[postgis-users] Newbie - get total distance travelled from query of points
woodbri at swoodbridge.com
Sun Jun 22 15:25:15 PDT 2008
Joe Roberts wrote:
> I am very new to PostGIS, and I'm stuck with what I hope is an easy
> problem to solve.
> I have a table which contains a list of points, originating from a GPS
> device's lat/long output. I want to get the distance travelled. I have
> the following query, but the result (distance) looks far to low:
> SELECT ST_Length(St_MakeLine(point))/1000 as distance
> FROM (
> SELECT point FROM points
> ORDER BY timestamp
> ) as p
> I know the route was over 5 miles (probably about about 8.4 km), but
> the query returns 0.000106496.
> Can anyone see what I am doing wrong? Is it my query, or could it be
> my data?
> Many thanks
So assuming your GOS is return LAT, LON in WGS84
ST_Length is returning the length in degrees (IE: the Cartesian distance
in the units of the geometry) and that return 0.106496 degrees. If you
want miles you can multiple that by the number of miles/degree and
assuming a great circle like path there are approximate 69.172 miles per
degree along the Equator so you distance is 7.366541312 miles which
seems to fit with your statements above.
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