mysql - Why I obtain different values calculating distance between two point in the space using this "Great-circle distance" formula and Google Earth tool? -


i not database , gis , have following doubt function used on mysql database calculate distance between 2 points.

i started tutorial there function used calculate distance between 2 point field:

create function earth_circle_distance(point1 point, point2 point) returns double     deterministic begin   declare lon1, lon2 double;   declare lat1, lat2 double;   declare td double;   declare d_lat double;   declare d_lon double;   declare a, c, r double;    set lon1 = x(geomfromtext(astext(point1)));   set lon2 = x(geomfromtext(astext(point2)));   set lat1 = y(geomfromtext(astext(point1)));   set lat2 = y(geomfromtext(astext(point2)));    set d_lat = radians(lat2 - lat1);   set d_lon = radians(lon2 - lon1);    set lat1 = radians(lat1);   set lat2 = radians(lat2);    set r = 6372.8; -- in kilometers    set = sin(d_lat / 2.0) * sin(d_lat / 2.0) + sin(d_lon / 2.0) * sin(d_lon / 2.0) * cos(lat1) * cos(lat2);   set c = 2 * asin(sqrt(a));    return r * c; end

i think should return value in km unit measurment.

then have used calculate distance between 2 point in space (taken google maps example rome , milan cities, here in italy).

i obtaining reasonable values. doubt that, have check distance using ruler tool in gooogle earth , obtaining different values.

foe example considering 2 points near (2 km of distance) obtain 300 m of difference between difference calculated google earth , previous function.

considering 2 points distant (for example rome , milan) obtain 100 km of difference between value calculated google earth , 1 calculated previous function.

the previous function give bigger value.

so have use calculate on small distance (maximum 30 km) error should little , ok pourpose.

but why obtain these difference between funcion , distance calculated using google earth?

sql server returns 534.728893656916km using code: declare @g geography = geography::point(9.191884,45.464090,4326) -- latitude, longitude, srid declare @h geography = geography::point(12.492359,41.890365,4326) select @g.stdistance(@h)/1000.00


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