Understanding Geometry Types

What is Geometry in PostGIS?

In PostGIS, geometry is the backbone of spatial data. Every spatial feature—whether it's a city point, a river line, or a land parcel polygon—is stored as a geometry object. Geometries are stored in a special column type called geometry, and can represent simple or complex shapes.

PostGIS supports a rich set of geometry types compliant with the OGC Simple Features Specification, enabling you to model virtually any spatial feature you can imagine.

Points – The Simplest Geometry

Points represent a single location in space defined by X and Y coordinates. Optionally, a point can also have Z (elevation) and M (measure) values.

CREATE TABLE landmarks (
  id SERIAL PRIMARY KEY,
  name TEXT,
  geom GEOMETRY(Point, 4326)
);

INSERT INTO landmarks (name, geom) VALUES
('City Hall', ST_GeomFromText('POINT(77.59 12.97)', 4326)),
('Library', ST_GeomFromText('POINT(77.61 12.95)', 4326)),
('Hospital', ST_GeomFromText('POINT(77.58 12.99)', 4326));

Common Use Cases:

• Representing locations like schools, offices, fire stations
• Mapping sensors, wells, or weather stations
• GPS tracking of vehicles or people
• Store locations and points of interest

LineStrings – Connecting Points

A LineString is a linear geometry made of a series of connected points. It represents features with a direction or length.

CREATE TABLE roads (
  id SERIAL PRIMARY KEY,
  name TEXT,
  geom GEOMETRY(LineString, 4326)
);

INSERT INTO roads (name, geom) VALUES
('Main Road', ST_GeomFromText('LINESTRING(77.58 12.96, 77.60 12.98)', 4326)),
('Highway 1', ST_GeomFromText('LINESTRING(77.55 12.95, 77.58 12.96, 77.60 12.98, 77.62 13.00)', 4326));

Common Use Cases:

• Mapping roads, rivers, pipelines, or flight paths
• Measuring distances and path optimization
• Visualizing movement or connectivity between points
• Network analysis and routing

Polygons – Enclosed Areas

Polygons are geometries with a closed ring of coordinates. They define an area with boundaries, optionally with holes.

CREATE TABLE parks (
  id SERIAL PRIMARY KEY,
  name TEXT,
  geom GEOMETRY(Polygon, 4326)
);

INSERT INTO parks (name, geom) VALUES
('Central Park', ST_GeomFromText('POLYGON((77.58 12.96, 77.60 12.96, 77.60 12.98, 77.58 12.98, 77.58 12.96))', 4326)),
('Lake Park', ST_GeomFromText('POLYGON((77.55 12.93, 77.57 12.93, 77.57 12.95, 77.55 12.95, 77.55 12.93))', 4326));

Common Use Cases:

• Defining land parcels, buildings, zones, and habitats
• Calculating area and perimeter
• Overlay analysis in urban and environmental planning
• Administrative boundaries and districts

Multi-Geometries – Collections

Collections group multiple geometries of the same or different type:

-- MultiPoint example
SELECT ST_GeomFromText('MULTIPOINT((77.59 12.97), (77.61 12.95), (77.58 12.99))');

-- MultiLineString example  
SELECT ST_GeomFromText('MULTILINESTRING((77.58 12.96, 77.60 12.98), (77.55 12.95, 77.57 12.97))');

-- MultiPolygon example
SELECT ST_GeomFromText('MULTIPOLYGON(((0 0,0 1,1 1,1 0,0 0)),((2 2,2 3,3 3,3 2,2 2)))');

-- GeometryCollection (mixed types)
SELECT ST_GeomFromText('GEOMETRYCOLLECTION(POINT(77.59 12.97), LINESTRING(77.58 12.96, 77.60 12.98))');

Common Use Cases:

• Country borders with multiple islands
• Roads with multiple disconnected segments
• Aggregated results from spatial queries
• Complex geographic features

3D Geometry – The Z Coordinate

Z coordinates allow geometries to model 3D space by adding elevation or height information:

-- 3D Point with elevation
SELECT ST_GeomFromText('POINT Z(77.59 12.97 250)', 4326);

-- 3D LineString (flight path)
SELECT ST_GeomFromText('LINESTRING Z(77.58 12.96 100, 77.60 12.98 150, 77.62 13.00 200)', 4326);

-- 3D Polygon (building footprint with height)
SELECT ST_GeomFromText('POLYGON Z((77.58 12.96 0, 77.60 12.96 0, 77.60 12.98 0, 77.58 12.98 0, 77.58 12.96 0))', 4326);

Common Use Cases:

• 3D city models and building height data
• Geological surveys and topographic models
• Drone and aircraft flight path analysis
• Underground utility mapping

Measured Geometry – The M Coordinate

The M value is a measure used for storing extra information along a geometry:

-- Point with measure
SELECT ST_GeomFromText('POINT M(77.59 12.97 100)', 4326);

-- LineString with measures (linear referencing)
SELECT ST_GeomFromText('LINESTRING M(77.58 12.96 0, 77.59 12.97 1000, 77.60 12.98 2000)', 4326);

Common Use Cases:

• Linear referencing systems for roads and rivers
• Store event times along a route (bus stops)
• Distance measurements along paths
• Combine with Z to support 4D geometries

Advanced Geometry Types

Polyhedral Surfaces and TINs

These represent 3D surfaces by combining multiple triangle faces:

-- Triangulated Irregular Network (TIN)
SELECT ST_GeomFromText('TIN(((0 0 0,0 1 0,1 0 1,0 0 0)), ((0 1 0,1 1 1,1 0 1,0 1 0)))');

-- Polyhedral Surface
SELECT ST_GeomFromText('POLYHEDRALSURFACE(((0 0 0,0 1 0,1 1 0,1 0 0,0 0 0)))');

Curved Geometries

PostGIS supports curved geometries for features that require arc segments:

-- Circular String
SELECT ST_GeomFromText('CIRCULARSTRING(0 0, 1 1, 1 0)', 4326);

-- Compound Curve
SELECT ST_GeomFromText('COMPOUNDCURVE(CIRCULARSTRING(0 0, 1 1, 1 0), (1 0, 0 0))', 4326);

-- Curve Polygon
SELECT ST_GeomFromText('CURVEPOLYGON(CIRCULARSTRING(0 0, 4 0, 4 4, 0 4, 0 0))', 4326);

Spatial Metadata – Catalog Tables

PostGIS provides catalog tables to store metadata about geometry columns:

-- View geometry column information
SELECT 
  f_table_name,
  f_geometry_column,
  type,
  srid,
  coord_dimension
FROM geometry_columns;

-- View geography column information
SELECT 
  f_table_name,
  f_geography_column,
  type,
  srid,
  coord_dimension
FROM geography_columns;

Best Practices

1. Choose the right geometry type: Use the simplest type that meets your needs
2. Specify SRID consistently: Always define the spatial reference system
3. Validate geometries: Use ST_IsValid() to check geometry validity
4. Index spatial columns: Create spatial indexes for better performance
5. Consider data precision: Balance precision with storage requirements

Real-World Applications

• Urban Planning: Model buildings, roads, and zoning areas
• Environmental Analysis: Track habitats, watersheds, and protected areas
• Transportation: Route optimization and network analysis
• Utilities: Map infrastructure like power lines and pipelines
• Emergency Services: Dispatch optimization and coverage analysis