PostGIS Length Calculation – Using ST_Length (GEOM, true) for Calculating Distances in Geographic Reference System

coordinate systemlengthpostgis-2.0

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: https://postgis.net/docs/ST_Length.html

EPSG:3395 reference: https://spatialreference.org/ref/epsg/3395/

Best Answer

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.

Related Solutions

ProjNet Transformations – How to Perform Transformations Using ProjNet

I had the same error (with a different SRID). Solution was to use the/an Esri WKT on spatialreference.org, which does not cause an issue with the ProjNet CoordinateSystemWktReader

SRID 4326 – Understanding Lon/Lat or Lat/Lon Coordinate Systems

In principle, it should always be lat/lon as that is what the current EPSG database defines it as. Unfortunately, over the years computer scientists have visited and made a decision to use lon/lat as that works for their high school maths mapping to X,Y and is easy.

So whenever you receive a file of coordinates in EPSG:4326 you need to check who sent them to you, if either of the columns exceeds 90 or plot them on a map and see if they are in the right place. If you are using someone else's code you should look inside the code and see what they are doing. Ideally you will see something like:

double ulLon, ulLat;
// Let's get upper-left corner coords
CRS.AxisOrder aorder = CRS.getAxisOrder(reqExtentInTileCrs.getCoordinateReferenceSystem());
switch (aorder) {
    case EAST_NORTH:
        ulLon = reqExtentInTileCrs.getMinX();
        ulLat = reqExtentInTileCrs.getMaxY();
        break;
    case NORTH_EAST:
        if (LOGGER.isLoggable(Level.FINE)) LOGGER.log(Level.FINE, "Inverted tile coords!");
        ulLon = reqExtentInTileCrs.getMinY();
        ulLat = reqExtentInTileCrs.getMaxX();
        break;
    default:
        LOGGER.log(Level.WARNING, "unexpected axis order " + aorder);
        return ret;
}

Best Answer

Related Solutions

ProjNet Transformations – How to Perform Transformations Using ProjNet

SRID 4326 – Understanding Lon/Lat or Lat/Lon Coordinate Systems

Related Question