id	summary	reporter	owner	description	type	status	priority	milestone	component	version	resolution	keywords	cc
18842	great circle arcs (orthodromy)	verdyp@…	team	"For now JOSM has tools to draw circles and straight lines, these are fine, but only for small distances or along the equator: 

* Except along the equator, loxodromies are in fact spirals converging two the two poles with the same angle relative to the North.

* ""straight lines"" are in fact loxodomies, but they are not the shortest route between two nodes. They just appear to be ""straight"" on a Mercator projection (like the default WGS84 projection used in JOSM), because this preserves angles, but not distances (real distances along a parallel with the same longitudinal angle difference are getting smaller when you escape away from the equator). So the ""circles"" in JOSM are not real circles: if you center the circle anywhere else then on the Equator in the Northern Hemsipere, the ""radius"" to the south is in fact smaller than the radius to the north, objects will be significantly distorted if your objects span a large enough difference of latitude (more than 1 degree).

We need another geometric ""primitive"" for large objects, notably routes, but also some legal definitions of maritime borders, that use ""orthodromies"" (i.e. effective distance along a great circle) and not ""loxodromies"", even if in a Mercator project they will show as smooth curves.

Lowodromies and orthodromies between two arbitrary nodes are equivalent if they are *both* on the Equator, or *both* on the same meridian. In all other cases, they are different, and ""straight lines"" are not the shortest routes (this also affects precise measurement of long distances in JOSM): to limit the error, we need to be able to add more nodes to represent these curves (JOSM warns us for straight loxodromic arcs betwee ntwo nodes that are longer than 10 kilometers, i.e. about 5 minutes of longitude, but the deviation error from the real great circle arc to the loxodromic arc can reach up to 1 minute measured orthograonally, i.e. about 700 meters, in the middle of the arc; this is not precise enough for some circonstances; the error depends on the angulare cape between the two nodes and the latitude difference).

Is there a plugin or integrated tool to approximate orthodromic routes between to nodes, by adding enough nodes so the deviation angle along all nodes of the generated polyline goes below 0.1 degree, or the deviation distance betwween loxodromy and orthodromy goes below a given precision (e.g. 10 meters) ?

For now, these orthodromic arcs have been drawn with incorrect estimations (e.g. incorrect radius for territorial waters, computed from a simple loxodromic distance) or based on external maps (such as WPS maps) where these arcs were computed by someone else ?

Note that we could compute routes externally and then import the generated ways.

Finally, how are distances really measured in JOSM ? I suppose that what is displayed is just a loxodromic distnace, but I really suspect these distances are incorrect estimations of real distances (even if we ignore the effect of altitude and relief, and suppose that all is measured on the projection sphere and not the real ground)."	enhancement	closed	normal		Plugin		wontfix