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Description:

proj4dart

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: any # or the latest version on Pub
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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 use Named Projection which has the following signature:
var projection = Projection.get('EPSG:4326')!;
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User-defined Projection #
Proj4dart supports Proj4 definition strings, OGC WKT definitions and ESRI WKT definitions. They can be obtained from epsg.io.
With Proj4 string definition
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
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The signature is:
var def = '+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';

// Named Projection signature, later find it from anywhere via Projection.get('EPSG:23700')
var namedProjection = Projection.add('EPSG:23700', def);
// Projection without name signature
var projection = Projection.parse(def);
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For full example visit example/proj4dart_example.dart
With OGC WKT definition
If you wish to define your own projection you can create it with a valid OGC WKT string (here for EPSG:23700):
PROJCS["HD72 / EOV",GEOGCS["HD72",DATUM["Hungarian_Datum_1972",SPHEROID["GRS 1967",6378160,298.247167427,AUTHORITY["EPSG","7036"]],TOWGS84[52.17,-71.82,-14.9,0,0,0,0],AUTHORITY["EPSG","6237"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4237"]],PROJECTION["Hotine_Oblique_Mercator_Azimuth_Center"],PARAMETER["latitude_of_center",47.14439372222222],PARAMETER["longitude_of_center",19.04857177777778],PARAMETER["azimuth",90],PARAMETER["rectified_grid_angle",90],PARAMETER["scale_factor",0.99993],PARAMETER["false_easting",650000],PARAMETER["false_northing",200000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Y",EAST],AXIS["X",NORTH],AUTHORITY["EPSG","23700"]]
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The signature is:
var def = 'PROJCS["HD72 / EOV",GEOGCS["HD72",DATUM["Hungarian_Datum_1972",SPHEROID["GRS 1967",6378160,298.247167427,AUTHORITY["EPSG","7036"]],TOWGS84[52.17,-71.82,-14.9,0,0,0,0],AUTHORITY["EPSG","6237"]],PRIMEM["Greenwich",0,AUTHORITY["EPSG","8901"]],UNIT["degree",0.0174532925199433,AUTHORITY["EPSG","9122"]],AUTHORITY["EPSG","4237"]],PROJECTION["Hotine_Oblique_Mercator_Azimuth_Center"],PARAMETER["latitude_of_center",47.14439372222222],PARAMETER["longitude_of_center",19.04857177777778],PARAMETER["azimuth",90],PARAMETER["rectified_grid_angle",90],PARAMETER["scale_factor",0.99993],PARAMETER["false_easting",650000],PARAMETER["false_northing",200000],UNIT["metre",1,AUTHORITY["EPSG","9001"]],AXIS["Y",EAST],AXIS["X",NORTH],AUTHORITY["EPSG","23700"]]';

// Named Projection signature, later find it from anywhere via Projection.get('EPSG:23700')
var namedProjection = Projection.add('EPSG:23700', def);
// Projection without name signature
var projection = Projection.parse(def);
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For full example visit example/proj4dart_ogc_wkt_example.dart
With ESRI WKT definition
If you wish to define your own projection you can create it with a valid ESRI WKT string (here for EPSG:23700):
PROJCS["HD72_EOV",GEOGCS["GCS_HD72",DATUM["D_Hungarian_1972",SPHEROID["GRS_1967",6378160,298.247167427]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Hotine_Oblique_Mercator_Azimuth_Center"],PARAMETER["latitude_of_center",47.14439372222222],PARAMETER["longitude_of_center",19.04857177777778],PARAMETER["azimuth",90],PARAMETER["scale_factor",0.99993],PARAMETER["false_easting",650000],PARAMETER["false_northing",200000],UNIT["Meter",1]]
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The signature is:
var def = 'PROJCS["HD72_EOV",GEOGCS["GCS_HD72",DATUM["D_Hungarian_1972",SPHEROID["GRS_1967",6378160,298.247167427]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Hotine_Oblique_Mercator_Azimuth_Center"],PARAMETER["latitude_of_center",47.14439372222222],PARAMETER["longitude_of_center",19.04857177777778],PARAMETER["azimuth",90],PARAMETER["scale_factor",0.99993],PARAMETER["false_easting",650000],PARAMETER["false_northing",200000],UNIT["Meter",1]]');

