean}

src/ol/proj.js, line 317

Checks if two projections are the same, that is every coordinate in one projection does represent the same geographic point as the same coordinate in the other projection.

Name Type Description
projection1 ol.proj.Projection

Projection 1.
projection2 ol.proj.Projection

Projection 2.
Returns:
Equivalent.

ol.proj.fromLonLat(coordinate, opt_projection){ol.Coordinate}

src/ol/proj.js, line 257

Transforms a coordinate from longitude/latitude to a different projection.

Name Type Description
coordinate ol.Coordinate

Coordinate as longitude and latitude, i.e. an array with longitude as 1st and latitude as 2nd element.
projection ol.ProjectionLike

Target projection. The default is Web Mercator, i.e. 'EPSG:3857'.
Returns:
Coordinate projected to the target projection.

ol.proj.get(projectionLike){ol.proj.Projection}

src/ol/proj.js, line 287

Fetches a Projection object for the code specified.

Name Type Description
projectionLike ol.ProjectionLike

Either a code string which is a combination of authority and identifier such as "EPSG:4326", or an existing projection object, or undefined.
Returns:
Projection object, or null if not in list.

ol.proj.getPointResolution(projection, resolution, point){number}

src/ol/proj.js, line 61

Get the resolution of the point in degrees or distance units. For projections with degrees as the unit this will simply return the provided resolution. For other projections the point resolution is by default estimated by transforming the 'point' pixel to EPSG:4326, measuring its width and height on the normal sphere, and taking the average of the width and height. A custom function can be provided for a specific projection, either by setting the getPointResolution option in the ol.proj.Projection constructor or by using ol.proj.Projection#setGetPointResolution to change an existing projection object.

Name Type Description
projection ol.ProjectionLike

The projection.
resolution number

Nominal resolution in projection units.
point ol.Coordinate

Point to find adjusted resolution at.
Returns:
Point resolution at point in projection units.

ol.proj.getTransform(source, destination){ol.TransformFunction}

src/ol/proj.js, line 342

Given the projection-like objects, searches for a transformation function to convert a coordinates array from the source projection to the destination projection.

Name Type Description
source ol.ProjectionLike

Source.
destination ol.ProjectionLike

Destination.
Returns:
Transform function.

ol.proj.setProj4(proj4)

src/ol/proj.js, line 37

Register proj4. If not explicitly registered, it will be assumed that proj4js will be loaded in the global namespace. For example in a browserify ES6 environment you could use:

import ol from 'openlayers';
import proj4 from 'proj4';
ol.proj.setProj4(proj4);
Name Type Description
proj4 Proj4

Proj4.

ol.proj.toLonLat(coordinate, opt_projection){ol.Coordinate}

src/ol/proj.js, line 272

Transforms a coordinate to longitude/latitude.

Name Type Description
coordinate ol.Coordinate

Projected coordinate.
projection ol.ProjectionLike

Projection of the coordinate. The default is Web Mercator, i.e. 'EPSG:3857'.
Returns:
Coordinate as longitude and latitude, i.e. an array with longitude as 1st and latitude as 2nd element.

ol.proj.transform(coordinate, source, destination){ol.Coordinate}

src/ol/proj.js, line 440

Transforms a coordinate from source projection to destination projection. This returns a new coordinate (and does not modify the original).

See ol.proj.transformExtent for extent transformation. See the transform method of ol.geom.Geometry and its subclasses for geometry transforms.

Name Type Description
coordinate ol.Coordinate

Coordinate.
source ol.ProjectionLike

Source projection-like.
destination ol.ProjectionLike

Destination projection-like.
Returns:
Coordinate.

ol.proj.transformExtent(extent, source, destination){ol.Extent}

src/ol/proj.js, line 456

Transforms an extent from source projection to destination projection. This returns a new extent (and does not modify the original).

Name Type Description
extent ol.Extent

The extent to transform.
source ol.ProjectionLike

Source projection-like.
destination ol.ProjectionLike

Destination projection-like.
Returns:
The transformed extent.

Type Definitions

ol.proj.Units{string}

Projection units: 'degrees', 'ft', 'm', 'pixels', 'tile-pixels' or 'us-ft'.