Unfortunately I've had some problems using the available GIS plugins that convert coordinates from the local grasshopper coordinate system to WGS84. Digging into the topic, I found that for my needs, a simple conversion using an "orthographic map projection" would be enough. The precision should be good enough for any project smaller than 1 square km.
If you want to hand over your local coordinates in true north to some planner using UTM grid north (https://www.google.com/search?q=grid+north&tbs=imgo:1&udm=2) this little definition might save your day ;)
For some reason, the Rhino Earth Anchor Point didn't work reliably in the scripts I used, maybe because the Plugins were made for GH5-7 or so... You do need to provide these definitions with an Earth Anchor Point with a known WGS84 coordinate, but directly inside GH. Not using the RhinoEAP here.
BEWARE: the Earth Anchor Point in this definition will always be at {0,0,0}. I might extend the capability of this in the future (or you can help?), but for now I neither have the time, not the need for that.
So there you go, the formulas from Wikipedia (https://en.wikipedia.org/wiki/Orthographic_map_projection) are implemented in these simple definition using off the shelf GH components, no magic. You can edit the clusters as you like.
Please do not expect any support from me, since I'm no GIS expert.
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