# Reverse parameters of a projection map / guess which geographic project is used

I'm user of a soaring simulator named "Condor Soaring".

This simulator is at the center of a community of enthusiastic people who build sceneries (landscapes) which represents quite accurately real world.

Glider pilots can train to achieve some tasks (such as flying a triangular circuit or other shapes).

Track distance can be quite long (more than 200 km). Time to achieve task is also important. Tactical glide computer can be used (like XCSoar) (both for real gliding but also for simulated gliding)

Condor Soaring tasks are stored in a text file (like a INI file). XCSoar circuits are stored in a text file (XML file).

I wrote Python script to convert Condor task file to XCSoar https://github.com/scls19fr/pycondor

XCSoar is using geographical coordinates but Condor soaring is using X, Y coordinates. Maximal values for X and Y depend of landscape considered.

A DLL named `NaviCon.dll` provides some functions to get theses informations about a given landscape (`GetMaxX`, `GetMaxY`)

This DLL also provides functions `XYToLat`, `XYToLon` to convert respectively XY coordinates to latitude and to longitude.

This part of my work development is working fine but I would like to be able to convert XY coordinates to Lat, Lon coordinates without the use of this DLL.

I wrote a script which output using DLL calls, for each installed landscape, `max_x` and `max_y` and have coordinates (lat, lon) of the 4 corners.

``````    PosX    PosY    Lat         Lon
0   0       0       43,22330093 7,807540417
1   207360  0       43,1682663  5,257740498
2   0       345600  46,33418655 7,741717339
3   207360  345600  46,272892   5,0517416
``````

I also did a script which output this kind of plot (for 20x20 regularly xy spaced points)

a scatter plot with 40x40 points

CSV data can be found here

I wonder how I can know what kind of projection was done, what geodetic datum was "probably" used. I also would like to know what parameters for this projection should be set to be able to calculate myself Lat, Lon without DLL calls but with the use of a Python projection library such as pyproj or GDAL... )

• Really good question. Do you need a projection though? A projection is a method of transforming the surface of a sphere (or nearly) to a flat surface. What you're describing seems to be a rendering of the sphere itself, suggesting a projection is not required?
– Phil
Apr 20, 2015 at 8:10
• I guess the easiest way would be to contact the Condor developers and ask them about the conversion algorithms. Otherwise you could check if you could somehow decompile the DLL. Maybe it includes just simple math formulas... Apr 20, 2015 at 11:26
• I ever contact Condor support but I didn't get any reply. In my mind it's a projection as it's a method which "transform the surface of a sphere (or nearly) to a flat surface". More exactly here it's a reverse projection as x and y are given. I need to reverse parameters of this reverse projection.
– scls
Apr 20, 2015 at 13:12
• There's a rotation involved which complicates matters. Apr 20, 2015 at 16:13