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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/

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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.

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  • In my opinion, HeyOverThere's answer is more precise that mkennedy's one. And also it makes me notice that I was using the wrong data type for working with Global Data, I should use "geography" type instead of "geometry". Anyway thanks to both of you for your answers. Jan 24 '14 at 9:09
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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.

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  • This answer is also correct because doing ST_Length (geom, true) is equivalent to cast from geometry to geography like doing ST_Length (geom::geography) Jan 24 '14 at 13:23

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