[Proj] Custom nadgrid and .lla format again

Michal Seidl michal.seidl at gmail.com
Sun Apr 20 14:15:29 EST 2014

1) I spent some time testing and studying proj code and this is my
conclusion about *.lla format that is used by nad2bin

The data are organized:
<line number>: <dx> <dy> <ddx> <ddy> .....

<ddx> means difference between current dx and dx at previous position(left).
So to get dx at some position needs get dx from begging of line plus sum of

Can someone confirm or explain?

2) As I am playing with nadgrids another question rise up. There is just one
nadgrids file. In what coordinate system it should be? In WGS84 LatLong or
Local LatLong? If you go from local system to WGS84 how can you interpolate
in gird tha is in WGS84 system?
   a) some iteration or other clever algorithm
   b) do not care, correction should not change so dramatically

Probably the same question applies to geoidgrids?

Best regards Michal

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