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PostgREST + PostGIS API Tutorial – Geospatial API in 5 Minutes

PostgREST tutorials

This tutorial is part of our PostgREST tutorial series:

How to Build a Powerful Spatial REST API with PostgREST, PostgreSQL and PostGIS

GeoJSONs over New York

Haftungsausschluss: This tutorial was developed on Mac OSX 10.14.6 and tested on Ubuntu 18.04. Windows compatibility cannot be guaranteed.

In this tutorial you will learn how to build a spatial REST API with the powerful PostgREST library utilizing PostGIS under its hood - in mere minutes!

We will implement a range of different API endpoints with the following functionality

  • Calculate the length of a LineString
  • Calculate the area of a polygon
  • Derive the intersection of two polygons

In case you have never heard of PostgREST before, let's have a look at what the authors say:

PostgREST is a standalone web server that turns your PostgreSQL database directly into a RESTful API. The structural constraints and permissions in the database determine the API endpoints and operations.

It couldn't be easier.

Prerequisites

To implement all steps of this tutorial it's required to install PostgreSQL, PostGIS and PostgREST. You can use our installation tutorial which will help you get a docker container up and running which PostgREST will use in the subsequent steps.

Installing and setting up PostgreSQL, PostGIS and PostgREST

Step 1 - Let's return a simple GeoJSON Object

As mentioned earlier, we will implement stored procedures which will calculate spatial information on the fly. You as a user will have the ability to post GeoJSON objects as the payload to the API. For the sake of demonstration, we will start with a simple procedure which will return the payload without changing it.

Let's bring back our psql prompt:

sudo docker exec -it postgrest_tut psql -U postgres

We will add a simple function to the api schema. It takes a single JSON argument and just returns it again as-is.

CREATE OR REPLACE FUNCTION api.singlegeojsonparam(single_param json) RETURNS json

    LANGUAGE sql
    AS $_$
    SELECT single_param;

$_$;

Hit enter... That's it, let's try it out!

curl -X POST \
  http://localhost:3000/rpc/singlegeojsonparam \
  -H 'Content-Type: application/json' \
  -H 'Prefer: params=single-object' \
  -d '{"type":"FeatureCollection","features":[{"type":"Feature","geometry":{"type":"Polygon","coordinates":[[[100.0,0.0],[101.0,0.0],[101.0,1.0],[100.0,1.0],[100.0,0.0]]]},"properties":{"prop0":"value1","prop1":{"this":"that"}}},{"type":"Feature","geometry":{"type":"Polygon","coordinates":[[[100.0,0.0],[101.0,0.0],[101.0,1.0],[100.0,1.0],[100.0,0.0]]]},"properties":{"prop0":"value2","prop1":{"this":"that"}}}]}'

If you're not too familiar with POST requests sending JSON data, this might be a little unfamiliar to you. The body is just one JSON object, and usually its keys would be interpreted as body parameters. However, by specifying the request header Prefer: params=single-object, we can let the backend know to intepret the entire POST body as the value of a single parameter. Quite handy for endpoints only expecting one single JSON object.

Step 2 - Calculating the Length of a LineString

Now we can start with some more fun stuff. Let's go ahead and write a function that is able to calculate the length of a LineString provided in EPSG:4326. Note, that the GeoJSON we are providing is in degrees, so we will have to transform to pseudo-mercator (or something similar). Additionally, we will cast it to numeric and after dividing it by 1.000 (to obtain a result in kilometers) we will round it to 2 decimals.

CREATE OR REPLACE FUNCTION api.calc_length(linestring json) RETURNS numeric AS $$

    SELECT ROUND(
      CAST(
        ST_Length(
          ST_Transform(
            ST_GeomFromGeoJSON(LineString), 54030
          )
        )/1000 AS numeric
      ),2
    )

$$ LANGUAGE SQL;

Easy enough. Let's give this a try.

curl -X POST \
  http://localhost:3000/rpc/calc_length \
  -H 'Content-Type: application/json' \
  -H 'Prefer: params=single-object' \
  -d '{"type":"LineString","coordinates":[[-101.744384,39.32155],[-101.552124,39.330048],[-101.403808,39.330048],[-101.332397,39.364032],[-101.041259,39.368279],[-100.975341,39.304549],[-100.914916,39.245016],[-100.843505,39.164141],[-100.805053,39.104488],[-100.491943,39.100226],[-100.437011,39.095962],[-100.338134,39.095962],[-100.195312,39.027718],[-100.008544,39.010647],[-99.865722,39.00211],[-99.684448,38.972221],[-99.51416,38.929502],[-99.382324,38.920955],[-99.321899,38.895308],[-99.113159,38.869651],[-99.0802,38.85682],[-98.822021,38.85682],[-98.448486,38.848264],[-98.206787,38.848264],[-98.020019,38.878204],[-97.635498,38.873928]],"crs":{"type":"name","properties":{"name":"EPSG:4326"}}}'

Voila! For our route through Kansas, USA it returns:

367.15

kilometers!

