[GRASS5] A problem with the proj library?

Eric G. Miller egm2 at jps.net
Sat Feb 9 21:13:38 EST 2002

On Sat, 9 Feb 2002 09:10:14 -0700, Roger Miller <rgrmill at rt66.com> wrote:

> Also, the input UTM coordinates in this case are supposed to be significant 
> to about 8 digits.  The surveyors actually quoted the northings to 9 digits.  
> The conversion constant of 0.3048000000 that GRASS stuck in my PROJ_UNITS 
> files would introduce a usually unacceptable error in the meters->feet 
> conversion.  We probably need a more precise constant built into the source.  
> I think 0.3048006096012 is about right.

If we're going to be precise, then we'd need to differentiate which type
of foot we're talking about:

  foot_type       to_meters
Survey Foot   |     (1200/3937) (derived from: 1 m = 39.37 in)
  Int. Foot   |     0.3048      (derived from: 1 yd. = 0.9144 m)

1 Int. Foot = 0.999 998 Survey Foot

True, though, that for geodetic measures, it is usually the old foot, or
survey foot, that is meant.

Eric G. Miller <egm2 at jps.net>

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