Geographic Information Systems Asked by CarlosRodLop on April 20, 2021
I use (1) ST_Length (ST_Transform (geom, 3395)) to get distances in meters from points in Geographic Reference Systems EPSG: 4326. The thing is I found that with (2) ST_Length (geom, true) you can get also the distance in meters but using the spheroid.
If I am right, is the option (2) more accurate if the GIS project use a Geographic Reference System for Global data, especially near to the poles?
ST_Length reference: http://postgis.org/docs/ST_Length.html
EPSG:3395 reference: http://spatialreference.org/ref/epsg/3395/
You can if your data is stored in a geography column, or cast from geometry to geography. If you run ST_Length against geometry it will return the units the data is stored in (feet, meters, degrees), if you run ST_Length against geography it will return the answer in meters.
See this workshop for details on the difference.
Correct answer by HeyOverThere on April 20, 2021
EPSG:3395 uses the Mercator projection. Mercator is not designed for distance calculations, instead it preserves shapes/angles, so yes, option 2, will give more accurate distances.
Answered by mkennedy on April 20, 2021
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