[postgis-users] decimal degrees -> miles?

[Dave Blasby]

Alex Rice wrote:
> 
> Hi, does PostGIS have a function for converting decimal degrees
> to miles? I'm using a -1 projection and my data is in decimal
> degrees.

There are some functions that do calculations on ellipsoids.  Otherwise,
I would just transform() the data to a rectangular spatial referencing
system, then do a length().

 From the help files:

---------------------

length_spheroid(geometry,spheroid) :: 
Calculates the length of of a geometry on an elipsoid. This is useful if
the coordinates of the geometry are in latitude/longitude and a length
is desired without reprojection. The elipsoid is a separate database
type and can be constructed as follows:

SPHEROID[<NAME>,<SEMI-MAJOR AXIS>,<INVERSE FLATTENING>]

Eg:

SPHEROID["GRS_1980",6378137,298.257222101]

An example calculation might look like this: 

SELECT
 length_spheroid(
            geometry_column,
            'SPHEROID["GRS_1980",6378137,298.257222101]'
      )
 from geometry_table;

length3d_spheroid(geometry,spheroid) ::
Calculates the length of of a geometry on an elipsoid, taking the
elevation into account. This is just like length_spheroid except
vertical coordinates (expressed in the same units as the spheroid axes)
are used to calculate the extra distance vertical displacement adds.

----------------------

So, you could:

SELECT
 length_spheroid(
            'LINESTRING(x1 y1, x2 y2)' ,
            'SPHEROID["GRS_1980",6378137,298.257222101]'
      )
;

OR 

SELECT 
	length( transform(setSRID('LINESTRING(x1 y1, x2 y2)',
<latlong>),<rectangular>) ) ;

	where latlong - SRID in the spatial_ref_system table of the latlong
projection
	      rectangular - SRID in the spatial_ref_system table of an
appropriate projection

NOTE: the results are probably in meters (if you use a standard
ellipsoid definition or a meters projection), so you'll have to convert
meters to miles.