[OSRS-PROJ] Donald Elliptic Projection for AT&T V&H Coordinates?

Clifford J Mugnier cjmce at lsu.edu
Fri Nov 29 15:21:33 EST 2002

```I am opposed to making "improvements" to the AT&T V&H math model for their
projection implementation.  Almost a century ago, the American Coast &
Geodetic Survey made some "improvements" on French Battle Maps (WWI) that
supposedly were cast on an "inferior" implementation of the Lambert
Conformal Conic.  Rather than match what was already a fact, higher-order
terms were used to "improve" the original French version.

Nothing would fit the existing maps.  The new "improved" maps had to be
discarded because consistency and continuity were more important than
theoretically "correct."  For decades, American Geodesists were derided by
their European peers even though our Army won the war for them.

I say that if such a projection is implemented, the original should be done
first so that old data can match new data.  A second "improved" alternative
would be acceptable as long as the user was able to judge (for themself)
which is preferable for the task at hand.

Cliff Mugnier
LOUISIANA STATE UNIVERSITY

-----------------------------------------

Out of curiosity I down loaded the stuff from Openmap but I still have not
figured out where the projection math is for anything.  I have *no*
experience
with Java so this is not helping.

To call this a projection for the ellipsoid (not to be confused with the
alternate
name of Elliptic Projection) seems a bit specious.  It says that the
latitude
on the
sphere is obtained from the ellipsoid latitude---but which one: the
conformal
latitude or authalic latitude?  I'd guess the former.  Then the remainder
of
the
calculations are done in the spherical system.  The problem is, that great
circle
distances (required in the Two Point Equidistant) cannot be accurately
computed by merely computing the conformal latitude and using the
relatively
simple spherical formula.

If I were going to make this a part of PROJ I would use (proj=tpeqd) as
the basis and make an overlaying entry for Donald which assigns the
constants
for the two base points (in Kentucky and Utah) and fiddles with the inane
rectagular transformations as well as the ellipsoid/sphere latitude
conversions.
Much like utm calls tmerc except a little more hairy.

If anyone can tell me where the math is, please let me know.  Thanks.

Now back to fixing the turkey.   :-)

Y'all have a good Thanksgiving.

Duncan Agnew wrote:

> There is a description of this projection at
>
> http://openmap.bbn.com/doc/api/com/bbn/openmap/util/VHTransform.html
>
> (at the bottom of the page). The code used is open-source (it appears)
>
> The actual conversion code contained inside the Java is from other
sources,
> and it is clear that this is something that even AT&T had difficulty
> sorting out later.
>
> I leave it up to the more expert to add this to PROJ, and to reverse-
> engineer the ellipsoid used.
>
> Don't thank me, thank Google.
>
> Duncan Agnew

----------------------------------------
PROJ.4 Discussion List
See http://www.remotesensing.org/proj for subscription, unsubscription
and other information.

```