Step 3 - Calculating the Area of a Polygon

By now you have most likely picked up the gist of how easy it is to implement custom functions. Similary to the LineString example we could also calculate the area of a polygon. Let's make this a little trickier and let the user send a GeoJSON FeatureCollection. To keep it a little simpler, the endpoint can only process the first feature of the FeatureCollection, so only one polygon.

CREATE OR REPLACE FUNCTION api.calc_area(featurecollection json) RETURNS numeric AS $$

    SELECT ROUND(
      CAST(
        ST_Area(
          ST_Transform(
            ST_GeomFromGeoJSON(
              featurecollection->'features'->0->'geometry'
            ), 54030
          )
        )/1000000 AS numeric
      ),2
    )

$$ LANGUAGE SQL;

Let's see the result:

curl -X POST \
  http://localhost:3000/rpc/calc_area \
  -H 'Content-Type: application/json' \
  -H 'Prefer: params=single-object' \
  -d '{"type":"FeatureCollection","features":[{"type":"Feature","properties":{},"geometry":{"type":"Polygon","coordinates":[[[-73.6962890625,41.02135510866602],[-73.8446044921875,41.253032440653186],[-74.168701171875,41.20758898181025],[-74.44335937499999,40.88444793903562],[-74.520263671875,40.60144147645398],[-74.2840576171875,40.53050177574321],[-73.8885498046875,40.63479884404164],[-72.83935546875,40.805493843894155],[-72.6800537109375,40.950862628132775],[-73.6962890625,41.02135510866602]]],"crs":{"type":"name","properties":{"name":"EPSG:4326"}}}}]}'

Our polygon covering part of New York is of size

6017.49

square kilometers. Easy.

Step 4 - Calculating the Intersection of 2 Polygons

Last but not least let's implement one more function which will calculate the intersection of 2 polygons. The function by now should be self-explanatory. We just have to make sure we are returning a GeoJSON instead of a numeric value.

CREATE OR REPLACE FUNCTION api.calc_intersection(featurecollection json) RETURNS json AS $$

      SELECT ST_AsGeoJSON(
        ST_Intersection(
          ST_GeomFromGeoJSON(
            featurecollection->'features'->0->'geometry'
          ),
          ST_GeomFromGeoJSON(
            featurecollection->'features'->1->'geometry'
          )
        )
      )::json;

$$ LANGUAGE SQL;

We'll call this API with a feature collection comprising 2 polygons.

curl -X POST \
  http://localhost:3000/rpc/calc_intersection \
  -H 'Content-Type: application/json' \
  -H 'Prefer: params=single-object' \
  -d '{"type":"FeatureCollection","features":[{"type":"Feature","properties":{},"geometry":{"type":"Polygon","coordinates":[[[-73.6962890625,41.02135510866602],[-73.8446044921875,41.253032440653186],[-74.168701171875,41.20758898181025],[-74.44335937499999,40.88444793903562],[-74.520263671875,40.60144147645398],[-74.2840576171875,40.53050177574321],[-73.8885498046875,40.63479884404164],[-72.83935546875,40.805493843894155],[-72.6800537109375,40.950862628132775],[-73.6962890625,41.02135510866602]]],"crs":{"type":"name","properties":{"name":"EPSG:4326"}}}},{"type":"Feature","properties":{},"geometry":{"type":"Polygon","coordinates":[[[-73.6798095703125,41.56203190200195],[-74.3280029296875,41.75492216766298],[-74.34997558593749,41.74262728637672],[-74.7894287109375,41.21998578493921],[-74.29504394531249,40.87614141141369],[-73.4326171875,40.91766362458114],[-73.6798095703125,41.56203190200195]]],"crs":{"type":"name","properties":{"name":"EPSG:4326"}}}}]}'

Which will respond with the GeoJSON intersection.

{
  "type": "Polygon",
  "coordinates": [
      [
          [
              -74.3926939034368,
              40.944056911669
          ],
          [
              -74.168701171875,
              41.2075889818102
          ],
          [
              -73.8446044921875,
              41.2530324406532
          ],
          [
              -73.6962890625,
              41.021355108666
          ],
          [
              -73.4662745356238,
              41.0053998532882
          ],
          [
              -73.4326171875,
              40.9176636245811
          ],
          [
              -74.2950439453125,
              40.8761414114137
          ],
          [
              -74.3926939034368,
              40.944056911669
          ]
      ]
  ]
}

Wrap-up

Congratulations for completing this tutorial. By now you will have learnt how to set up and use the PostgREST web API for spatial purposes using PostGIS.

In comparison to other frameworks it's probably the most straight-forward web framework we have ever stumbled across. This obviously only touches the surface of what is possible, but if you are looking for a quick way to set up an API to handle spatial computations on-the-fly we can definitely recommend using PostgREST.

Please feel free to get in touch with us if you have any further questions, need support or have ideas for other tutorials on PostgREST!

TRITT DER GIS-OPS COMMUNITY BEI!

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