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

Gerald Evenden gerald.evenden at verizon.net
Thu Nov 28 13:15:31 EST 2002

```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

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