// Named Projection signature, later find it from anywhere via Projection.get('EPSG:23700')
var namedProjection = Projection.add('EPSG:23700', def);
// Projection without name signature
var projection = Projection.parse(def);
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For full example visit example/proj4dart_esri_wkt_example.dart
Grid Based Datum Adjustments
To use +nadgrids= in a proj definition, first read your NTv2 .gsb file into an Uint8List, then pass to Projection.nadgrid. E.g:
Dart:
final bytes = await File(
'assets/my_grid.gsb',
).readAsBytes();

Projection.nadgrid('key', bytes);
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Flutter:
import 'package:flutter/services.dart' show rootBundle;
final bytes = (await rootBundle.load(fileName)).buffer.asUint8List();

Projection.nadgrid('key', bytes);
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then use the given key in your definition, e.g. +nadgrids=@key,null. See Proj4 General Parameters.
Nadgrid example:
import 'package:flutter/services.dart' show rootBundle;

final bytes = (await rootBundle.load('assets/nzgd2kgrid0005.gsb')).buffer.asUint8List();
Projection.nadgrid('nzgd2kgrid0005.gsb', bytes);

var def = '+proj=longlat +datum=nzgd49 +towgs84=59.47,-5.04,187.44,0.47,-0.1,1.024,-4.5993 +nadgrids=nzgd2kgrid0005.gsb +no_defs';
var namedProjection = Projection.add('EPSG:4272', def);
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For full example visit example/proj4dart_shift_grid_example.dart
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 projSrc = Projection.get('EPSG:4326')!;

// Find Projection by name or define it if not exists
var projDst = Projection.get('EPSG:23700') ??
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',
);

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

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Transform between Projections using ProjectionTuple #
import 'package:proj4dart/proj4dart.dart';

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

// Define ProjectionTuple which makes vice versa conversions even easier
var tuple = ProjectionTuple(
// Use built-in projection
fromProj: Projection.get('EPSG:4326')!,
// Define custom projection
toProj: Projection.parse(
'+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 = tuple.forward(pointSrc);
print(
'FORWARD: Transform point ${pointSrc.toArray()} from EPSG:4326 to EPSG:23700: ${pointForward.toArray()}');
// FORWARD: Transform point [17.888058560281515, 46.89226406700879] from EPSG:4326 to EPSG:23700: [561651.8408065987, 172658.61998377228]

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

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Accuracy #
Proj4dart was tested with

8444 Proj4 definitions (test/data/all_proj4_defs.dart)
5642 OGC WKT definitions (test/data/all_proj4_ogc_wkt_defs.dart)
7426 ESRI WKT definitions (test/data/all_proj4_esri_wkt_defs.dart)

Forward and inverse transformations were both performed and checked in each case. The expected forward and inverse results were pre-calculated using proj4js (test/results/all_proj4_results.dart, test/results/all_proj4_ogc_wkt_results.dart, test/results/all_proj4_esri_wkt_results.dart).
The following table shows the avarage deltas of different Projections (using EPSG:4326 point of [17.888058560281515, 46.89226406700879] and transformed to Custom CRS defined with Proj4 definition string which gave the forward result then transformed back the received forward result which gave the inverse result):

Projection
No. tests
avg delta forward_x
avg delta forward_y
avg delta inverse_x
avg delta inverse_y

Albers Projection
58
2.8260822953849e-9
1.6378431484617036e-9
1.0719394720518753e-14
1.0903155772870503e-14

