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Geospatial data structures (features, geometry and metadata) and parsers (GeoJSON, WKT) for Dart.

Geospatial data structures and parsers

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Geospatial data structures (features, geometry and metadata) and utilities to parse GeoJSON and WKT (Well-known text representation of geometry) data.

The package supports representing both geographic (decimal degrees or longitude-latitude) and projected (or cartesian XYZ) coordinates in 2D and 3D. There are also classes for temporal data (instants and intervals) and feature objects (or geospatial entities) containing properties and geometry.

This package is at BETA stage, interfaces not fully final yet.

Introduction #

You might first want to learn basics of geospatial geometry types on the Wikipedia page about WKT representation of geometry.

Geometry primitives supported by this library package (with samples adapted from the samples of the Wikipedia source):

GeometryShapeSamples to create instances
PointPoint2(x: 30.0, y: 10.0)
Point2.from([30.0, 10.0])
Point2.parse('30 10')
LineStringLineString.parse('30 10, 10 30, 40 40', Point2.geometry)
PolygonPolygon.parse('(30 10, 40 40, 20 40, 10 20, 30 10)', Point2.geometry)
Polygon (with a hole)Polygon.parse('(35 10, 45 45, 15 40, 10 20, 35 10), (20 30, 35 35, 30 20, 20 30)', Point2.geometry)

Also multipart geometry classes are provided:

GeometryShapeSamples to create instances
MultiPointMultiPoint.parse('10 40, 40 30, 20 20, 30 10', Point2.geometry)
MultiLineStringMultiLineString.parse('(10 10, 20 20, 10 40), (40 40, 30 30, 40 20, 30 10)', Point2.geometry)
MultiPolygonMultiPolygon.parse('((30 20, 45 40, 10 40, 30 20)), ((15 5, 40 10, 10 20, 5 10, 15 5))', Point2.geometry)
MultiPolygon (with a hole)MultiPolygon.parse('((40 40, 20 45, 45 30, 40 40)), ((20 35, 10 30, 10 10, 30 5, 45 20, 20 35),(30 20, 20 15, 20 25, 30 20))', Point2.geometry)
GeometryCollectionGeometryCollection.from(<Geometry>[Point2i(x: 40, y: 10), LineString.make([[10, 10], [20, 20], [10, 40]], Point2i.geometry), Polygon.parse('(40 40, 20 45, 45 30, 40 40)', Point2i.geometry)])

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

The next section describes alternative representations for points, with either projected or geographic coordinates. Also other geometry types, metadata classes, and feature objects or geospatial entities are discussed below.

Usage #

Cartesian or projected points #

The abstract base class for all point geometries is Point. It's implemented by following concrete classes to represent projected or cartesian (XYZ) coordinates with an optional measure (m) coordinate:

ClassCoordinatesxyzm
Point2num++
Point2mnum+++
Point3num+++
Point3mnum++++
Point2iint++
Point3iint+++

Points are created by geometry parsers or point factory implementations. Each point geometry class has also multiple factory constructors.

For example Point3 can be created in many ways:

  // Projected point with X, Y and Z coordinates in two ways.
  Point3(x: 708221.0, y: 5707225.0, z: 45.0);
  Point3.xyz(708221.0, 5707225.0, 45.0);

  // The same point created from `Iterable<num>`.
  Point3.from([708221.0, 5707225.0, 45.0]);

  // The same point parsed from WKT compatible text.
  // Actually WKT representation would be : "POINT (708221.0 5707225.0 45.0)",
  // but this parser takes only coordinate data between paranthesis.
  Point3.parse('708221.0 5707225.0 45.0');

  // The `parse` method throws when text is invalid, but `tryParse` returns null
  // in such case. This can be utilized for fallbacks.
  Point3.tryParse('nop') ?? Point3.parse('708221.0 5707225.0 45.0');

  // The same point parsed using the WKT parser for projected geometries.
  // Here `wktProjected` is a global constant for a WKT factory implementation.
  wktProjected.parse('POINT Z (708221.0 5707225.0 45.0)');

All other point classes have similar constructors.

Geographic points #

The base class for all geographic point geometries is GeoPoint, that extends also Point. Geographic coordinates are longitude (lon) and latitude (lat), in degrees and preferable always in this order. Elevation (elev) in meters and measure (m) coordinates are optional.

