PostGIS is a powerful spatial extension for PostgreSQL that allows you to store, query, and manipulate geographic objects. If you're working with maps, geometry, or location-based data, PostGIS is an essential tool. In this article, we’ll explore what a polygon is in PostGIS, what the parameters in a polygon definition mean, and the role of ST_ functions in spatial queries.
🧱 What is a Polygon in PostGIS?
A polygon in PostGIS is a two-dimensional geometric object that represents an enclosed area defined by one or more rings:
- The outer ring defines the exterior boundary.
- Inner rings (optional) define holes within the polygon.
Each ring is made up of at least four coordinate points, and the first and last point must be the same to form a closed loop.
📐 Example: Creating a Polygon
Consider the following SQL query:
SELECT ST_GeomFromText('POLYGON((0 0, 0 5, 5 5, 5 0, 0 0))');
This uses WKT (Well-Known Text) format to define a simple square polygon.
Breaking down the parameters:
POLYGON((0 0, 0 5, 5 5, 5 0, 0 0))
| Point | Coordinates | Description |
|---|---|---|
| 1 | 0 0 |
Bottom-left corner (starting point) |
| 2 | 0 5 |
Top-left corner |
| 3 | 5 5 |
Top-right corner |
| 4 | 5 0 |
Bottom-right corner |
| 5 | 0 0 |
Closes the polygon (must match point 1) |
This creates a square with sides of 5 units.
🧭 What Does ST_ Mean in PostGIS?
The ST_ prefix stands for "Spatial Type" and is used for all spatial functions in PostGIS. It follows standards defined by the Open Geospatial Consortium (OGC).
Why use ST_?
- To clearly distinguish spatial functions from regular SQL functions.
- To comply with OGC naming conventions.
🛠️ Common ST_ Functions
| Function | Purpose |
|---|---|
ST_GeomFromText() |
Converts WKT into a PostGIS geometry |
ST_AsText() |
Converts geometry back to WKT |
ST_Area() |
Calculates the area of a polygon |
ST_Length() |
Measures the length of a linestring |
ST_Intersects() |
Checks if two geometries intersect |
ST_Contains() |
Tests whether one geometry completely contains another |
ST_Buffer() |
Expands a geometry outward by a given distance |
ST_Distance() |
Measures the distance between two geometries |
🧪 Example Usage
-- Create a polygon from WKT
SELECT ST_GeomFromText('POLYGON((0 0, 0 5, 5 5, 5 0, 0 0))');
-- Compute the area
SELECT ST_Area(ST_GeomFromText('POLYGON((0 0, 0 5, 5 5, 5 0, 0 0))'));
This example returns 25, the area of the 5×5 square.
📦 Summary
- A polygon in PostGIS is a closed shape defined by coordinate points.
- The parameters in
'POLYGON((x y, x y, ..., x y))'represent the vertices. - The
ST_prefix identifies spatial functions that handle geometry or geography types.
Whether you're building map-based applications or analyzing spatial data, understanding how polygons and ST_ functions work is key to leveraging the full power of PostGIS.
Top comments (1)
Really clear explanation, thanks!