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CRS support

Starting with version 0.15.0, pygeoapi fully supports OGC API - Features - Part 2: Coordinate Reference Systems by Reference. This enables the import and export of any data according to dedicated projections. A "projection" is specified with a Coordinate Reference System (CRS) identifier. These are in URI formats like http://www.opengis.net/def/crs/OGC/1.3/CRS84 (basically WGS84 in longitude, latitude axis order) or the "OpenGIS" format like http://www.opengis.net/def/crs/EPSG/0/4258. Note that the "EPSG:" format like EPSG:4326 is outside the scope of the OGC standard.

In particular CRS support allows for the following:

  • to specify the CRS in which the data is stored, in pygeoapi the storageCRS config option
  • to specify the list of CRSs in which Feature data can be retrieved, in pygeoapi the crs config option
  • to publish these CRSs in the collection metadata
  • the crs= query parameter for a collection or collection item
  • the bbox-crs= query parameter to indicate that the bbox= parameter is encoded in that CRS
  • the HTTP response header Content-Crs denotes the CRS of the Feature(s) in the data returned

So although GeoJSON mandates WGS84 in longitude, latitude order, the client and server may still agree on other CRSs.

Under the hood, pygeoapi uses the well-known pyproj Python wrapper to the PROJ library.

Read more in the pygeoapi documentation in the CRS Chapter.

Exercise

Adding CRS support to pygeoapi collections for the provider type feature is as simple as for example extending the Exercise 2 config with this snippet:

  crs:
      - http://www.opengis.net/def/crs/OGC/1.3/CRS84
      - http://www.opengis.net/def/crs/EPSG/0/4258
      - http://www.opengis.net/def/crs/EPSG/0/3857
      - http://www.opengis.net/def/crs/EPSG/0/4326
  storage_crs: http://www.opengis.net/def/crs/OGC/1.3/CRS84

Axis

Axis order (are coordinates it longitude, latitude or latitude, longitude order?) in projections is often a source of confusion. However the URI format is quite clear on this, at least more than the EPSG: format. So http://www.opengis.net/def/crs/OGC/1.3/CRS84 is longitude, latitude order, while http://www.opengis.net/def/crs/EPSG/0/4326 is latitude, longitude order.

In the config below, we basically indicate that the data is stored in WGS84 (longitude, latitude axis order) and can be retrieved in CRSs like http://www.opengis.net/def/crs/EPSG/0/4258 (ETRS89 latitude, longitude axis order) etc.

Add CRS to a pygeoapi configuration

Open the pygeoapi configuration file in a text editor. Find the line # START - EXERCISE 2 - firenze-terrain

Update the dataset section with CRS support by replacing it with the snippet below:

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firenze-terrains-vec:
    type: collection
    title: Administrative boundaries before 2014
    description: Cadastral parcels (terrains) from the cadastre. Territory Agency; SIT and Information Networks;
    keywords:
        - Cadastral parcels
    links:
        - type: text/html
          rel: canonical
          title: Administrative boundaries before 2014
          href: http://dati.cittametropolitana.fi.it/geonetwork/srv/metadata/cmfi:c539d359-4387-4f83-a6f4-cd546b3d8443
          hreflang: it
    extents:
        spatial:
            bbox: [11.23,43.75,11.28,43.78]
            crs: http://www.opengis.net/def/crs/OGC/1.3/CRS84
    providers:
        - type: feature
          name: SQLiteGPKG
          data: /data/firenze_terrains.gpkg # place correct path here
          id_field: fid
          crs:
            - http://www.opengis.net/def/crs/OGC/1.3/CRS84
            - http://www.opengis.net/def/crs/EPSG/0/4258
            - http://www.opengis.net/def/crs/EPSG/0/3857
            - http://www.opengis.net/def/crs/EPSG/0/4326
          storage_crs: http://www.opengis.net/def/crs/OGC/1.3/CRS84
          title_field: codbo
          table: firenze_terrains

Now we can inspect the collection metadata and retrieve Features in various CRSs. We can even do this in the Swagger UI, but using the browser is quite fast and clear.

Metadata

Collection Metadata

Open the URL: http://localhost:5000/collections/firenze-terrains-vec Your configured CRSs are displayed at the bottom of the page: "Reference Systems" and "Storage CRS".

See these in JSON format, also at the bottom: http://localhost:5000/collections/firenze-terrains-vec?f=json 

   .
   .
   "crs":[
    "http://www.opengis.net/def/crs/OGC/1.3/CRS84",
    "http://www.opengis.net/def/crs/EPSG/0/4258",
    "http://www.opengis.net/def/crs/EPSG/0/3857",
    "http://www.opengis.net/def/crs/EPSG/0/4326"
   ],
   "storageCRS":"http://www.opengis.net/def/crs/OGC/1.3/CRS84"
 }

Reproject Features

Using the CRS query parameter

Open the URL: http://localhost:5000/collections/firenze-terrains-vec/items?f=json&crs=http://www.opengis.net/def/crs/EPSG/0/4258

This is ETRS89, similar to WGS84, but for the European Continent (Datum) and in lat,lon order. This is e.g. used in INSPIRE.

See these in JSON format, also at the bottom:

"type":"FeatureCollection",
  "features":[
      {
          "type":"Feature",
          "geometry":{
              "type":"MultiPolygon",
              "coordinates":[
                  [
                      [
                          [
                              43.77805936835436,
                              11.23486287997071
                          ],
                          [
                              43.77809089595012,
                              11.2348943159564
                          ],
                          [
                              43.77810038978989,
                              11.23491359066035
                          ],
                          [
                              43.77705757917591,
                              11.2368990806804
   .
   .
   "crs":[
    "http://www.opengis.net/def/crs/OGC/1.3/CRS84",
    "http://www.opengis.net/def/crs/EPSG/0/4258",
    "http://www.opengis.net/def/crs/EPSG/0/3857",
    "http://www.opengis.net/def/crs/EPSG/0/4326"
   ],
   "storageCRS":"http://www.opengis.net/def/crs/OGC/1.3/CRS84"
 }

If you open the browser development console, you can observe the HTTP response header:

Content-Crs: <http://www.opengis.net/def/crs/EPSG/0/4258>

(The CRS URI is always enclosed in < >)