GeoJSON output

Madrona provides GeoJSON output for by default. As expected, these are exposed through the REST API using an alternate generic link relation:

/features/generic-links/links/geojson/{{instance.uid}}/

Nested vs Flat

Unfortunately the GeoJSON spec does not allow FeatureCollection to be nested within other FeatureCollections as Madrona does. Given this nested feature structure:

folder1
 |- mpa1
 |- folder2
     | - mpa2

You have two strategies for GeoJSON representation:

1. flat: (Default) Flatten the geojson output and loose information about the heirarchical structure:

   FeatureCollection
    |- mpa1
    |- mpa2

2. nest_feature_set: The collection is represented as an empty geometry with a special feature_set property - a list of UIDs of the contained features. This is within the GeoJSON spec but fetching those contained features requires a client with knowledge of this convention:

   FeatureCollection
    |- mpa1
    |- folder2 (with a `feature_set` list of uids and null geom)

You can specify the geojson strategy with a URL parameter like so:

/features/generic-links/links/geojson/{{instance.uid}}/?strategy=nest_feature_set

Specifying the spatial reference system

By default, the GeoJSON geometries will be in the coordinate system of your database (defined by your GEOMETRY_DB_SRID setting). You can set a different default by using the GEOJSON_SRID setting:

GEOJSON_SRID = 3357  # default to web mercator

If you need a different coordinate system at runtime, you can specify an srid parameter on the URL:

/features/generic-links/links/geojson/{{instance.uid}}/?srid=4326

Custom GeoJSON output

To override the default geojson representation of a feature, you can specify a geojson method on your feature class which returns string containing a geojson feature (without a trailing comma):

{
  "type": "Feature",
  "geometry": {"type": "Point", "coordinates": [102.0, 0.5]},
  "properties": {"prop0": "value0"}
}

Important

Your geojson method must take an srid argument and transform the geometry accordingly.

In order to construct the geojson, there are several utility functions available. The example below show how you could add an additional property to the geojson feature:

import json
from madrona.common.jsonutils import get_properties_json, get_feature_json

class AOI(PolygonFeature):
    ...

    def geojson(self, srid):
        props = get_properties_json(self)
        props['absolute_url'] = self.get_absolute_url()
        json_geom = self.geometry_final.transform(srid, clone=True).json
        return get_feature_json(json_geom, json.dumps(props))

Geometry field

Unless you provide a custom geojson method on your feature class, the GeoJSON view will look for a geometry_final attribute. If it doesn’t exist for whatever reason, the GeoJSON geometry will be null. You can fake a geometry_final field by mirroring with a property:

@property
def geometry_final(self):
    return self.my_custom_geometry_field

But most likely you’ll want to create a custom GeoJSON property.

Properties

The GeoJSON feature properties are serialized directly from the feature instance. Not all fields are included though; Madrona strips out the following:

unwanted_properties = [
    'geometry_final',
    'geometry_orig',
    'content_type',
    'object_id',
]

... and adds a uid property. This behavior can be overridden with a custom geojson method.

Downloading

The default behavior is to handle as a download (i.e. prompt the user to save the json file and give it a reasonable filename). This is done through an HTTP Content-Disposition: attachment; filename={{instance_uid}}.geojson header. The default behavior can be controlled through a setting:

GEOJSON_DOWNLOAD = True  # Default
GEOJSON_DOWNLOAD = False  # don't treat like a downloadable attachment

Additionally, you can override the setting at runtime by adding an attach or noattach parameter to the URL:

/features/generic-links/links/geojson/{{instance.uid}}/?attach
/features/generic-links/links/geojson/{{instance.uid}}/?noattach

Turning off GeoJSON

You can specify an Option on your feature class to turn off geojson export:

class AOI(PolygonFeature):
    ...
    class Options:
        export_geojson = False