Geometry vs Geography

PostGIS geometry uses planar math in a projected coordinate system. geography uses a spheroid model on WGS84 for geodesic distances in meters. Pick the type that matches your SRID and distance accuracy needs.

Recipe

-- Same lon/lat, different types
SELECT
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326) AS geom_4326,
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326)::extensions.geography AS geog;
 
-- Distance NYC to LA
SELECT extensions.ST_Distance(
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326)::extensions.geography,
  extensions.ST_SetSRID(extensions.ST_MakePoint(-118.2437, 34.0522), 4326)::extensions.geography
) AS meters_geog;
 
SELECT extensions.ST_Distance(
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326),
  extensions.ST_SetSRID(extensions.ST_MakePoint(-118.2437, 34.0522), 4326)
) AS degrees_geom_wrong_for_meters;

When to reach for this: Choosing column types for new tables, debugging distance bugs, or optimizing indexes for ST_DWithin.

Working Example

CREATE TABLE delivery_zones (
  id       serial PRIMARY KEY,
  name     text,
  -- City-scale polygon: local UTM for accurate geometry math
  zone_geom extensions.geometry(Polygon, 32618),  -- UTM 18N for NYC area
  -- Driver location queries: geography for meter radius from GPS
  service_geog extensions.geography(Polygon, 4326)
);
 
-- Store both when needed: geometry for editing in local projection,
-- geography for "within 2 km of driver" queries
CREATE INDEX delivery_zones_geog_gist
  ON delivery_zones USING gist (service_geog);
 
SELECT name
FROM delivery_zones
WHERE extensions.ST_DWithin(
  service_geog,
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.99, 40.75), 4326)::extensions.geography,
  2000
);

What this demonstrates:

Local projected geometry for city planning polygons
geography for GPS radius filters in meters
GiST on geography supports ST_DWithin
Mixed models are valid when transforms are documented

Deep Dive

Comparison Table

Aspect	geometry	geography
Coordinate system	Projected (meters in plane)	WGS84 spheroid
Distance units	Map units (meters if projected)	Meters on spheroid
Performance	Faster for local areas	Slower, more accurate globally
Functions	Full planar suite	Subset; casts often needed
SRID	Must be projected for meter math	Typically 4326

SRID Quick Reference

SRID	Name	Use
4326	WGS84 lat/lon	GPS, web maps, geography
3857	Web Mercator	Display only; poor for distance
32618	UTM 18N	NYC region planar work
0	Unknown	Avoid in production

SELECT srid, auth_name, auth_srid, srtext
FROM spatial_ref_sys
WHERE srid IN (4326, 3857, 32618);

Transforms

-- Reproject geometry between SRIDs
SELECT extensions.ST_Transform(
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326),
  32618
);
 
-- Geography from geometry (4326 only for geog cast)
SELECT extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326)::extensions.geography;

Index Considerations

GiST works on both geometry and geography.
Use geometry with correct local projection for large polygon overlays (faster).
Use geography for consistent meter radii on lat/lon inputs without projection math in app.

Gotchas

4326 geometry distance in meters - ST_Distance on 4326 geometry returns degrees. Fix: Cast to geography or transform to UTM.
3857 for analytics - Web Mercator distorts area and distance. Fix: 3857 for tiles only; store 4326 or local UTM.
SRID 0 insert - ST_GeomFromText('POINT(0 0)') without SRID. Fix: Typmod geometry(Point,4326) rejects wrong SRID when enforced.
geography on non-4326 - Limited support; wrong results. Fix: Transform to 4326 before geography cast.
Mixing types in ST_Intersects - Implicit cast surprises. Fix: Explicit cast and document pipeline.
Poles and dateline - geography handles; bad polygons in naive geometry. Fix: Validate with ST_IsValid.

Alternatives

Alternative	Use When	Don't Use When
geometry + UTM	City/county fixed region	Global roaming queries
geography only	Simple GPS radius apps	Heavy polygon overlay performance pain
Haversine in app	One-off distance	Spatial joins and indexes needed
Raster tiles	Display-only maps	Vector feature storage

FAQs

Which type for delivery radius?

geography on 4326 for meter ST_DWithin from GPS points.

Polygon storage?

Local city: projected geometry. Global service areas: geography or multi-zone UTM splits.

ST_Length units?

geometry: map units; geography: meters on spheroid.

Area calculations?

Use appropriate projection; geography area is spheroid-based.

Typmod enforcement?

geometry(Point,4326) rejects wrong SRID inserts in recent PostGIS.

Performance tip?

Bounding box && operator on geometry geography casts uses index prefilter.

EWKT?

SRID=4326;POINT(...) in COPY for explicit SRID ingest.

3D geography?

Rare; usually drop Z for web apps.

QGIS export?

Match SRID in export settings to column typmod.

Testing?

Known distance pairs (NYC-LA ~3935 km) sanity check queries.

PostGIS Basics - when PostGIS vs tiles
Spatial Indexes - GiST usage
PostGIS Performance - bounds prefilter
Data Types JSONB - composite patterns
PostGIS Best Practices - SRID on every geometry

Stack versions: This page was written for PostgreSQL 18.4 (stable 18, maintenance 17), pgvector 0.8+, PostGIS 3.5+, pgbouncer 1.x, and Patroni 3.x.

