[Proj] Problems with Pittman geodesic??
Hamish
hamish_b at yahoo.com
Sun Jan 18 23:00:36 EST 2009
Gerald wrote:
> While checking out the accuracy of Vincenty vs. Pittman
> I was fairly consistently getting agreement to micron or
> better.
>
> BUT not always!!!
>
> I would appreciate anyone who has a Pittman geodesic routine
> to test the following inverse (WGS84 ellipsoid):
>
> point 1 at 0 lat, 0 lon and point 2 at 45N 90E.
>
> With my version of Pittman I get a distance of 9993541.5348708
> AND I get the same distance while changing the longitude of the
> second point from about 89.6 to a hair over 90.
>
> At 0,0/45,89.5:
> Pittman: 9970963.0100082
> Vincenty:9970963.01000801
>
> At 0,0/45,89.6
> Pittman: 9993541.5348708
> Vincenty:9978847.65947167
>
> Pittman remained constant in this longitude interval
>
> At 0,0/45,90
> Pittman: 9993541.5348708
> Vincenty:10010386.3610382
>
> Between 90.0000001 and 90.000001 Pittman finally came back
> into near agreement with Vincenty.
>
> Note: I did nothing to the Pittman FORTRAN loaned from Mugnier
> other than compile it with gfortran and link to a C driver.
I would guess that some internal variable is defined as a REAL (float)
instead of a DOUBLE PRECISION (double), and so the effect you are seeing
is quantization due to variable precision.
Hamish
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