Guide to Convert Multipolygon to Polygon in PostGIS

Introduction

Understanding how to effectively convert multipolygons to individual polygons in PostGIS is a crucial skill for spatial data management and analysis.

In many cases, spatial datasets store multiple distinct areas as a single multipolygon geometry. While this format is useful for grouping related areas, individual polygons may be needed for more precise map visualization, geospatial analysis, or integration into logistics and business intelligence systems.

For businesses relying on geospatial data and using boundaries databases for map visualization or postal databasesfor defining service areas, the ability to split multipolygons into separate polygons is essential. Whether you’re working with detailed regional boundaries, ZIP code zones, or delivery areas, converting multipolygon geometries into individual polygons allows for better spatial querying, improved mapping accuracy, and enhanced data integration in applications like business intelligence dashboards or logistics planning tools.

💡 Use accurate location and boundaries data to convert multipolygons to polygons in PostGIS. Our worldwide ZIP code and boundaries data is updated weekly, relying on over 1,500 sources. Browse Geopostcodes datasets and download a free sample here.

This guide will walk you through the process of converting multipolygons to polygons in PostGIS, covering key concepts, best practices, and practical solutions to common challenges.

Understanding Polygons and Multipolygons in GIS

In Geographic Information Systems (GIS), polygons and multipolygons serve as fundamental building blocks for representing real-world geographic features, though they differ significantly in their structure and application. A polygon is a two or three-dimensional shape defined by a series of ordered x,y coordinates that form a closed boundary. Think of a polygon as a single parcel of land or a building footprint – one continuous area with clear boundaries.

Multipolygons, on the other hand, are more complex structures that consist of multiple polygon features grouped together as a single entity. A classic example would be an archipelago, where each island is represented as a separate polygon, but collectively they form one multipolygon feature. This distinction is particularly important when working with spatial data that represents discontinuous geographic features.

For spatial analysts and GIS professionals, understanding these differences is not merely academic – it directly impacts how we store, analyze, and visualize spatial data. The ability to convert between these formats enables more flexible data analysis and can solve various spatial computing challenges.

Representing Polygons in PostGIS

PostGIS extends PostgreSQL’s capabilities by introducing the specialized geom geometry data type, which provides robust support for storing and manipulating spatial data. When working with polygons in PostGIS, you have multiple options for data representation and creation.

The most common method for creating polygons is using the ST_GeomFromText function, which accepts Well-Known Text (WKT) representation. Here’s an example:

REATE TABLE land_parcels (
    id SERIAL PRIMARY KEY,
    parcel_geometry geometry(Polygon, 4326)
);

INSERT INTO land_parcels (parcel_geometry)
VALUES (
    ST_GeomFromText('POLYGON((0 0, 0 1, 1 1, 1 0, 0 0))', 4326)
);

In this example we have insert in our table ‘land_parcels’ perfect square.

Alternatively, you can use the ST_GeomFromWKB function for creating polygons from Well-Known Binary (WKB) format, which is particularly useful when working with binary spatial data:

INSERT INTO land_parcels (parcel_geometry)
VALUES (
    ST_GeomFromWKB(E'\\\\001...')
);

Converting Multipolygons to Separate Polygons

The ST_Dump function in PostGIS is the primary tool for converting multipolygons into their constituent polygons. This function returns a set of geometry_dump objects, each containing a path and a geometry component. Details of the function. Here’s a practical example:

WITH multipolygon_data AS (
    SELECT ST_GeomFromText('MULTIPOLYGON(((0 0, 0 1, 1 1, 1 0, 0 0)),((2 2, 2 3, 3 3, 3 2, 2 2)))') as geom
)
SELECT (ST_Dump(geom)).geom AS single_polygon
FROM multipolygon_data;

In this example, the multipolygon consists of a single record containing two squares. By using ST_Dump, we obtain two separate records, each representing one square.

Working with Geom Geometry in PostGIS

The geom geometry data type in PostGIS is incredibly versatile, supporting various spatial data types and operations. Here are some essential functions for working with geometry objects:

-- Calculate area of a polygon
SELECT ST_Area(geometry_column) FROM your_table;

-- Calculate perimeter
SELECT ST_Perimeter(geometry_column) FROM your_table;

-- Check geometry type
SELECT ST_GeometryType(geometry_column) FROM your_table;

These functions provide crucial information about your spatial features and help ensure data integrity throughout the conversion process.

Best Practices for Converting Multipolygons

When converting multipolygons to individual polygons, following best practices ensures data quality and prevents issues downstream:

1. Always validate your geometries before and after conversion:

SELECT ST_IsValid(geometry_column) FROM your_table;

1. If your geometry is not valid you check the reason with ST_IsValidReason (see more)

SELECT ST_IsValidReason(geometry_column) FROM your_table;

1. Check for self-intersections:

SELECT ST_Intersects(a.geometry, b.geometry)
FROM your_table a, your_table b
WHERE a.id < b.id;

1. Maintain spatial indexes for better performance:

CREATE INDEX spatial_idx ON your_table USING GIST (geometry_column);

Troubleshooting Common Issues

Invalid geometries and self-intersecting polygons are common challenges when working with spatial data. Here’s how to address these issues:

For invalid geometries:

UPDATE your_table
SET geometry_column = ST_MakeValid(geometry_column)
WHERE NOT ST_IsValid(geometry_column);

The function ST_MakeValid aims to generate a valid representation of an invalid geometry without discarding any of the input vertices (more information in the function)

Conclusion

Converting multipolygons to separate polygons is a fundamental operation in spatial data management that requires a solid understanding of both the underlying concepts and the practical implementation in PostGIS. The process involves using specialized functions like ST_Dump while maintaining data integrity through validation and proper geometry handling.

Success in this area depends on:

• Understanding the fundamental differences between polygons and multipolygons
• Proficiency with PostGIS functions and the geom geometry data type
• Following best practices for geometry validation and error handling
• Implementing proper troubleshooting techniques when issues arise

By mastering these concepts and techniques, you’ll be better equipped to handle complex spatial data operations and maintain high-quality geographic information in your database systems. To learn how to perform data-driven spatial analysis and techniques such as mapping, GIS analysis, and remote sensing, consider leveraging accurate and up-to-date geospatial data from GeoPostcodes. Prepared to convert multipolygons to individual polygons in PostGIS now? Contact us today.

FAQ