Hi Jacob, 

well — the geo distance computes the distance of the direct line between the two coordinates:
       <gml:coordinates>52.519881,13.407338</gml:coordinates>
and
   <gml:coordinates>51.507351,-0.12766</gml:coordinates>

Which happens to be 13.572817977888896  <=> https://en.wikipedia.org/wiki/Euclidean_distance

Yet, the earth is spherical, so you need to take that into account, https://en.wikipedia.org/wiki/Geographical_distance

I think what you are actually looking for is the great-circle distance — the length of a line traveling along the surface between two points on a circle.

A somehow good approximation for that distance can be computed using the haversine formula:
http://www.movable-type.co.uk/scripts/latlong.html

I rewrote the Javascript as XQuery and the result seems to be about right.

import module namespace geo = "http://expath.org/ns/geo";
declare namespace gml='http://www.opengis.net/gml';


(: formula taken from here: 
  https://www.movable-type.co.uk/scripts/latlong.html
:) 
declare function local:haversine(
  $point1 as element(gml:Point),
  $point2 as element(gml:Point)
) as xs:double {
  let $r    := 6371  (: earth radius in KM :)
  let $lat1 := geo:x($point1)
  let $lat2 := geo:x($point2)
  let $lon1 := geo:y($point1)
  let $lon2 := geo:y($point2)
  let $φ1   := $lat1 => local:radians()
  let $φ2   := $lat2 => local:radians()
  let $Δφ   := ($lat2 - $lat1) => local:radians()
  let $Δλ   := ($lon2 - $lon1) => local:radians()

  let $a    := math:pow(math:sin($Δφ div 2), 2) +
               math:cos($φ1) * math:cos($φ2) *
               math:pow(math:sin($Δλ div 2), 2)
  let $c   := 2 * math:asin(math:sqrt($a))
  
  return $c * $r
};


declare function local:radians(
  $deg as xs:double
) as xs:double {
  $deg * (math:pi() div 180)
};


let $berlin :=
 <gml:Point>
       <gml:coordinates>52.519881,13.407338</gml:coordinates>
 </gml:Point>
let $london :=
 <gml:Point>
   <gml:coordinates>51.507351,-0.12766</gml:coordinates>
 </gml:Point>
return local:haversine($berlin, $london)
(: returns 931.7188794140812 :)



Maybe you should make yourself comfortable with the maths behind that formula to better understand its shortcomings, nevertheless, here’s what I came up with in a more readable version:

https://gist.github.com/micheee/e47166d0947f925e395504c1e220fbea

Best
Michael






Am 26.02.2019 um 13:19 schrieb Jacob Borisov <jacobborisov@gmail.com>:

Hi! For example, it's need to get the distance between Berlin and
London, which is 934 km.

So I've put the coordinates as they can be found in Google Maps, etc.:

let $berlin :=
  <gml:Point>
        <gml:coordinates>52.519881,13.407338</gml:coordinates>
  </gml:Point>
let $london :=
  <gml:Point>
    <gml:coordinates>51.507351,-0.12766</gml:coordinates>
  </gml:Point>
return geo:distance($berlin, $london)

And the result is 13.572817977888896

Also I had trying to change srsName attribute of gml:Point's to
different values like "3857", "EPSG:3857" (it's uses meter units as
mentioned here https://epsg.io/3857), but practically this changes
nothing. Thanks.

вт, 26 февр. 2019 г. в 19:33, Michael Seiferle <ms@basex.org>:

Hi Jacob,

I’ve never really used the geo:module but according to the documentation:
in the units of the spatial reference system of geometry1
If I understand it correctly, you would have to convert your coordinates to map them to miles or kilometers for example.

Maybe you can provide an example?

Given this very simple query:
import module namespace geo = "http://expath.org/ns/geo";
declare namespace gml='http://www.opengis.net/gml';
let $point1 :=<gml:Point><gml:coordinates>1,1</gml:coordinates></gml:Point>
let $point2 := <gml:Point><gml:coordinates>2,2</gml:coordinates></gml:Point>
return geo:distance($point1, $point2)

The result is math:sqrt(2),  the length of a straight line from P(1,1) — P’(2,2) but this length does not contain any unit of measurement.



Best
Michael



Am 26.02.2019 um 12:12 schrieb Jacob Borisov <jacobborisov@gmail.com>:

Hello. Could anybody explain how to get a result from geo:distance()
in kilometers or miles? Thanks.