[OSRS-PROJ] Significant digits in parameters

[Craig Bruce]

Gerald Evenden <gerald.evenden at verizon.net> wrote:

> Using a larger number of digits in projection parameters will, for a
> variety of reasons, not increase the accuracy of the results For example,
> the major axis is only spec'd to a 7 digit integer, thus the accuracy
> of the x-y values are also only to the nearest meter regardless of the
> significant digits of the other parameters.

I realize that what you are saying is geomatically correct, but talking
about software issues.  It's advantageous if different software or even the
same software produces results that are either the same or very close to
being the same for the same input.  For this reason, the best plan is to
represent all parameters and calculations using full 'double' precision,
even if it contains many more digits than are geomatically significant.
If the rest of the system computes values using full double precision,
why would you want to unnecessarily remove consistency with other systems
especially when the solution is as simple as changing the "%.9f" to a
"%.16g" when auto-generating the tables.

There are many places where round-off error control is applied and
full precision allows programs to produce the same results (e.g., in
error-controlling the translation of projection coordinates to integer
pixel coordinates) and to make optimizations when computed coordinates
align properly.  A lot of software doing error control may not even know
that it is dealing with ground coordinates, so it may assume that there
are 12-15 significant digits (or "consistent digits" in our case).

--------------------------+------------------------+--------------------------
Dr. Craig S. Bruce        | Tel: 819-771-8303 x205 |             CubeWerx Inc.
Senior Software Developer |   Home: 613-565-1344   |  Gatineau, Québec, Canada
csbruce at cubewerx.com      | http://www.csbruce.com |  http://www.cubewerx.com/
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    "Everything should be built from the top down, except the first time."
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