How to convert result of Presto `ST_Distance` to meters?

Question

I am trying to figure out a way to convert the result of presto geo spatial function ST_DISTANCE to meters.

If I run the this example query: SELECT ST_Distance(ST_Point(0.3476, 32.5825),ST_Point(0.0512, 32.4637))

The result I get from Presto is: 0.3193217812802629. The actual distance between these two places is 40,000m.

The presto documentation states that ST_DISTANCE: Returns the 2-dimensional cartesian minimum distance (based on spatial ref) between two geometries in projected units.

What I can understand about spatial ref is at links such as these: http://webhelp.esri.com/arcgiSDEsktop/9.3/index.cfm?TopicName=Defining_a_spatial_reference

Which leads me to believe I need to figure you what spatial-ref Presto is using.

If I check the prest docs here: https://github.com/prestodb/presto/blob/master/presto-geospatial/src/main/java/com/facebook/presto/geospatial/GeoFunctions.java

I can guess that is using the ESRI libraries so I assume the ESRI spatial ref? But that is where I get a bit lost as to where to proceed?

Thank you for your help..

Answer 1

I would recommend using Presto’s great_circle_distance() function instead of ST_Distance(). It will interpret your coordinates as WGS84 (aka EPSG:4326), and find the distance between them in kilometres by treating the shape of the earth as a sphere.

ST_Distance() would be appropriate if the coordinate system being used was already projected into a system that used metres or miles or some other unit, but there's no trivial way to do that in Presto.