Latitude and Longitude of the Earth

ClassCoordinateslonlatelevm
GeoPoint2double++
GeoPoint2mdouble+++
GeoPoint3double+++
GeoPoint3mdouble++++

See below how to create GeoPoint3m instances (other classes in similar ways):

  // Geographic point with longitude, latitude, elevation and measure.
  GeoPoint3m(lon: -0.0014, lat: 51.4778, elev: 45.0, m: 123.0);
  GeoPoint3m.lonLatElevM(-0.0014, 51.4778, 45.0, 123.0);

  // Someone might want to represent latitude before longitude, it's fine too.
  GeoPoint3m.latLonElevM(51.4778, -0.0014, 45.0, 123.0);

  // When creating from value array, the order is: lon, lat, elev, m.
  GeoPoint3m.from([-0.0014, 51.4778, 45.0, 123.0]);

  // Also here it's possible to parse from WKT compatible text.
  GeoPoint3m.parse('-0.0014 51.4778 45.0 123.0');

  // The WKT parser for geographic coordinates parses full representations.
  wktGeographic.parse('POINT ZM (-0.0014 51.4778 45.0 123.0)');

Point series #

Other geometries are composed of point geometries in different structures. PointSeries is a collection class with a series of points and it can represent a geometry path, a line string, an outer or inner linear ring of a polygon, a multi point, a vertex array or any any other collection for points.

  // A point series of `Point2` composed of list of points that are of `Point2`
  // or it's sub classes.
  PointSeries<Point2>.from([
    Point2(x: 10.0, y: 10.0),
    Point2(x: 20.0, y: 20.0),
    Point2m(x: 30.0, y: 30.0, m: 5.0),
    Point3(x: 40.0, y: 40.0, z: 40.0),
    Point3m(x: 50.0, y: 50.0, z: 50.0, m: 5.0),
  ]);

  // Making a point series of `Point3` from a list of a list of nums.
  PointSeries.make(
    // three points each with x, y and z coordinates
    [
      [10.0, 11.0, 12.0],
      [20.0, 21.0, 22.0],
      [30.0, 31.0, 32.0],
    ],
    // This is `PointFactory` that converts `Iterable<num>` to a point instance,
    // in this example using a factory creating `Point3` instances.
    Point3.geometry,
  );

  // Parsing a point series of `GeoPoint` from WKT compatible text with
  // `GeoPoint3` as a concrete point class.
  PointSeries<GeoPoint>.parse(
      '10.0 11.0 12.0, 20.0 21.0 22.0, 30.0 31.0 32.0', GeoPoint3.geometry);

The PointSeries class is not extending the Geometry class, but it's used by actual geometry classes, described in following sections, as a building block.

Line strings #

A line string contains a chain of points, implemented using PointSeries.

You can use LineString.any factory constructor to create a line string with any chain, or LineString.ring constructor to create a linear ring with a closed chain. Both constructors simply take an instance of PointSeries.

Or below are examples of more direct ways to construct line strings:

  // This makes a a line string of `Point3m` from a list of points.
  LineString.make(
    [
      [10.0, 11.0, 12.0, 5.1],
      [20.0, 21.0, 22.0, 5.2],
      [30.0, 31.0, 32.0, 5.3],
    ],
    Point3m.geometry,
  );

  // Parsing using the WKT factory produces the result as the previous sample.
  wktProjected.parse<Point3m>('LINESTRING ZM (10.0 11.0 12.0 5.1, '
      '20.0 21.0 22.0 5.2, 30.0 31.0 32.0 5.3)');

Polygons #

A polygon contains one exterior boundary and optional interior boundaries (representing holes). All boundaries are linear rings implemented as LineString instances each with a closed chain of points as a ring.

The default constructor of Polygon takes a series of LineString instances, with at least an exterior boundary at index 0.

Other ways to construct polygons are familiar from previous samples:

  // Making a polygon of `GeoPoint2` from a list of a list of a list of nums:
  Polygon.make(
    [
      // this is an exterior boundary or an outer ring
      [
        [35, 10],
        [45, 45],
        [15, 40],
        [10, 20],
        [35, 10]
      ],
      // this is an interior boundary or an inner ring representing a hole
      [
        [20, 30],
        [35, 35],
        [30, 20],
        [20, 30]
      ],
    ],
    GeoPoint2.geometry,
  );

  // The same polygon geometry as above, but parsed from a WKT compatible text.
  Polygon.parse(
      '(35 10, 45 45, 15 40, '
      '10 20, 35 10) (20 30, 35 35, 30 20, 20 30)',
      GeoPoint2.geometry);

Multi geometries #

Multi points, multi line strings and multi polygons can also be constructed in similar ways described already for other geometries. Also parsed from text:

  // A multi point of `GeoPoint2` with four lon-lat points.
  MultiPoint.parse('10 40, 40 30, 20 20, 30 10', GeoPoint2.geometry);

  // A multi line string of `Point2` with two line strings.
  MultiLineString.parse(
      '(10 10, 20 20, 10 40), (40 40, 30 30, 40 20, 30 10)', Point2.geometry);

