[Proj] Local Level

[Greg Troxel]

  Can I convert a point from geocentric to Local Level Projection (also called
  Local Tangent Plane) ?
  It's only Rotation and Shift from the geocentric representation.

I tried to do that earlier, and found that it was somewhat difficult.
First, wheter you are coming from ECEF XYZ (assuming that's what you
mean by geocentric) or geodetic shouldn't matter much.  In my case I
started with WGS84 geodetic coordinates.

I had a local coordinate system in a parking lot used for testing a
radio system, defined by survey nails.  One nail was the origin at 0,0,
and another nail defined the angle to 100,0.  But my 'easting' (first
coordinate) was close to north, but not quite.

I felt that my options were to define my local coordinate system as a
proj string or to convert to something planar and then transform.  I
didn't figure out the first option, so what I did was

  transform to UTM zone 19
  subtract UTM coordinates of my reference station (at (40,30) it so happens)
  rotate from MSP north to my grid y axis
  add back in my (40,30) reference station coordinates

This was not a lot of code, and it's here:

  http://acert.ir.bbn.com/viewvc/adroit/mrp/src/share/grid.py?revision=1.12&view=markup

Also, search for PROJ in:

  http://acert.ir.bbn.com/viewvc/acert-geospatial/xplot/xplot.c?revision=1.9&view=markup

for C code to take "ADROIT grid" and transform to Mass State Plane
(because that's what my orthophotos were in).


I think it would be nice for proj to be able to do these kinds of
transformations (offset/rotate/offset) from projected coordinates, but
representing is probably a bit messy.
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