[postgis-users] distance between two points and elevation

[MWilson Matthew]

Good evening,

  I'm a total postGIS newbie and I've done some searching but I haven't found an answer yet.

 

  I've found and tried using the st_distance_spheroid function, and that seems to return a result as I would expect.  However, I have a project where my goal is to determine what points fall within ¼ mile of a point that a user would specify as well as determining the elevation difference to the selected points from the user specified point.

 

  I've defined my geometry field with the Z value that we've pre-calculated, and that seems to show up properly when looking at the values using the st_AsEWKT function.

 

  I was initially thinking I would have to generate a line object for the two points in question using ST_Makeline, use that distance with the st_length3d_spheroid function to compare with the 2d distance returned by st_distance_spheroid and calculate for the other side of the triangle to get the elevation difference, but that doesn't seem to match the elevation numbers I have.

 

  I did have to transform everything into latlong to get the distance functions to work...but I have a feeling that I'm over thinking this.  There should be some kind of function to return the Z value difference between two geometries right?  

 

  Could someone please point me in that direction?

 

Thanks,

Matthew

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