geodata 0.11.0-dev.0

A geospatial client to read GeoJSON and OGC API Features data sources.

Geospatial feature service Web APIs with support for GeoJSON and OGC API Features clients for Dart.

Features

✨ New: Updated with latest geobase version 0.3.0 based on Dart SDK 2.17, and no longer with dependency on geocore.

Key features:

• 🪄 Client-side data source abstraction for geospatial feature service Web APIs
• 🌐 The GeoJSON client to read features from static web resources and local files
• 🌎 The OGC API Features client to access metadata and feature items from a compliant geospatial Web API providing GeoJSON data

The client-side support for the OGC API Features standard is not complete, however key functionality of Part1 : Core of the standard is supported.

Usage

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:
  geodata: ^0.11.0-dev.0

Import it:

import `package:geodata/geodata.dart`

There are also partial packages containing only a certain subset. See the Packages section below.

Other documentation:

📚 Concepts: If coordinates, geometries, features and feature collections are unfamiliar concepts, you might want to read more about geometries, geospatial features and GeoJSON in the documentation of the geobase package.

🚀 Samples: The Geospatial demos for Dart repository contains more sample code showing also how to use this package! But read the documentation below first.

GeoJSON client

The GeoJSON client allows fetching and reading geospatial feature collections with their geometry objects (ie. point, line string, polygon, multi point, multi line string, multi polygon and geometry collection) from following resource types:

• a web resource (by URL) containing GeoJSON content - data is fetched using the HTTP client (as provided by the http package)
• custom resources, ie. a local file or an app bundled containing valid GeoJSON data

The sample below shows to read GeoJSON features from a web resource using the HTTP client.

import 'package:geodata/geojson_client.dart';

Future<void> main(List<String> args) async {
  // read GeoJSON for earthquakes from web using HTTP(S)
  await _readFeatures(
    GeoJSONFeatures.http(
      location: Uri.parse(
        'https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/'
        '2.5_day.geojson',
      ),
    ),
  );
}

Future<void> _readFeatures(BasicFeatureSource source) async {
  // read features with error handling
  try {
    // get items or features from a source, maximum 5 features returned
    final items = await source.itemsAll(limit: 5);

    // do something with features, in this sample just print them out
    for (final f in items.collection.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]}');
      }
    }
  } on ServiceException<FeatureFailure> catch (e) {
    print('Reading GeoJSON resource failed: ${e.failure.name}');
    if (e.cause != null) {
      print('Cause: ${e.cause}');
    }
    if (e.trace != null) {
      print(e.trace);
    }
  } catch (e, st) {
    print('Reading GeoJSON resource failed: $e');
    print(st);
  }
}

OGC API Features client

The GeoJSON client discussed above allows reading data from a static web resource or a local file. However most often geospatial APIs contains huge datasets, and data items to be queried must be selected and filtered.

The OGC API Features standard by the Open Geospatial Consortium (or OGC) specifies this - how data is discovered and accessed, or as described by the standard itself:

OGC API Features provides API building blocks to create, modify and query features on the Web. OGC API Features is comprised of multiple parts, each of them is a separate standard. This part, the "Core" specifies the core capabilities and is restricted to fetching features where geometries are represented in the coordinate reference system WGS 84 with axis order longitude/latitude. Additional capabilities that address more advanced needs will be specified in additional parts.

A compliant (according to Part 1: Core) API service should provide at least following resources:

Resource	Path	Description
Landing page	/	Metadata about the API.
Conformance classes	/conformance	Conformance classes supported by the API.
Feature collections	/collections	Metadata about all feature collections provided by the API.
Feature collection	/collections/{collectionId}	Metadata about a single feature collection provided by the API.
Features	/collections/{collectionId}/items	Feature items (with geometry and property data) in a specified feature collection provided by the API.
Feature (by id)	/collections/{collectionId}/items/{featureId}	A single feature item (with geometry and property data) in a specified feature collection provided by the API.

This package supports key features of the Part1 : Core specification of the OGC API Features standard.

You can use ogcApiFeaturesHttpClient function to access a feature service instance by giving at least an endpoint URL for an API service you want to use. The class for service instances has following methods:

/// A feature service compliant with the OGC API Features standard.
abstract class OGCFeatureService {
  /// Get meta data (or "landing page" information) about this service.
  Future<ResourceMeta> meta();

  /// Conformance classes this service is conforming to.
  Future<Iterable<String>> conformance();

  /// Get metadata about feature collections provided by this service.
  Future<Iterable<CollectionMeta>> collections();

  /// Get a feature source for a feature collection identified by [id].
  Future<OGCFeatureSource> collection(String id);
}

Actual data, features with geometries and properties in a feature collection, is accessed by collection method. This returns a future for a feature source (OGCFeatureSource) that let's you access feature items by following methods:

  /// Fetches a single feature by id (set in [query]) from this source.
  Future<OGCFeatureItem> item(ItemQuery query);

  /// Fetches features matching [query] from this source.
  ///
  /// This call accesses only one set of feature items (number of returned items
  /// can be limited).
  Future<OGCFeatureItems> items(BoundedItemsQuery query);

  /// Fetches features as paged sets matching [query] from this source.
  ///
  /// This call returns a first set of feature items (number of returned items
  /// can be limited), with a link to an optional next set of feature items.
  Future<Paged<OGCFeatureItems>> itemsPaged(BoundedItemsQuery query);

See geodata_example.dart for a sample how to read metadata and feature items from an API service conforming to OGC API Features.

Reference

Packages

The geodata library contains also following partial packages, that can be used to import only a certain subset instead of the whole geodata package:

Package	Exports also	Description
common		Common data structures and helpers (for links, metadata, paged responses).
core		Metadata and data source abstractions of geospatial Web APIs (ie. features).
geojson_client	common, core	A client-side data source to read GeoJSON data from web and file resources.
ogcapi_features_client	common, core	A client-side data source to read features from OGC API Features services.

External packages geodata is depending on:

• equatable for equality and hash utils
• geobase for base geospatial data structures
• http for a http client
• meta for annotations

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.