Hi Melita Kennedy - sorry to bother again on this matter 😕
I have tried a little more and I am close to be happy. The case is to make local reference systems on smaller airports (one runway). So I use the ARP (Airport Reference Point) as my origo and wish to use the centreline (CL) of the runway as my North/Y. Grabbing lon/lat for the start and end-points to calculate the azimuth needed. A perfect result would be to have a constant Y for the CL in the new reference system - but for the 2 tests I have done I always end up with a difference in Y coordinates between start- and stop of the runway CL in the range 5-12 cm over a runway typically 1 - 2 km long.
I guess this would be related to the azimuth being a little off. So my question would be which of the possible azimuths should be calculated and (as a bonus) if You have a preferred formula.
best
pH