[OSRS-PROJ] Significant digits in parameters

Frank Warmerdam warmerdam at pobox.com
Mon Jul 29 08:53:54 EDT 2002

Craig Bruce wrote:
> Paul Ramsey <pramsey at refractions.net> pointed out that the EPSG, et al.
> coordinate-system definitions used in PROJ.4 look like the following:
> +proj=lcc +lat_1=47.500000000 +lat_2=48.733333333 +lat_0=47.000000000
> +lon_0=-120.833333333 +x_0=609601.219 +y_0=0.000 +ellps=clrk66
> +datum=NAD27 +to_meter=+0.3048006096
> A significant issue here is that all of the numbers appear to be given
> with a fixed format of only nine decimal places (C "%.9f").  I strongly
> recommend that a floating format of 16 significant digits be used instead
> (C "%.16g") because most computations are performed using 'double'
> values and sixteen significant digits is the (close-decimal-equivalent)
> accuracy of this type.  If computations are carried out with only nine
> or so significant digits, then the calculations will be unnecessarily
> imprecise, and this can lead to problems with code that deals with
> floating-point rounding control.


Without taking a position in the "precision" discussions I agree to review
the OGR code used to generate the nad/epsg file to pass through numbers in a
form closer to their original values.  It was not my intention to alter the
EPSG numbers in conversion to the nad/epsg initialization file.

I have created bug 185 to address this issue:


As implied by his and other issues raised, the nad/epsg file is intended to
be helpful, but is incomplete and in at least some cases inaccurate.  Hopefully
through use we can evolve it to a reasonable accurate representation of the
underlying EPSG coordinate systems.

Best regards,
I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
light and sound - activate the windows | http://pobox.com/~warmerdam
and watch the world go round - Rush    | Geospatial Programmer for Rent

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