  // A multi polygon of `GeoPoint2` with two polygon (both with exterior 
  // boundary without holes).
  MultiPolygon.parse(
      '((30 20, 45 40, 10 40, 30 20)), ((15 5, 40 10, 10 20, 5 10, 15 5))',
      GeoPoint2.geometry);

There is also GeometryCollection:

  // A geometry collection can contain any other geometry types. Items for such
  // a collection can be constructed using different ways.
  GeometryCollection.from(<Geometry>[
    // A point with integer values using a constructor with named parameters.
    Point2i(x: 40, y: 10),
    // A line string made from a list of points (each a list of nums).
    LineString.make([
      [10, 10],
      [20, 20],
      [10, 40]
    ], Point2i.geometry),
    // A polygon parsed from WKT compatible text.
    Polygon.parse('(40 40, 20 45, 45 30, 40 40)', Point2i.geometry)
  ]);

Spatial bounds #

Bounding boxes or spatial bounds objects can be represented in 2D or 3D, and with an optional measure coordinates.

Bounds samples with projected or cartesian coordinates:

  // Bounds (2D) or bounding box from minimum and maximum 2D projected points.
  Bounds.of(min: Point2(x: 10.0, y: 10.0), max: Point2(x: 20.0, y: 20.0));

  // Bounds (3D) made from a list of list of nums.
  Bounds.make([[10.0, 10.0, 10.0], [20.0, 20.0, 20.0]], Point3.geometry);

  // Bounds (3D with measure) parsed from WKT compatible text.
  Bounds.parse('10.0 10.0 10.0 5.0, 20.0 20.0 20.0 5.0', Point3m.geometry);

Bounds samples with geographic coordinates:

  // Geographical bounds (-20.0 .. 20.0 in longitude, 50.0 .. 60.0 in latitude).
  GeoBounds.bboxLonLat(-20.0, 50.0, 20.0, 60.0);

  // The same bounds created of 2D geographic point instances.
  GeoBounds.of(
      min: GeoPoint2(lon: -20.0, lat: 50.0),
      max: GeoPoint2(lon: 20.0, lat: 60.0));

Temporal instants and intervals #

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

  // Temporal 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');

  // Temporal 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');

Coordinate reference system identifiers #

A Coordinate reference system (CRS) is a coordinate-based local, regional or global system used to locate geographical entities.

Currently the support for CRSs in this library is very limited. However it's possible to create identifiers (with more support coming in future releases):

  • CRS object created with an identifier: CRS.id('urn:ogc:def:crs:EPSG::4326')
  • CRS84 constant refers to http://www.opengis.net/def/crs/OGC/1.3/CRS84
    • WGS 84 longitude-latitude
  • CRS84h constant refers to http://www.opengis.net/def/crs/OGC/0/CRS84h
    • WGS 84 longitude-latitude-height

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.
  Extent.single(
    crs: CRS84,
    bounds: GeoBounds.bboxLonLat(-20.0, 50.0, 20.0, 60.0),
    interval: Interval.parse('../2020-10-31'),
  );

  // An extent with multiple spatial bounds and temporal interval segments.
  Extent.multi(crs: CRS84, allBounds: [
    GeoBounds.bboxLonLat(-20.0, 50.0, 20.0, 60.0),
    GeoBounds.bboxLonLat(40.0, 50.0, 60.0, 60.0),
  ], allIntervals: [
    Interval.parse('2020-10-01/2020-10-05'),
    Interval.parse('2020-10-27/2020-10-31'),
  ]);

Geospatial features #

According to the OGC Glossary a geospatial feature is a digital representation of a real world entity. It has a spatial domain, a temporal domain, or a spatial/temporal domain as one of its attributes. Examples of features include almost anything that can be placed in time and space, including desks, buildings, cities, trees, forest stands, ecosystems, delivery vehicles, snow removal routes, oil wells, oil pipelines, oil spill, and so on.

Below is an illustration of features in a simple vector map. Wells are features with a point geometry, rivers with a line string (or polyline) geometry, and finally lakes are features with a polygon geometry. Features normally contain also an identifier and other attributes (or properties) along with a geometry.

The external attributes package has a Entity class that represents a structured data entity that has an optional identification by an Identifier object and contains associated property values in a PropertyMap object.

The Feature class of this package extends Entity, and has also geospatial geometry and bounds as fields along with id and properties fields. That is a feature is a geospatial entity object.

  // Geospatial feature with an identification, a point geometry and properties.
  Feature.view(
    id: 'ROG',
    geometry: GeoPoint3(lon: -0.0014, lat: 51.4778, elev: 45.0),
    properties: <String, dynamic>{
      'title': 'Royal Observatory',
      'place': 'Greenwich',
      'city': 'London',
      'isMuseum': true,
      'measure': 5.79,
    },
  );

Naturally, the geometry field could also contain other geometries described earlier, not just points.

