geobase 0.2.0-dev.6

Geospatial coordinates (geographic and projected), projections and data writers (GeoJSON, WKT) for Dart.

Geospatial coordinates, projections and writers

The package provides geospatial coordinates (geographic and projected), projections, and data writers for geospatial features, geometries, coordinates and properties.

Features

• 🔢 enums for geospatial coordinate and geometry types
• 🌐 geographic positions and bounding boxes (longitude-latitude-elevation)
• 🗺️ projected positions and bounding boxes (cartesian XYZ)
• 🏗️ coordinate transformations and projections (initial support)
• 📅 temporal data structures (instant, interval)
• 📃 geospatial data writers for features, geometries, coordinates, properties:
  • 🌎 supported formats: GeoJSON
• 📃 geospatial data writers for geometries and coordinates:
  • 🪧 supported formats: WKT

Getting started

The package requires at least Dart SDK 2.17, and it supports all Dart and Flutter platforms.

Add the dependency in your pubspec.yaml:

dependencies:
  geobase: ^0.2.0-dev.6

Import it:

import `package:geobase/geobase.dart`

A sample to write a Point geometry with a geographic position to GeoJSON:

  // geometry writer for GeoJSON, with number of decimals for text output set
  final writer = geoJsonFormat().geometriesToText(decimals: 1);

  // prints:
  //    {"type":"Point","coordinates":[10.1,20.3]}
  print(
    writer
      ..geometryWithPosition(
        type: Geom.point,
        coordinates: const Geographic(lon: 10.123, lat: 20.25),
      )
      ..toString(),
  );

User guide

About coordinates

Coordinate types supported are defined by the Coords enum.

Type	Description
xy	Projected or cartesian coordinates (x, y)
xyz	Projected or cartesian coordinates (x, y, z)
xym	Projected or cartesian coordinates (x, y, m)
xyzm	Projected or cartesian coordinates (x, y, z, m)
lonLat	Geographic coordinates (longitude, latitude)
lonLatElev	Geographic coordinates (longitude, latitude, elevation)
lonLatM	Geographic coordinates (longitude, latitude, m)
lonLatElevM	Geographic coordinates (longitude, latitude, elevation, m)

The m coordinate represents a measurement or a value on a linear referencing system (like time).

There are base interfaces (abstract classes) for positions and bounding boxes.

Interface	Description
Position	A base interface for geospatial positions.
Box	A base interface for axis-aligned bounding boxes with min & max coordinates.

This package provides four classes (extending these interface) for representing coordinates for positions and bounding boxes. These classes can act also as interfaces (sub implementations allowed) or as concrete classes to represent data.

Class	Description
Geographic	A geographic position with longitude, latitude and optional elevation and m.
Projected	A projected position with x, y, and optional z and m coordinates.
GeoBox	A geographic bounding box with west, south, east and north coordinates.
ProjBox	A bounding box with minX, minY, maxX and maxY coordinates.

Geographic coordinates

Geographic positions:

  // Geographic position with longitude and latitude
  const Geographic(lon: -0.0014, lat: 51.4778);

  // Geographic position with longitude, latitude and elevation.
  const Geographic(lon: -0.0014, lat: 51.4778, elev: 45.0);

  // Geographic position with longitude, latitude, elevation and measure.
  const Geographic(lon: -0.0014, lat: 51.4778, elev: 45.0, m: 123.0);

  // The last sample also from num iterable or text.
  Geographic.fromCoords(const [-0.0014, 51.4778, 45.0, 123.0]);
  Geographic.fromText('-0.0014,51.4778,45.0,123.0');
  Geographic.fromText('-0.0014 51.4778 45.0 123.0', delimiter: ' ');

Geographic bounding boxes:

  // Geographic bbox (-20.0 .. 20.0 in longitude, 50.0 .. 60.0 in latitude).
  const GeoBox(west: -20.0, south: 50.0, east: 20.0, north: 60.0);

  // Geographic bbox with limits on elevation coordinate too.
  const GeoBox(
    west: -20.0,
    south: 50.0,
    minElev: 100.0,
    east: 20.0,
    north: 60.0,
    maxElev: 200.0,
  );

  // The last sample also from num iterable or text.
  ProjBox.fromCoords(const [-20.0, 50.0, 100.0, 20.0, 60.0, 200.0]);
  ProjBox.fromText('-20.0,50.0,100.0,20.0,60.0,200.0');

  // Geographic bbox with limits on elevation and measure coordinates too.
  const GeoBox(
    west: -20.0,
    south: 50.0,
    minElev: 100.0,
    minM: 5.0,
    east: 20.0,
    north: 60.0,
    maxElev: 200.0,
    maxM: 6.0,
  );

