[OSRS-PROJ] Pseudo X/Y values, forward and reverse

Duncan Agnew agnew at bilby.ucsd.edu
Fri Jan 31 16:22:15 EST 2003

For the interested onlookers, perhaps the original poster (Mr. Hersman)
could explain why one would want to get (eg) Alaska SPCS Zone 4 coordinates
when in Zone 10? Since the grid will be very distorted, it's not as though
you can use the X&Y to get distances or direction--if you want to get these,
why not do what is usual over large distances: use lat and long, and solve
for the geodesic?

As far as the software goes, it should probably function so that the direct
and inverse do in fact invert each other to mm precision, to avoid confusion
--but this is not the same as their being "right" for these large distances
(that is, agreeing with an exact expression for the TM).  Does being right
matter, and if so, why? To forestall the obvious answer (which is, to agree
with other, exact, computations), I would ask, who makes such computations?
Given the rarity of TM-5-241-10, not many people, it would appear.

BTW, the Snyder's discussion of the TM equations in USGS PP 1395 clearly
states their limitations and error bounds, with references to where the
more exact formulae can be found (namely, TM-5-241-10).

Duncan Agnew
dagnew at ucsd.edu

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