proj4dart 1.0.0

Proj4dart is a Dart library to transform point coordinates from one coordinate system to another, including datum transformations (Dart version of proj4js/proj4js).

PROJ4DART

Proj4dart is a Dart library to transform point coordinates from one coordinate system to another, including datum transformations (Dart version of proj4js/proj4js).

Installing

Add proj4dart to pubspec.yml (dependencies section), then run pub get to download the new dependencies.

dependencies:
  proj4dart: latest

Using

Predefined Projection

There are 3 predefined Projections and 5 aliases by default:

• EPSG:4326, which has the following alias:
  • WGS84
• EPSG:4269
• EPSG:3857, which has the following aliases:
  • EPSG:3785
  • GOOGLE
  • EPSG:900913
  • EPSG:102113

If you wish to use one of the predefined ones the signature is:

  var projection = Projection('EPSG:4326');

User-defined Projection

If you wish to define your own projection you can create it with a valid Proj4 string (here for EPSG:23700):

+proj=somerc +lat_0=47.14439372222222 +lon_0=19.04857177777778 +k_0=0.99993 +x_0=650000 +y_0=200000 +ellps=GRS67 +towgs84=52.17,-71.82,-14.9,0,0,0,0 +units=m +no_defs

The signature is:

var projection = Projection.add('EPSG:23700',
      '+proj=somerc +lat_0=47.14439372222222 +lon_0=19.04857177777778 +k_0=0.99993 +x_0=650000 +y_0=200000 +ellps=GRS67 +towgs84=52.17,-71.82,-14.9,0,0,0,0 +units=m +no_defs');

At the moment proj4dart cannot consume WKT definitions (which is on the roadmap though).

Transform between Projections

import 'package:proj4dart/proj4dart.dart';

void main() {
  // Define Point
  var pointSrc = Point(x: 17.888058560281515, y: 46.89226406700879);

  // Use built-in projection
  var epsgSrc = 'EPSG:4326';
  var projSrc = Projection(epsgSrc);

  // Define custom projection
  var epsgDst = 'EPSG:23700';
  var projDst = Projection.add(epsgDst,
      '+proj=somerc +lat_0=47.14439372222222 +lon_0=19.04857177777778 +k_0=0.99993 +x_0=650000 +y_0=200000 +ellps=GRS67 +towgs84=52.17,-71.82,-14.9,0,0,0,0 +units=m +no_defs');

  // Forward transform (lonlat -> projected crs)
  var pointForward = projSrc.transform(projDst, pointSrc);
  print(
      'FORWARD: Transform point [${pointSrc.x}, ${pointSrc.y}] from $epsgSrc to $epsgDst: [${pointForward.x}, ${pointForward.y}]');
  // FORWARD: Transform point [17.888058560281515, 46.89226406700879] from EPSG:4326 to EPSG:23700: [561651.8408065987, 172658.61998377228]

  // Inverse transform (projected crs -> lonlat)
  var pointInverse = projDst.transform(projSrc, pointForward);
  print(
      'INVERSE: Transform point [${pointForward.x}, ${pointForward.y}] from $epsgDst to $epsgSrc: [${pointInverse.x}, ${pointInverse.y}]');
  // INVERSE: Transform point [561651.8408065987, 172658.61998377228] from EPSG:23700 to EPSG:4326: [17.888058565574845, 46.89226406698969]
}

Accuracy

Proj4dart was tested with almost 4000 different Proj4 definitions. Forward and inverse transformations were both performed and checked in each case. All the tested Proj4 definitions can be found in all_proj4_defs.dart. The expected forward and inverse results were pre-calculated using proj4js (all_proj_tests.dart).

Acceptance criteria was:

• precision delta of 0.000001 in case of LonLat
• precision delta of 0.00001 in case of projected CRS.

Example (using EPSG:4326 point of [17.888058560281515, 46.89226406700879] and transformed to EPSG:23700):

LIBRARY	forward_x	forward_y	inverse_x	inverse_y
proj4js	561651.8408065989	172658.61998377228	17.888058565574852	46.89226406698969
proj4dart	561651.8408065987	172658.61998377228	17.888058565574845	46.89226406698969
delta	0.0000000002	0.0	0.000000000000007	0.0

In some cases also manual PostGIS testing (PostgreSQL 12.1, PostGIS 3.0.0 r17983) was performed such as the following (using EPSG:4326 point of [17.888058560281515, 46.89226406700879] and transformed to EPSG:23700):

SELECT
  ST_X(
    ST_Transform(
      ST_GeomFromText('POINT(17.888058560281515 46.89226406700879)', 4326),
      23700
    )
  ) AS forward_x,
  ST_Y(
    ST_Transform(
      ST_GeomFromText('POINT(17.888058560281515 46.89226406700879)', 4326),
      23700
    )
  ) AS forward_y,
  ST_X(
    ST_Transform(
      ST_Transform(
        ST_GeomFromText('POINT(17.888058560281515 46.89226406700879)', 4326),
        23700
      ),
      4326
    )
  ) AS inverse_x,
  ST_Y(
    ST_Transform(
      ST_Transform(
        ST_GeomFromText('POINT(17.888058560281515 46.89226406700879)', 4326),
        23700
      ),
      4326
    )
  ) AS inverse_y
;

which results compared to proj4dart results:

LIBRARY	forward_x	forward_y	inverse_x	inverse_y
PostGIS	561651.8408065987	172658.6199837724	17.88805856557482	46.8922640683514
proj4dart	561651.8408065987	172658.61998377228	17.888058565574845	46.89226406698969
delta	0.0	0.00000000012	0.0	0.00000000136171

Authors

Proj4dart was ported from proj4js by @maRci002 and Gergely Padányi-Gulyás (@fegyi001) at Ulyssys Ltd, Budapest, Hungary.