[Proj] HARN/HPGN

[Greg Troxel]

  So my question is basically - is there really a distinct HARN datum for
  each state?  Does it make sense to treat HARN as a datum, or should it be
  treated as ~52 datums?

I think the answer is "in theory, each HARN project is a more accurate
realization of NAD83, but in practice, each state network's coordinates
have a different relationship to CORS/ITRFxx/etc.".  But I don't have a
good basis for claiming that.

Surprisingly little of the information on the web addressses this point,
but this paper is interesting.  It talks about minor misalignments
between HARNs of different states (on the order of 10 cm), due to
choices of what data to hold constrained during adjustments:

http://www.ngs.noaa.gov/PUBS_LIB/HARN_CORS_COMP/eval_harn_to_cors.html
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