13

In PostGIS, the ST_length() function returns the length of a linestring in some units, but not in meters.

Is there a function like ST_Length() but in meters?

11

St_Length() returns the length in the units of its Spatial Reference System. If you want to get meters, you will need to transform your feature to a different SRS that uses meters as units. This can be done on-the-fly, so don't worry about writing out another data set.

SELECT ST_AsText(ST_Transform(the_geom,26915)) from my_table;

In this example, SRID 26915 is UTM Zone 15 N, NAD83.

  • Or simply add your own field and manage the calculation yourself. NOTE you have to write your own trigger or recalculate it on changes in your lengths. – Brad Nesom Oct 11 '12 at 4:12
17

It looks like since PostGIS 2.0 (maybe before) you can also just do this with ST_Length by recasting the geometry to geography:

ST_Length(geom::geography)

For example, the distance between 50°N, 12°E and 51°N, 13°E:

SELECT ST_Length(geom) AS deg,
       ST_Length(geom::geography)/1000 AS km_1,
       ST_Length_Spheroid(geom, 'SPHEROID["WGS 84",6378137,298.257223563]')/1000 as km_2
FROM   ST_GeomFromText('LINESTRING(12 50, 13 51)' , 4326) AS geom

which gives

       deg       |       km_1       |       km_2       
-----------------+------------------+------------------
 1.4142135623731 | 131.935962780384 | 131.935962780384
2

Examples of SQL request: how-to-calculate-length-of-polyline-geometry-for-several-tables-in-postgis

SQL request (length in km (/1000 -delete for meters)):

SELECT 
sum(ST_Length_Spheroid(the_geom,'SPHEROID["WGS 84",6378137,298.257223563]'))/1000 AS km_roads 
FROM 
"D1_r";
0

Use the following formula :

select ST_Length(ST_Transform(ST_GeomFromEWKT('SRID=4326;'||st_astext(the_geom)),26986)) as geolength from my_table;

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.