PostGIS Cheatsheet

A curated list of useful PostGIS commands with PostgreSQL.
We will be primaryly dealing with GEOGRAPHY data types.

Note: A GEOGRAPHY type is same as GEOMETRY type with SRID of 4326. So anywhere you see 4326 that refers to SRID
If you run these commands in pgAdmin4, you get the opportunity to view these geo data in a map.

Table of contents

• Why use PostGIS
• Installing PostGIS
• Enable extension
• Verify installation
• Upgrade PostGIS
• Create table
• Add a Geo column to an existing table
• Insert data into table
• Build point from coordinates
• Build polygon from coordinates
• Build circle from a point and radius
• Parse WKB
• Point in polygon example
• Bounding box example
• Useful links

---

Why PostGIS

If your application has any geofence feature where you provide information about a place to users or requires that you save some geolocation data to your database, you might consider using PostGIS if you are not already using it.
This is an interesting article comparing Geofencing from within code vs using PostGIS.
Beating Uber geofencing with PostGIS

---

Installation

To install PostGIS on ubuntu, run this command
sudo apt install postgis

---

Enable extension

Enable PostGIS extension for the currently selected database
CREATE EXTENSION postgis;

---

Verify installation

Verify that the PostGIS extension is enabled properly
SELECT postgis_full_version();
OR
SELECT postgis_version();

---

Upgrade PostGIS

ALTER EXTENSION postgis UPDATE TO "VERSION_NUMBER";

---

Create table

This is a sample SQL command to create a table that has geography supported column

CREATE TABLE regions(
    id SERIAL PRIMARY KEY,
    name VARCHAR(50),
    center  GEOMETRY(POINT, 4326),
    boundary GEOMETRY(POLYGON, 4326)
);

OR

CREATE TABLE regions(
    id SERIAL PRIMARY KEY,
    name VARCHAR(50),
    center  GEOGRAPHY(POINT),
    boundary GEOGRAPHY(POLYGON)
);

---

Add Column

Use this command to add a geo column type to an existing table.

ALTER TABLE cities ADD COLUMN center GEOMETRY(POINT, 4326);

OR

AddGeometryColumn(cities, center, 4326, POINT, 2);

---

Insert data

INSERT INTO regions(center, boundary)
VALUES(
    ST_GeogFromText('POINT(-0.2577338 5.5548604)'),
    ST_GeogFromText('POLYGON((-0.269149 5.548880, -0.266059 5.571860, -0.246404 5.566393, -0.240053 5.554946, -0.255159 5.543413, -0.269149 5.548880))')
);

If you are using Laravel Eloquent, you can save a geometry type like this

$geom = sprintf('POINT(%f %f)', $lon, $lat);
$city = new City;
$city->name = 'Accra';
$city->center = DB::raw("ST_GeogFromText($geom)");
$city->save();

---

Point from coordinates

Given a pair of coordinates ie. latitude and longitude, you can construct a geometry point using below query

SELECT ST_GeogFromText('POINT(longitude latitude)');

OR

SELECT ST_GeomFromText('POINT(longitude latitude)', 4326);

OR

SELECT ST_SetSRID(ST_MakePoint(longitude, latitude), 4326);

OR

SELECT ST_GeomFromEWKT('SRID=4326;POINT(-2.5441288 10.0611456)');

Polygon from coordinates

You can construct a polygon from a set of latitudes and longitudes. The starting set points must be same as the last points. ie The polygon must be closed.

SELECT ST_GeogFromText('POLYGON((lon1 lat1, lon2 lat2, lon3 lat3, lon1 lat1))');

---

Circle from radius

This example shows how to construct a circle given a lat and lon coordinates together with a radius

SELECT ST_Buffer(ST_MakePoint(lon, lat)::geography, radius);

Note: Radius is in meters

---

Parse WKB

These are some examples showing how to parse Well Known binaries(WKB)

• Parse to text

SELECT ST_AsEWKT('0101000020E610000012D90759164CD0BF3F19E3C3EC351640');

• Parse to json

SELECT ST_AsGeoJSON('0101000020E610000012D90759164CD0BF3F19E3C3EC351640');

---

Point in Polygon

There are instances where you want to determine if a point is inside a polygon. Use this query

SELECT ST_Contains(polygon_column, point_column);

Example:

SELECT * FROM cities WHERE ST_Contains(boundary, 'SRID=4326;POINT(lon lat)');

Select points ordered by how close they are to a specific point

SELECT * FROM towns
ORDER BY geom_column <-> (SETSRID(ST_MakePoint(lon, lat)), 4326) LIMIT 10

---

Bounding box example

One common use case when working with geolocation data is, you may want to return all points that lie within a bounding box.
Maybe you want to render points onto an area of a Google or Mapbox that is currently visible to the user.

• Use this query when your coordinates are not stored as geo types ie you have float type columns for lat and lon.

SELECT * FROM cities
WHERE ST_Contains(ST_MakeEnvelope(minLon, minLat, maxLon, maxLat, 4326), ST_SetSRID(ST_MakePoint(longitude, latitude), 4326));

Note: If your column types is VARCHAR, you will have to cast them to floats like longitude::float and latitude::float

• In the example below, the center column is already a geometry type

SELECT * FROM cities
WHERE ST_Contains(ST_MakeEnvelope(minLon, minLat, maxLon, maxLat, 4326), cities.center);

If you are using leafletjs on your frontend, you can get the min and max coordinates using below snippet

const boundBox = map.getBounds()
const northEast = boundBox.getNorthEast()
const southWest = boundBox.getSouthWest()

const minLat = southWest.lat
const maxLat = northEast.lat
const minLng = northEast.lng
const maxLng = southWest.lng

You can then send this via ajax to your backend.