Basics of PostGIS

This tutorial has quite a bit of data and takes a sec to load so be a little patient.

PostGIS is one of the most powerful extensions for PostgreSQL and it can turn a database into a GIS (Geographic Information System).

For this tutorial, we’ve loaded a data bundle of 2020 New York City Census data. This data is also part of the PostGIS.net tutorial if you’d like to dig in deeper there.

Finding single points:

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SELECT name, ST_AsText(geom)
  FROM nyc_subway_stations
  LIMIT 10;

Calculating area

In square meters

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SELECT ST_Area(geom)
  FROM nyc_neighborhoods
  WHERE name = 'West Village';

Calculating length

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SELECT ST_Length(geom)
  FROM nyc_streets
  WHERE name = 'Columbus Cir';

What is the length of streets in New York City, summarized by type?

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SELECT type, Sum(ST_Length(geom)) AS length
FROM nyc_streets
GROUP BY type
ORDER BY length DESC;

The SRID

When transforming coordinate data (such as with ST_GeomFromText()) you need a standardized way of transforming from latitude/longitude to internal representations.  PostGIS comes with a spatial_ref_sys spatial reference table upon installation that contains the most common spatial references, standardized across GIS offerings.  In this case we are using the id of 26918 which is a common projection for projections centered around North America.

Extrapolating from a point

Find the point

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SELECT name, ST_AsText(geom)
FROM nyc_subway_stations
WHERE name = 'Broad St';

name   |                 st_astext                  
----------+--------------------------------------------
 Broad St | POINT(583571.9059213118 4506714.341192182)
(1 row)

Find the district and borough name for that point.

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SELECT name, boroname
FROM nyc_neighborhoods
WHERE ST_Intersects(geom, ST_GeomFromText('POINT(583571 4506714)',26918));

Spatial Joins

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SELECT
subways.name AS subway_name,
neighborhoods.name AS neighborhood_name,
neighborhoods.boroname AS borough
FROM nyc_neighborhoods AS neighborhoods
JOIN nyc_subway_stations AS subways
ON ST_Contains(neighborhoods.geom, subways.geom)
WHERE subways.name = 'Broad St';

Distance

From one point to another

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SELECT ST_Distance(a.geom, b.geom)
FROM nyc_streets a, nyc_streets b
WHERE a.name = 'Columbus Cir'
AND b.name = 'Atlantic Commons';

Or distance to find something close. For example the streets with 10 meters of the Broad Street station (distance from a point).

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SELECT name
FROM nyc_streets
WHERE ST_DWithin(
        geom,
        ST_GeomFromText('POINT(583571 4506714)',26918),
        10
      );

Want to go further and test QGIS or some of our API web tools, see our [PostGIS for Newbies blog](https://www.crunchydata.com/blog/postgis-for-newbies).