Azimuthal Equidistant Projection
12
7.761021455128987e-11
1.5522042910257974e-10
1.7763568394002505e-15
1.1842378929335002e-15

Cassini Projection
33
3.9510654680656665e-10
3.3866275440562856e-10
3.2297397080004555e-16
0.0

Central Cylindrical Projection
5
0.0
0.0
0.0
0.0

Equidistant Conic Projection
9
1.2935035758548312e-11
0.0
1.578983857244667e-15
0.0

Equidistant Cylindrical Projection
25
0.0
0.0
0.0
0.0

Geocentric Projection
196
0.0
0.0
9.06304509898087e-17
0.0

Gnomic Projection
2
0.0
0.0
3.552713678800501e-15
0.0

Hotine Oblique Mercator Projection
51
4.460304683330012e-9
8.017819959158991e-9
6.339155779820502e-15
3.901018941428001e-15

Krovak Projection
9
9.054525030983819e-10
1.448724004957411e-9
1.578983857244667e-15
3.157967714489334e-15

Lambert Azimuthal Equal-Area Projection
20
5.820766091346741e-11
0.0
5.329070518200751e-16
0.0

Lambert Conformal Conic Projection
2028
1.8960008225716005e-8
1.2588414830372427e-8
4.613097105712702e-14
8.064379757884726e-14

LongLat
1152
1.6961740653995446e-16
0.0
2.312964634635743e-16
2.7755575615628914e-16

Miller Cylindrical Projection
2
0.0
0.0
0.0
0.0

Mollweide Projection
2
0.0
0.0
0.0
0.0

New Zealand Map Grid Projection
1
0.0
0.0
0.000003294908452744494
0.000008533214440831216

Orthographic Projection
2
0.0
0.0
7.105427357601002e-15
3.552713678800501e-15

Polyconic Projection
10
0.0
0.0
1.1842378929335002e-15
0.0

Pseudo Mercator Projection
25
0.0
3.7252902984619143e-10
1.4210854715202004e-16
2.842170943040401e-16

Robinson Projection
2
0.0
0.0
0.0
0.0

Sinusoidal Projection
3
0.0
0.0
0.0
0.0

Stereographic North Projection
21
2.217434701465425e-11
2.993536846978324e-10
4.906128413581644e-15
1.3534147347811431e-15

Stereographic South Projection
44
2.43414258446509e-10
2.063731004268861e-10
1.3726393759001935e-15
0.0

Swiss Oblique Mercator Projection
5
4.656612873077393e-11
0.0
1.4210854715202005e-15
0.0

Transverse Mercator Projection
3445
7.527289676908484e-12
5.01819311793899e-12
3.966550829771659e-13
3.0942861377693824e-13

Universal Transverse Mercator Projection
1279
3.4847229055075836e-10
3.429655454604303e-10
3.4304936617033767e-15
2.4999548951684524e-15

Van der Grinten Projection
2
3.3760443329811096e-9
2.3283064365386963e-9
0.0
0.0

Example (using EPSG:4326 point of [17.888058560281515, 46.89226406700879] and transformed to EPSG:23700 defined with Proj4 definition string):

LIBRARY
forward_x
forward_y
inverse_x
inverse_y

proj4dart
561651.8408065987
172658.61998377228
17.888058565574845
46.89226406698969

proj4js
561651.8408065989
172658.61998377228
17.888058565574852
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 defined with Proj4 definition string):

LIBRARY
forward_x
forward_y
inverse_x
inverse_y

proj4dart
561651.8408065987
172658.61998377228
17.888058565574845
46.89226406698969

PostGIS
561651.8408065987
172658.6199837724
17.88805856557482
46.8922640683514

delta
0.0
0.00000000012
0.000000000000025
0.00000000136171

Used by #

Proj4dart is currently the default projection handler of flutter_map.

Authors #
Proj4dart was ported from proj4js by @maRci002 and Gergely Padányi-Gulyás (@fegyi001).