[Proj] Re: how to compare 2 proj4 definitions? or: how to normalizeproj4 definition?

Richard Greenwood richard.greenwood at gmail.com
Thu Mar 1 22:24:01 EST 2007

On 3/1/07, Ed McNierney <ed at topozone.com> wrote:
> Sergey -
> I never generated any code - I simply worked on trying to define the
> problem.  The problem with a user-configurable "epsilon" value is
> whether that is what the creator of the definition intended.  We can
> provide tools for the user to adjust, but it's hard for the user to
> determine whether they did the right thing or simply succeeded in
> forcing two DIFFERENT definitions together.  But having some kind of
> tolerance value will certainly be needed if you want to consider 3.28
> and 3.281 as both being the "same" multiplier when converting meters to
> feet.

Seems like what would be better would be to compare the results of a
transformation using the two different sets of parameters that you are
comparing. For example, if you wanted to determine if two definitions
were identical (or similar enough) you would transform the same point
coordinates to longlat wgs84 using the two definitions, and then
compare the results to determine if they were close enough to meet
your needs. Obviously you would need to pick a test point that was
within the realm of the defined coordinate system, and ideally you
would pick  several points.

What's close enough varies wildly between different users needs.


Richard Greenwood
richard.greenwood at gmail.com

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