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Introduction

As already mentioned Geospatial tools for Dart provides two stable Dart packages:

  • 🌐 geobase : Geospatial data structures (coordinates, geometries, features, metadata), spherical geodesy, projections and tiling schemes. Vector data format support for GeoJSON, WKT and WKB.
  • 🌎 geodata : Geospatial feature service Web APIs with support for GeoJSON and OGC API Features clients.

This chapter introduces some of the key capabilities how these packages help Dart and Flutter developers to build their own geospatial-powered applications.

🌐 Coordinates (with geobase)

General purpose positions, series of positions and bounding boxes:

// A position as a view on a coordinate array containing x and y.
Position.view([708221.0, 5707225.0]);
// The sample above shorted.
[708221.0, 5707225.0].xy;
// A bounding box.
Box.view([70800.0, 5707200.0, 70900.0, 5707300.0]);
// A series of positions from an array of position objects.
PositionSeries.from(
[
[70800.0, 5707200.0].xy, // position 0 with (x, y) coordinate values
[70850.0, 5707250.0].xy, // position 1 with (x, y) coordinate values
[70900.0, 5707300.0].xy, // position 2 with (x, y) coordinate values
],
type: Coords.xy,
);

Geographic and projected positions and bounding boxes:

// A geographic position without and with an elevation.
Geographic(lon: -0.0014, lat: 51.4778);
Geographic(lon: -0.0014, lat: 51.4778, elev: 45.0);
// A projected position without and with z.
Projected(x: 708221.0, y: 5707225.0);
Projected(x: 708221.0, y: 5707225.0, z: 45.0);
// Geographic and projected bounding boxes.
GeoBox(west: -20, south: 50, east: 20, north: 60);
GeoBox(west: -20, south: 50, minElev: 100, east: 20, north: 60, maxElev: 200);
ProjBox(minX: 10, minY: 10, maxX: 20, maxY: 20);
// Positions and bounding boxes can be also built from an array or parsed.
Geographic.build([-0.0014, 51.4778]);
Geographic.parse('-0.0014,51.4778');
Geographic.parse('-0.0014 51.4778', delimiter: ' ');
Geographic.parseDms(lon: '0° 00′ 05″ W', lat: '51° 28′ 40″ N');
GeoBox.build([-20, 50, 100, 20, 60, 200]);
GeoBox.parse('-20,50,100,20,60,200');
GeoBox.parseDms(west: '20°W', south: '50°N', east: '20°E', north: '60°N');

Read more about coordinates on the documentation.

🧩 Simple geometries (with geobase)

As a quick sample, this is how geometry objects with 2D coordinate are created using geobase:

GeometryShapeDart code to build objects
PointPoint.build([30.0, 10.0])
LineStringLineString.build([30, 10, 10, 30, 40, 40])
PolygonPolygon.build([[30, 10, 40, 40, 20, 40, 10, 20, 30, 10]])
Polygon (with a hole)Polygon.build([[35, 10, 45, 45, 15, 40, 10, 20, 35, 10], [20, 30, 35, 35, 30, 20, 20, 30]])
MultiPointMultiPoint.build([[10, 40], [40, 30], [20, 20], [30, 10]])
MultiLineStringMultiLineString.build([[10, 10, 20, 20, 10, 40], [40, 40, 30, 30, 40, 20, 30, 10]])
MultiPolygonMultiPolygon.build([[[30, 20, 45, 40, 10, 40, 30, 20]], [[15, 5, 40, 10, 10, 20, 5, 10, 15, 5]]])
MultiPolygon (with a hole)MultiPolygon.build([[[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]]])
GeometryCollectionGeometryCollection([Point.build([30.0, 10.0]), LineString.build([10, 10, 20, 20, 10, 40]), Polygon.build([[40, 40, 20, 45, 45, 30, 40, 40]])])

Primitive geometries introduced above contain geographic or projected positions:

  • Point with a single position
  • LineString with a chain of positions (at least two positions)
  • Polygon with an array of linear rings (exactly one exterior and 0 to N interior rings with each ring being a closed chain of positions)

In previous samples position data (chains of positions) is NOT modeled as iterables of position objects, but as a flat structure represented by arrays of coordinate values, for example:

  • 2D position arrays: [x0, y0, x1, y1, x2, y2, ...]
  • 3D position arrays: [x0, y0, z0, x1, y1, z1, x2, y2, z2, ...]

To distinguish between arrays of different spatial dimensions you can use Coords enum:

LineString.build([30, 10, 10, 30, 40, 40]); // default type == Coords.xy
LineString.build([30, 10, 10, 30, 40, 40], type: Coords.xy);
LineString.build([30, 10, 5.5, 10, 30, 5.5, 40, 40, 5.5], type: Coords.xyz);

Read more about simple geometries on the documentation.

