[PROJ] Strange values with NAVD88

[Greg Troxel]

Stepping back, some complexities, some of which need to addressed with
EPSG (once someone(tm) has dug in enough to be sure what to ask for).  I
realize some of this has been discussed before; hoping it is helpful for
Javier.

  NADCON5 would be a big help.

  EPSG database currently seems to not include a number of NAD83ish
  things, including the notion that you can convert among any NAD83
  flavor with a null transform, and this conversion is better than any
  transform involving WGS84.

  The current NAD83 label is not obviously NAD83(1986), at least to
  people who are not already well read in NAD83.

  NAD83 isn't listed as a datum ensemble.

  It's not clear how a datum ensemble should be handled when there are
  2D and 3D CRSs in the same family.  Perhaps two of them, one 2D and
  one 3D.  Maybe NAD83 is the only ensemble that isn't all the same
  kind.

  Perhaps proj should somehow have a validity flag for the third
  coordinate and throw errors if someone provides coordinates with the
  third value present as an input for a CRS which is 2D.

  As discussed before, accuracy values for transforms in EPSG are
  imported into EPSG at face value, and thus cannot be compared against
  each other.  Doing this right is hard, but seems necessary for proj's
  transform-search approach to be sound.

  I realize proj doesn't really want to go down this path, but it would
  be nice to be able to have proj add transforms/etc. locally that
  aren't in EPSG but should be.  There's a "too much work" aspect, and
  then there's perhaps a licensing issue, perhaps not.


I will try to understand the "NAD83 in EPSG" situation once I get caught
up on proj updates in pkgsrc.  That's probably the best way to get
improvement for hours spent as a first step.

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