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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
Using
Predefined Projection
There are 3 predefined Projections and 5 aliases by default:
- EPSG:4326, which has the following alias:
- 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')!;
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
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';
var namedProjection = Projection.add('EPSG:23700', def);
var projection = Projection.parse(def);
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"]]
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"]]';
var namedProjection = Projection.add('EPSG:23700', def);
var projection = Projection.parse(def);
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]]
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]]');
var namedProjection = Projection.add('EPSG:23700', def);
var projection = Projection.parse(def);
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);
Flutter:
import 'package:flutter/services.dart' show rootBundle;
final bytes = (await rootBundle.load(fileName)).buffer.asUint8List();
Projection.nadgrid('key', bytes);
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);
For full example visit example/proj4dart_shift_grid_example.dart
Transform between Projections
import 'package:proj4dart/proj4dart.dart';
void main() {
var pointSrc = Point(x: 17.888058560281515, y: 46.89226406700879);
var projSrc = Projection.get('EPSG:4326')!;
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',
);
var pointForward = projSrc.transform(projDst, pointSrc);
print(
'FORWARD: Transform point ${pointSrc.toArray()} from EPSG:4326 to EPSG:23700: ${pointForward.toArray()}');
var pointInverse = projDst.transform(projSrc, pointForward);
print(
'INVERSE: Transform point ${pointForward.toArray()} from EPSG:23700 to EPSG:4326: ${pointInverse.toArray()}');
}
Transform between Projections using ProjectionTuple
import 'package:proj4dart/proj4dart.dart';
void main() {
var pointSrc = Point(x: 17.888058560281515, y: 46.89226406700879);
var tuple = ProjectionTuple(
fromProj: Projection.get('EPSG:4326')!,
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',
),
);
var pointForward = tuple.forward(pointSrc);
print(
'FORWARD: Transform point ${pointSrc.toArray()} from EPSG:4326 to EPSG:23700: ${pointForward.toArray()}');
var pointInverse = tuple.inverse(pointForward);
print(
'INVERSE: Transform point ${pointForward.toArray()} from EPSG:23700 to EPSG:4326: ${pointInverse.toArray()}');
}
Accuracy
Proj4dart was tested with
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).