🔷 Geospatial features (with geobase)

Features represent geospatial entities with properties and geometries:

Feature(
id: 'ROG',
// a point geometry with a position (lon, lat, elev)
geometry: Point.build([-0.0014, 51.4778, 45.0]),
properties: {
'title': 'Royal Observatory',
},
);

The GeoJSON format is supported as text input and output for features:

final feature = Feature.parse(
'''
{
"type": "Feature",
"id": "ROG",
"geometry": {
"type": "Point",
"coordinates": [-0.0014, 51.4778, 45.0]
},
"properties": {
"title": "Royal Observatory"
}
}
''',
format: GeoJSON.feature,
);
print(feature.toText(format: GeoJSON.feature));

Geospatial feature collections can be instantiated easily too:

// A geospatial feature collection (with two features):
FeatureCollection([
Feature(
id: 'ROG',
// a point geometry with a position (lon, lat, elev)
geometry: Point.build([-0.0014, 51.4778, 45.0]),
properties: {
'title': 'Royal Observatory',
'city': 'London',
'isMuseum': true,
},
),
Feature(
id: 'TB',
// a point geometry with a position (lon, lat)
geometry: Point.build([-0.075406, 51.5055]),
properties: {
'title': 'Tower Bridge',
'built': 1886,
},
),
]);

Read more about geospatial features on the documentation.

📃 Vector formats (with geobase)

As already introduced the geobase package supports GeoJSON, WKT and WKB vector data formats.

GeoJSON, WKT and WKB formats are supported as input and output:

// Parse a geometry from GeoJSON text.
final geometry = LineString.parse(
'{"type": "LineString", "coordinates": [[30,10],[10,30],[40,40]]}',
format: GeoJSON.geometry,
);
// Encode a geometry as GeoJSON text.
print(geometry.toText(format: GeoJSON.geometry));
// Encode a geometry as WKT text.
print(geometry.toText(format: WKT.geometry));
// Encode a geometry as WKB bytes.
final bytes = geometry.toBytes(format: WKB.geometry);
// Decode a geometry from WKB bytes.
LineString.decode(bytes, format: WKB.geometry);

A sample showing more deeply how to handle WKB and EWKB binary data:

// to get a sample point, first parse a 3D point from WKT encoded string
final p = Point.parse('POINT Z(-0.0014 51.4778 45)', format: WKT.geometry);
// to encode a geometry as WKB/EWKB use toBytes() or toBytesHex() methods
// encode as standard WKB data (format: `WKB.geometry`), prints:
// 01e9030000c7bab88d06f056bfb003e78c28bd49400000000000804640
final wkbHex = p.toBytesHex(format: WKB.geometry);
print(wkbHex);
// encode as Extended WKB data (format: `WKB.geometryExtended`), prints:
// 0101000080c7bab88d06f056bfb003e78c28bd49400000000000804640
final ewkbHex = p.toBytesHex(format: WKB.geometryExtended);
print(ewkbHex);
// otherwise encoded data equals, but bytes for the geometry type varies
// there are some helper methods to analyse WKB/EWKB bytes or hex strings
// (decodeFlavor, decodeEndian, decodeSRID and versions with hex postfix)
// prints: "WkbFlavor.standard - WkbFlavor.extended"
print('${WKB.decodeFlavorHex(wkbHex)} - ${WKB.decodeFlavorHex(ewkbHex)}');
// when decoding WKB or EWKB data, a variant is detected automatically, so
// both `WKB.geometry` and `WKB.geometryExtended` can be used
final pointFromWkb = Point.decodeHex(wkbHex, format: WKB.geometry);
final pointFromEwkb = Point.decodeHex(ewkbHex, format: WKB.geometry);
print(pointFromWkb.equals3D(pointFromEwkb)); // prints "true"
// SRID can be encoded only on EWKB data, this sample prints:
// 01010000a0e6100000c7bab88d06f056bfb003e78c28bd49400000000000804640
final ewkbHexWithSRID =
p.toBytesHex(format: WKB.geometryExtended, crs: CoordRefSys.EPSG_4326);
print(ewkbHexWithSRID);
// if you have WKB or EWKB data, but not sure which, then you can fist check
// a flavor and whether it contains SRID, prints: "SRID from EWKB data: 4326"
if (WKB.decodeFlavorHex(ewkbHexWithSRID) == WkbFlavor.extended) {
final srid = WKB.decodeSRIDHex(ewkbHexWithSRID);
if (srid != null) {
print('SRID from EWKB data: $srid');
// after finding out CRS, an actual point can be decoded
// Point.decodeHex(ewkbHexWithSRID, format: WKB.geometry);
}
}

