I am trying to create a great circle line in order to ascertain if one was looking 'straight-out' from the end of a pier what country/place they would be viewing. I have the below PostGIS code written:

-- assume two points, calculate the angle, then extend it.
-- point 1 = -33.6307920509579, 115.338797474435  
-- point 2 = -33.6290969607648, 115.338316140276 
ST_Segmentize( -- break it up into segments so it looks better when re-projecting for display
        ST_SetSRID(ST_Point(115.338797474435, -33.6307920509579), 4326) -- starting point (ST_MakeLine expects projected coordinates)
        ST_Project( -- find a point a long way away to use  as the second point in the line
                ST_Point(115.338797474435, -33.6307920509579)::geography -- starting point again
                ,20000000 -- 1/2 circumference of earth at the equator (should get us far enough)
                ,ST_Azimuth(ST_Point(115.338797474435, -33.6307920509579)::geography, ST_Point(115.338316140276, -33.6290969607648)::geography) -- angle between my two points
,20000 -- break it into segments of 20km

enter image description here

When I create the linestring and open it in QGIS I get a great circle line but the angle is not correct (as shown below). What am I missing?

enter image description here

  • 1
    I'm not sure if the result is wrong- a great circle line on OSM that uses Web mercator - EPSG:3857 would result in a line like this: 4.bp.blogspot.com/-nJyLLfJXsQ8/Wj-FUthJhyI/AAAAAAAAECQ/… - if you just connect start- and endpoint, you get a straight line following a completely different path, but still the correct destination- see the second line on the linked image.
    – Babel
    Commented Nov 21, 2021 at 10:26
  • As you see, the direction at the start point is completely different, still the result for your question is the same: the country of the line's wnd point. So make sure if this is the case in your case. Test it by exporting the line to a kml file and open it in Google Earth: it is a very intuitive way to see where a great circle line passes through.
    – Babel
    Commented Nov 21, 2021 at 10:30


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Browse other questions tagged or ask your own question.