Projected coordinates

Projected positions:

  // Projected position with x and y.
  const Projected(x: 708221.0, y: 5707225.0);

  // Projected position with x, y and z.
  const Projected(x: 708221.0, y: 5707225.0, z: 45.0);

  // Projected position with x, y, z and m.
  const Projected(x: 708221.0, y: 5707225.0, z: 45.0, m: 123.0);

  // The last sample also from num iterable or text.
  Projected.fromCoords(const [708221.0, 5707225.0, 45.0, 123.0]);
  Projected.fromText('708221.0,5707225.0,45.0,123.0');
  Projected.fromText('708221.0 5707225.0 45.0 123.0', delimiter: ' ');

Projected bounding boxes:

  // Projected bbox with limits on x and y.
  const ProjBox(minX: 10, minY: 10, maxX: 20, maxY: 20);

  // Projected bbox with limits on x, y and z.
  const ProjBox(minX: 10, minY: 10, minZ: 10, maxX: 20, maxY: 20, maxZ: 20);

  // The last sample also from num iterable or text.
  ProjBox.fromCoords(const [10, 10, 10, 20, 20, 20]);
  ProjBox.fromText('10,10,10,20,20,20');

  // Projected bbox with limits on x, y, z and m.
  const ProjBox(
    minX: 10,
    minY: 10,
    minZ: 10,
    minM: 10,
    maxX: 20,
    maxY: 20,
    maxZ: 20,
    maxM: 20,
  );

Geometry types

Geometry types introduced above are based on the Simple Feature Access - Part 1: Common Architecture standard by The Open Geospatial Consortium.

The types are also compatible with Well-known text representation of geometry.

Geometry types supported are defined by the Geom enum.

Type	Description
point	The type for the POINT geometry
lineString	The type for the LINESTRING geometry.
polygon	The type for the POLYGON geometry.
geometryCollection	The type for the GEOMETRYCOLLECTION geometry.
multiPoint	The type for the MULTIPOINT geometry.
multiLineString	The type for the MULTILINESTRING geometry.
multiPolygon	The type for the MULTIPOLYGON geometry.

The geobase package does not however provide data structure classes for these geometry types, but types are used by geospatial data writers. Please see the geocore package also for geometry data structures.

GeoJSON writer

The geoJsonFormat() function can be used to access writers for coordinates, geometries and features producing GeoJSON compatible text.

A sample to write a LineString geometry to GeoJSON:

  // geometry writer for GeoJSON
  final writer = geoJsonFormat().geometriesToText();

  // prints (however without line breaks):
  //    {"type":"LineString",
  //     "bbox":[-1.1,-3.49,3.5,-1.1],
  //     "coordinates":[[-1.1,-1.1],[2.1,-2.5],[3.5,-3.49]]}
  print(
    writer
      ..geometryWithPositions1D(
        type: Geom.lineString,
        bbox: const GeoBox(west: -1.1, south: -3.49, east: 3.5, north: -1.1),
        coordinates: [
          const Geographic(lon: -1.1, lat: -1.1),
          const Geographic(lon: 2.1, lat: -2.5),
          const Geographic(lon: 3.5, lat: -3.49),
        ],
      )
      ..toString(),
  );

A sample to write a Feature geometry to GeoJSON:

  // feature writer for GeoJSON
  final writer = geoJsonFormat().featuresToText();

  // prints (however without line breaks):
  //    {"type":"Feature",
  //     "id":"fid-1",
  //     "geometry":
  //        {"type":"Point","coordinates":[10.123,20.25]},
  //     "properties":
  //        {"foo":100,"bar":"this is property value","baz":true}}
  print(
    writer
      ..feature(
        id: 'fid-1',
        geometries: (gw) => gw.geometryWithPosition(
          type: Geom.point,
          coordinates: const Geographic(lon: 10.123, lat: 20.25),
        ),
        properties: {
          'foo': 100,
          'bar': 'this is property value',
          'baz': true,
        },
      )
      ..toString(),
  );

WKT writer

The wktFormat() function can be used to access writers for coordinates and geometries producing WKT compatible text. However feature objects cannot be written to WKT even if supported by GeoJSON.

A sample to write a Point geometry to WKT (with z and m coordinates too):

  // geometry writer for WKT
  final writer = wktFormat().geometriesToText();

  // prints:
  //    POINT ZM(10.123 20.25 -30.95 -1.999)
  print(
    writer
      ..geometryWithPosition(
        type: Geom.point,
        coordType: Coords.xyzm,
        coordinates:
            const Geographic(lon: 10.123, lat: 20.25, elev: -30.95, m: -1.999),
      )
      ..toString(),
  );

Temporal data

Temporal data can be represented as instants (a time stamp) and intervals (an open or a closed interval between time stamps).