Using Newline-delimited GeoJSON (or “GeoJSONL”) is as easy as using the standard GeoJSON:

/// a feature collection encoded as GeoJSONL and containing two features that
/// are delimited by the newline character \n
const sample = '''
{"type":"Feature","id":"ROG","geometry":{"type":"Point","coordinates":[-0.0014,51.4778,45]},"properties":{"title":"Royal Observatory","place":"Greenwich"}}
{"type":"Feature","id":"TB","geometry":{"type":"Point","coordinates":[-0.075406,51.5055]},"properties":{"title":"Tower Bridge","built":1886}}
''';
// parse a FeatureCollection object using the decoder for the GeoJSONL format
final collection = FeatureCollection.parse(sample, format: GeoJSONL.feature);
// ... use features read and returned in a feature collection object ...
// encode back to GeoJSONL data
print(collection.toText(format: GeoJSONL.feature, decimals: 5));

Read more about vector data formats on the documentation.

📅 Metadata (with geobase)

Temporal instants and intervals, and geospatial extents:

// An instant and three intervals (open-started, open-ended, closed).
Instant.parse('2020-10-31 09:30Z');
Interval.parse('../2020-10-31');
Interval.parse('2020-10-01/..');
Interval.parse('2020-10-01/2020-10-31');
// An extent with spatial (WGS 84 longitude-latitude) and temporal parts.
GeoExtent.single(
crs: CoordRefSys.CRS84,
bbox: GeoBox(west: -20.0, south: 50.0, east: 20.0, north: 60.0),
interval: Interval.parse('../2020-10-31'),
);

Read more about metadata on the documentation.

📐 Spherical geodesy (with geobase)

Spherical geodesy functions for great circle (shown below) and rhumb line paths:

final greenwich = Geographic.parseDms(lat: '51°28′40″ N', lon: '0°00′05″ W');
final sydney = Geographic.parseDms(lat: '33.8688° S', lon: '151.2093° E');
// Distance (~ 16988 km)
greenwich.spherical.distanceTo(sydney);
// Initial and final bearing: 61° -> 139°
greenwich.spherical.initialBearingTo(sydney);
greenwich.spherical.finalBearingTo(sydney);
// Destination point (10 km to bearing 61°): 51° 31.3′ N, 0° 07.5′ E
greenwich.spherical.destinationPoint(distance: 10000, bearing: 61.0);
// Midpoint: 28° 34.0′ N, 104° 41.6′ E
greenwich.spherical.midPointTo(sydney);

Read more about spherical geodesy on the documentation.

ℹ Other capabilities (geobase)

Coordinate projections, tiling schemes (web mercator, global geodetic) and coordinate array classes are some of the more advanced topics not introduced here. Please see separate chapters about projections, tiling schemes and coordinate arrays to learn about them.

🌎 Web APIs for GeoJSON (with geodata)

The geodata package has the following diagram describing a decision flowchart how to select a client class to access GeoJSON features:

Quick start code to access a Web API service conforming to OGC API Features:

// 1. Get a client instance for a Web API endpoint.
final client = OGCAPIFeatures.http(endpoint: Uri.parse('...'));
// 2. Access/check metadata (meta, OpenAPI, conformance, collections) as needed.
final conformance = await client.conformance();
if (!conformance.conformsToFeaturesCore(geoJSON: true)) {
return; // not conforming to core and GeoJSON - so return
}
// 3. Get a feature source for a specific collection.
final source = await client.collection('my_collection');
// 4. Access (and check) metadata for this collection.
final meta = await source.meta();
print('Collection title: ${meta.title}');
// 5. Access feature items.
final items = await source.itemsAll(limit: 100);
// 6. Check response metadata.
print('Timestamp: ${items.timeStamp}');
// 7. Get an iterable of feature objects.
final features = items.collection.features;
// 8. Loop through features (each with id, properties and geometry)
for (final feat in features) {
print('Feature ${feat.id} with geometry: ${feat.geometry}');
}

Read more about GeoJSON client and OGC API Features client on the documentation.

🚀 Demos and samples

✨ See also the Geospatial demos for Dart code repository for demo and sample apps demonstrating the usage of geobase and geodata packages along with other topics.

CodeDescription
earthquake_mapShows earthquakes fetched from the USGS web service on a basic map view. The demo uses both geobase and geodata packages for geospatial data accesss. Discusses also state management based on Riverpod. The map UI is based on the Google Maps Flutter plugin.