Geometry vs Geography

Recipe

-- Same lon/lat, different types
SELECT
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326) AS geom_4326,
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326)::extensions.geography AS geog;
 
-- Distance NYC to LA
SELECT extensions.ST_Distance(
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326)::extensions.geography,
  extensions.ST_SetSRID(extensions.ST_MakePoint(-118.2437, 34.0522), 4326)::extensions.geography
) AS meters_geog;
 
SELECT extensions.ST_Distance(
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326),
  extensions.ST_SetSRID(extensions.ST_MakePoint(-118.2437, 34.0522), 4326)
) AS degrees_geom_wrong_for_meters;

When to reach for this: Choosing column types for new tables, debugging distance bugs, or optimizing indexes for ST_DWithin.

Working Example

CREATE TABLE delivery_zones (
  id       serial PRIMARY KEY,
  name     text,
  -- City-scale polygon: local UTM for accurate geometry math
  zone_geom extensions.geometry(Polygon, 32618),  -- UTM 18N for NYC area
  -- Driver location queries: geography for meter radius from GPS
  service_geog extensions.geography(Polygon, 4326)
);
 
-- Store both when needed: geometry for editing in local projection,
-- geography for "within 2 km of driver" queries
CREATE INDEX delivery_zones_geog_gist
  ON delivery_zones USING gist (service_geog);
 
SELECT name
FROM delivery_zones
WHERE extensions.ST_DWithin(
  service_geog,
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.99, 40.75), 4326)::extensions.geography,
  2000
);

What this demonstrates:

Local projected geometry for city planning polygons
geography for GPS radius filters in meters
GiST on geography supports ST_DWithin
Mixed models are valid when transforms are documented

Deep Dive

Comparison Table

Aspect	geometry	geography
Coordinate system	Projected (meters in plane)	WGS84 spheroid
Distance units	Map units (meters if projected)	Meters on spheroid
Performance	Faster for local areas	Slower, more accurate globally
Functions	Full planar suite	Subset; casts often needed
SRID	Must be projected for meter math	Typically 4326

SRID Quick Reference

SRID	Name	Use
4326	WGS84 lat/lon	GPS, web maps, geography
3857	Web Mercator	Display only; poor for distance
32618	UTM 18N	NYC region planar work
0	Unknown	Avoid in production

SELECT srid, auth_name, auth_srid, srtext
FROM spatial_ref_sys
WHERE srid IN (4326, 3857, 32618);

Transforms

-- Reproject geometry between SRIDs
SELECT extensions.ST_Transform(
  extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326),
  32618
);
 
-- Geography from geometry (4326 only for geog cast)
SELECT extensions.ST_SetSRID(extensions.ST_MakePoint(-73.9857, 40.7484), 4326)::extensions.geography;

Index Considerations

GiST works on both geometry and geography.
Use geometry with correct local projection for large polygon overlays (faster).
Use geography for consistent meter radii on lat/lon inputs without projection math in app.

Gotchas

4326 geometry distance in meters - ST_Distance on 4326 geometry returns degrees. Fix: Cast to geography or transform to UTM.
3857 for analytics - Web Mercator distorts area and distance. Fix: 3857 for tiles only; store 4326 or local UTM.
SRID 0 insert - ST_GeomFromText('POINT(0 0)') without SRID. Fix: Typmod geometry(Point,4326) rejects wrong SRID when enforced.
geography on non-4326 - Limited support; wrong results. Fix: Transform to 4326 before geography cast.
Mixing types in ST_Intersects - Implicit cast surprises. Fix: Explicit cast and document pipeline.
Poles and dateline - geography handles; bad polygons in naive geometry. Fix: Validate with ST_IsValid.

Alternatives

Alternative	Use When	Don't Use When
geometry + UTM	City/county fixed region	Global roaming queries
geography only	Simple GPS radius apps	Heavy polygon overlay performance pain
Haversine in app	One-off distance	Spatial joins and indexes needed
Raster tiles	Display-only maps	Vector feature storage

FAQs

Which type for delivery radius?

geography on 4326 for meter ST_DWithin from GPS points.

Polygon storage?

Local city: projected geometry. Global service areas: geography or multi-zone UTM splits.

ST_Length units?

geometry: map units; geography: meters on spheroid.

Area calculations?

Use appropriate projection; geography area is spheroid-based.

Typmod enforcement?

geometry(Point,4326) rejects wrong SRID inserts in recent PostGIS.

Performance tip?

Bounding box && operator on geometry geography casts uses index prefilter.

EWKT?

SRID=4326;POINT(...) in COPY for explicit SRID ingest.

3D geography?

Rare; usually drop Z for web apps.

QGIS export?

Match SRID in export settings to column typmod.

Testing?

Known distance pairs (NYC-LA ~3935 km) sanity check queries.

PostGIS Basics - when PostGIS vs tiles
Spatial Indexes - GiST usage
PostGIS Performance - bounds prefilter
Data Types JSONB - composite patterns
PostGIS Best Practices - SRID on every geometry

Stack versions: This page was written for PostgreSQL 18.4 (stable 18, maintenance 17), pgvector 0.8+, PostGIS 3.5+, pgbouncer 1.x, and Patroni 3.x.

Geometry vs Geography

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Geometry vs Geography