Parsing GeoJSON data #

GeoJSON, as described by Wikipedia, is an open standard format designed for representing simple geographical features, along with their non-spatial attributes.

See also the official GeoJSON website. As specified by the referenced RFC 7946 standard, all GeoJSON geometries use WGS 84 geographic coordinates. Alternative coordinate reference systems can also be used when involved parties have a prior arrangement of using other systems.

Below is an example with sample GeoJSON data and code to parse it.

Imports:

import 'package:geocore/parse_geojson.dart';

The sample code:

  // sample GeoJSON data
  const sample = '''
    {
      "type": "FeatureCollection",
      "features": [
        {
          "type": "Feature",
          "id": "ROG",
          "geometry": {
            "type": "Point",
            "coordinates": [-0.0014, 51.4778, 45.0]  
          },
          "properties": {
            "title": "Royal Observatory",
            "place": "Greenwich",
            "city": "London"
          }
        }  
      ]
    }
  ''';

  // parse FeatureCollection using the default GeoJSON factory
  final fc = geoJSON.featureCollection(sample);

  // loop through features and print id, geometry and properties for each
  for (final f in fc.features) {
    print('Feature with id: ${f.id}');
    print('  geometry: ${f.geometry}');
    print('  properties:');
    for (final key in f.properties.keys) {
      print('    $key: ${f.properties[key]}');
    }
  }

At this stage the package supports reading following GeoJSON elements:

  • FeatureCollection
  • Feature
  • Point, LineString and Polygon
  • MultiPoint, MultiLineString and MultiPolygon
  • GeometryCollection

Parsing WKT data #

Well-known text representation of geometry (WKT) is a text markup language for representing vector geometry objects. It's specified by the Simple Feature Access - Part 1: Common Architecture standard.

WKT representations for coordinate data has already been discussed on previous sections introducing geometry objects. Geometry classes have factory constructors that allows parsing coordinate values from WKT compatible text (like a point using Point2.parse('100.0 200.0') factory).

When parsing full WKT geometry text representations, with a geometry type id and coordinate values, the WktFactory class can be used. There are two global constants of class instances for different use cases:

Global constantUse cases
wktProjectedParsing geometries with projected or cartesian coordinates.
wktGeographicParsing geometries with geographic coordinates (like WGS 84).

Imports:

import 'package:geocore/parse_wkt.dart';

Samples to parse from WKT text representation of geometry:

  // Parse projected points from WKT (result is different concrete classes).
  wktProjected.parse('POINT (100.0 200.0)'); // => Point2
  wktProjected.parse('POINT M (100.0 200.0 5.0)'); // => Point2m
  wktProjected.parse('POINT (100.0 200.0 300.0)'); // => Point3
  wktProjected.parse('POINT Z (100.0 200.0 300.0)'); // => Point3
  wktProjected.parse('POINT ZM (100.0 200.0 300.0 5.0)'); // => Point3m

  // Parse geographical line string, from (10.0 50.0) to (11.0 51.0).
  wktGeographic.parse('LINESTRING (10.0 50.0, 11.0 51.0)');

  // Parse geographical polygon with a hole. 
  wktGeographic.parse('POLYGON ((40 15, 50 50, 15 45, 10 15, 40 15),'
      ' (25 25, 25 40, 35 30, 25 25))');

Supported WKT geometry types: POINT, LINESTRING, POLYGON, MULTIPOINT, MULTILINESTRING, MULTIPOLYGON. Please note that GEOMETRYCOLLECTION is not (yet) supported.

Package #

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

The package supports Dart null-safety and using it requires at least Dart 2.12 from the stable channel. Please see the official null-safety migration guide.

In the pubspec.yaml of your project add the dependency:

dependencies:
  geocore: ^0.7.0

All dependencies used by geocore are also ready for null-safety!

The package is associated with and depending on the attributes package containing non-geospatial data structures that are extended and utilized by the geocore to provide geospatial data structures and utilities.

Libraries #

The package contains following mini-libraries:

LibraryDescription
baseGeometry classes including points, bounds, line strings, polygons and more.
crsClasses to represent coordinate reference system (CRS) identifiers.
featureFeature and FeatureCollection to handle dynamic geospatial data objects.
geoGeographic points and bounds classes to represent longitude-latitude data
meta_extentMetadata structures to handle extents with spatial and temporal parts.
parse_factoryBase interfaces and implementations for geospatial data factories.
parse_geojsonGeospatial data factory to parse GeoJSON from text strings.
parse_wktGeospatial data factory to parse WKT from text strings.

For example to access a mini library you should use an import like:

import 'package:geocore/base.dart';

To use all libraries of the package:

import 'package:geocore/geocore.dart';

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.

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Geospatial data structures (features, geometry and metadata) and parsers (GeoJSON, WKT) for Dart.

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