  // Instants can be created from `DateTime` or parsed from text.
  Instant(DateTime.utc(2020, 10, 31, 09, 30));
  Instant.parse('2020-10-31 09:30Z');

  // Intervals (open-started, open-ended, closed).
  Interval.openStart(DateTime.utc(2020, 10, 31));
  Interval.openEnd(DateTime.utc(2020, 10, 01));
  Interval.closed(DateTime.utc(2020, 10, 01), DateTime.utc(2020, 10, 31));

  // Same intervals parsed (by the "start/end" format, ".." for open limits).
  Interval.parse('../2020-10-31');
  Interval.parse('2020-10-01/..');
  Interval.parse('2020-10-01/2020-10-31');

Geospatial extents

Extent objects have both spatial bounds and temporal interval, and they are useful in metadata structures for geospatial data sources.

  // An extent with spatial (WGS 84 longitude-latitude) and temporal parts.
  GeoExtent.single(
    crs: 'EPSG:4326',
    bbox: const GeoBox(west: -20.0, south: 50.0, east: 20.0, north: 60.0),
    interval: Interval.parse('../2020-10-31'),
  );

  // An extent with multiple spatial bounds and temporal interval segments.
  GeoExtent.multi(
    crs: 'EPSG:4326',
    boxes: const [
      GeoBox(west: -20.0, south: 50.0, east: 20.0, north: 60.0),
      GeoBox(west: 40.0, south: 50.0, east: 60.0, north: 60.0),
    ],
    intervals: [
      Interval.parse('2020-10-01/2020-10-05'),
      Interval.parse('2020-10-27/2020-10-31'),
    ],
  );

The crs property in extents above refer to a Coordinate reference system that is a coordinate-based local, regional or global system used to locate geographical entities.

This library does not define any crs constants, please refer to registries like The EPSG dataset.

Projections

Built-in coordinate projections (currently only between WGS84 and Web Mercator):

  // Geographic (WGS 84 longitude-latitude) to Projected (Web Mercator metric)
  final forward = wgs84ToWebMercator.forward();
  final projected =
      forward.project(const Geographic(lon: -0.0014, lat: 51.4778));

  // Projected (Web Mercator metric) to Geographic (WGS 84 longitude-latitude)
  final inverse = wgs84ToWebMercator.inverse();
  final unprojected = inverse.project(projected);

  print('$unprojected <=> $projected');

Coordinate projections based on the external proj4dart package:

// import the default geobase library
import 'package:geobase/geobase.dart';

// need also an additional import with dependency to `proj4dart` 
import 'package:geobase/projections_proj4d.dart';

// A projection adapter from WGS84 (EPSG:4326) to EPSG:23700 (with definition)
// (based on the sample at https://pub.dev/packages/proj4dart).
final adapter = proj4dart(
  'EPSG:4326',
  'EPSG:23700',
  toDef: '+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',
);

// Apply a forward projection to EPSG:23700 with points represented as Point2.
final forward = adapter.forward();
print(forward.project(const Geographic(lon: 17.8880, lat: 46.8922)));

Please see the documentation of proj4dart package about it's capabilities, and accuracy of forward and inverse projections.

Transforms

Projections described above project coordinates between Projected and Geographic positions.

Coordinate transformations transform coordinate value without changing the type.

This sample uses the built-int translatePoint function:

  // Create a point and transform it with the built-in translation that returns
  // `Position(x: 110.0, y: 220.0, z: 50.0, m: 1.25)` after transform.
  print(
    const Projected(x: 100.0, y: 200.0, z: 50.0, m: 1.25)
        .transform(translatePosition(dx: 10.0, dy: 20.0)),
  );

Geographic algorithms

Currently supported, a distance between geographic positions using the Haversine formula.

/// Returns a distance in meters between [position1] and [position2].
/// 
/// Given [earthRadius] is used for calculation with the approximate mean radius
/// as a default.
double distanceHaversine(
  Geographic position1,
  Geographic position2, {
  double earthRadius = 6371000.0,
});

Package

This is a Dart package named geobase under the geospatial code repository.

See also the geocore package for geometry and feature data structures, data parsers and other utilities.

Authors

This project is authored by Navibyte.

More information and other links are available at the geospatial repository from GitHub.

License

This project is licensed under the "BSD-3-Clause"-style license.

Please see the LICENSE.