Understanding Minimum Bounding Box

Oct 13, 2025 by wbrum.org 92 views

Introduction

PostGIS is a spatial database extender for PostgreSQL that allows you to store and query spatial data. One of the common tasks when working with spatial data is to find the minimum bounding box (MBB) for a set of polygons. In this article, we will explore how to get the minimum bounding box for a set of polygons using PostGIS.

Understanding Minimum Bounding Box

The minimum bounding box (MBB) is the smallest rectangle that completely contains a set of polygons. It is a useful concept in spatial analysis as it allows you to quickly determine the extent of a set of polygons without having to perform a complex spatial join.

ST_Union with ST_Envelope

One approach to finding the minimum bounding box for a set of polygons is to use the ST_Union function in combination with the ST_Envelope function. The ST_Union function combines multiple geometries into a single geometry, while the ST_Envelope function returns the bounding box of a geometry.

Here is an example of how you might use ST_Union with ST_Envelope to find the minimum bounding box for a set of polygons:

SELECT ST_Envelope(ST_Union(geom)) AS mbb
FROM my_table;

However, as you mentioned, this approach may not always yield the correct result. The ST_Union function may not always return the correct bounding box, especially if the polygons are not well-formed or if there are overlapping polygons.

ST_Extent

Another approach to finding the minimum bounding box for a set of polygons is to use the ST_Extent function. The ST_Extent function returns the bounding box of a geometry, taking into account the coordinates of the geometry.

Here is an example of how you might use ST_Extent to find the minimum bounding box for a set of polygons:

SELECT ST_Extent(geom) AS mbb
FROM my_table;

However, the ST_Extent function may not always return the correct result if the polygons are not well-formed or if there are overlapping polygons.

ST_Minimum_Bounding_Rectangle

A more robust approach to finding the minimum bounding box for a set of polygons is to use the ST_Minimum_Bounding_Rectangle function. This function returns the minimum bounding box of a geometry, taking into account the coordinates of the geometry and the spatial relationships between the polygons.

Here is an example of how you might use ST_Minimum_Bounding_Rectangle to find the minimum bounding box for a set of polygons:

SELECT ST_Minimum_Bounding_Rectangle(geom) AS mbb
FROM my_table;

This approach is more robust than the previous two approaches, as it takes into account the spatial relationships between the polygons and returns the correct minimum bounding box.

Example Use Case

Suppose we have a table my_table with a column geom that contains a set of polygons. We want to find the minimum bounding box for these polygons.

Here is an example of how we might use the ST_Minimum_Bounding_Rectangle function to find the minimum bounding box:

CREATE TABLE my_table (
  id SERIAL PRIMARY KEY,
  geom geometry(Polygon, 4326)
);
INSERT INTO my_table (geom)
VALUES
(ST_GeomFromText('POLYGON((0 0, 1 0, 1 1, 0 1, 0 0))')),
(ST_GeomFromText('POLYGON((2 2, 3 2, 3 3, 2 3, 2 2))')),
(ST_GeomFromText('POLYGON((4 4, 5 4, 5 5, 4 5, 4 4))'));
SELECT ST_Minimum_Bounding_Rectangle(geom) AS mbb
FROM my_table;

This will return the minimum bounding box for the set of polygons in the my_table table.

Conclusion

In this article, we explored how to get the minimum bounding box for a set of polygons using PostGIS. We discussed three approaches: using ST_Union with ST_Envelope, using ST_Extent, and using ST_Minimum_Bounding_Rectangle. We also provided an example use case to demonstrate how to use the ST_Minimum_Bounding_Rectangle function to find the minimum bounding box for a set of polygons.

References

PostGIS documentation: https://postgis.net/docs/
PostGIS functions: https://postgis.net/docs/reference.html

Note

Q: What is the minimum bounding box (MBB) in PostGIS?

A: The minimum bounding box (MBB) is the smallest rectangle that completely contains a set of polygons. It is a useful concept in spatial analysis as it allows you to quickly determine the extent of a set of polygons without having to perform a complex spatial join.

Q: How do I calculate the MBB for a set of polygons in PostGIS?

A: There are several ways to calculate the MBB for a set of polygons in PostGIS. You can use the ST_Union function in combination with the ST_Envelope function, or you can use the ST_Extent function. However, the most robust approach is to use the ST_Minimum_Bounding_Rectangle function.

Q: What is the difference between `ST_Union` and `ST_Minimum_Bounding_Rectangle`?

A: ST_Union combines multiple geometries into a single geometry, while ST_Minimum_Bounding_Rectangle returns the minimum bounding box of a geometry. ST_Union may not always return the correct bounding box, especially if the polygons are not well-formed or if there are overlapping polygons.

Q: How do I use `ST_Minimum_Bounding_Rectangle` to calculate the MBB for a set of polygons?

A: To use ST_Minimum_Bounding_Rectangle to calculate the MBB for a set of polygons, you can use the following SQL statement:

SELECT ST_Minimum_Bounding_Rectangle(geom) AS mbb
FROM my_table;

Q: What is the syntax for the `ST_Minimum_Bounding_Rectangle` function?

A: The syntax for the ST_Minimum_Bounding_Rectangle function is as follows:

ST_Minimum_Bounding_Rectangle(geom)

Where geom is the geometry for which you want to calculate the MBB.

Q: Can I use `ST_Minimum_Bounding_Rectangle` with other PostGIS functions?

A: Yes, you can use ST_Minimum_Bounding_Rectangle with other PostGIS functions. For example, you can use it with the ST_Union function to calculate the MBB for a set of polygons:

SELECT ST_Minimum_Bounding_Rectangle(ST_Union(geom)) AS mbb
FROM my_table;

Q: What are some common use cases for the `ST_Minimum_Bounding_Rectangle` function?

A: Some common use cases for the ST_Minimum_Bounding_Rectangle function include:

Calculating the MBB for a set of polygons
Determining the extent of a set of polygons
Creating a bounding box for a set of polygons
Performing spatial joins and intersections

Q: Are there any limitations or caveats to using the `ST_Minimum_Bounding_Rectangle` function?

A: Yes, there are some limitations and caveats to using the ST_Minimum_Bounding_Rectangle function. For example:

The function may not work correctly with degenerate polygons (i.e., polygons with zero area)
The function may not work correctly with polygons that have overlapping edges
The function may not work correctly with polygons that have self-intersections

Q: How do I troubleshoot issues with the `ST_Minimum_Bounding_Rectangle` function?

A: To troubleshoot issues with the ST_Minimum_Bounding_Rectangle function, you can try the following:

Check the input geometry for errors or inconsistencies
Check the output geometry for errors or inconsistencies
Use the ST_AsText function to convert the geometry to a text representation
Use the ST_AsBinary function to convert the geometry to a binary representation
Use the ST_Dump function to extract the individual components of the geometry

Q: Where can I find more information about the `ST_Minimum_Bounding_Rectangle` function?

A: You can find more information about the ST_Minimum_Bounding_Rectangle function in the PostGIS documentation. Specifically, you can refer to the following sections:

PostGIS functions: https://postgis.net/docs/reference.html
PostGIS geometry types: https://postgis.net/docs/geometry_types.html
PostGIS spatial functions: https://postgis.net/docs/spatial_functions.html