Custom CRSs

Projection

To define a Proj4Crs (custom CRS) you have to register a projection of proj4.Projection. For that you must import proj4dart library as follows:

import 'package:proj4dart/proj4dart.dart' as proj4;

You can create and register your custom projection in multiple ways, but the recommended is to use a Proj4 definition string from epsg.io. For example for EPSG:3413 (WGS 84 / NSIDC Sea Ice Polar Stereographic North) you can find it here. This is how a Proj4 definition string looks like:

+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m + no_defs

With this Proj4 definition string and a string identifier register your proj4.Projection like this:

var customProjection = proj4.Projection.add('EPSG:3413',
    '+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs');

For more possible ways to register proj4.Projection see proj4dart documentation.

Coordinate Reference System (CRS)

You can use your previously registered proj4.Projection to create a custom CRS of type Proj4Crs. You can use the following parameters:

• <String> code (required): string identifier for the selected CRS, e.g. EPSG:3413

• <proj4.Projection> proj4Projection (required): the proj4.Projection object you wish to use

• <Bounds> bounds: bounds of the CRS in projected coordinates

• <List<double>> resolutions: an array of zoom factors (projection units per pixel, eg. meters/pixel)

• <List<double>> scales: scale factors (pixels per projection unit); specify either scales or resolutions, but not both!

• <List<Point>> origins: tile origin in projected coordinates (for TileLayer). Why is it needed? GeoServer by default can define different origins (top left coordinates) for each zoom levels. In case of origin mismatch the tile will be drawn on the wrong place: the map will jump at such zoom levels. If your origins vary with zoom levels the number of origins must match the number of resolutions. You can get the desired origins from a GetCapabilities WMTS call from geoserver e.g. http://[ip:port]/geoserver/gwc/service/wmts?request=GetCapabilities. This results an XML, and you have to look up for the TopLeftCorners for each TileMatrix of your TileMatrixSet.

• <Transformation> transformation: the transformation to use when transforming projected coordinates into pixel coordinates

An example:

var epsg3413 = proj4.Projection.add('EPSG:3413',
    '+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45 +k=1 +x_0=0 +y_0=0 +datum=WGS84 +units=m +no_defs');

final resolutions = <double>[
  32768,
  16384,
  8192,
  4096,
  2048,
  1024,
  512,
  256,
  128,
];

final epsg3413Bounds = Bounds<double>(
  Point<double>(-4511619.0, -4511336.0),
  Point<double>(4510883.0, 4510996.0),
);

var maxZoom = (resolutions.length - 1).toDouble();

var epsg3413CRS = Proj4Crs.fromFactory(
  code: 'EPSG:3413',
  proj4Projection: epsg3413,
  resolutions: resolutions,
  bounds: epsg3413Bounds,
  origins: [Point(0, 0)],
  scales: null,
  transformation: null,
);

Usage

Proj4Crs has multiple uses:

• Set FlutterMap's default CRS:

    FlutterMap(
      options: MapOptions(
        // Set the default CRS
        crs: epsg3413CRS,
        center: LatLng(65.05166470332148, -19.171744826394896),
        zoom: 3.0,
        maxZoom: maxZoom,
      ),
      layers: [],
    );

• Set a WMS layer's CRS

    TileLayer(
      wmsOptions: WMSTileLayerOptions(
        // Set the WMS layer's CRS
        crs: epsg3413CRS,
        transparent: true,
        format: 'image/jpeg',
        baseUrl:
            'https://www.gebco.net/data_and_products/gebco_web_services/north_polar_view_wms/mapserv?',
        layers: ['gebco_north_polar_view'],
      ),
    );

For complete code (with point transformation from one projection to another) see the page source code. This is how it looks like: