[geos-devel] WKT Precision

Wed Jan 6 09:20:30 PST 2021

Right, I have now seen that std::setprecision switches to scinot if the
precision is less than the magnitude of the number.  Very much not ideal
IMO.  So some way of using std::fixed might be needed to solve this. (Not a
problem if decimalPlaces is the default 16 though, I think - numbers >
10^16 will still be in scinot, but that should be rare).

So agreed, your PR seems like the right direction.  Does it work with
negative numbers and numbers << 1 ?

On Wed, Jan 6, 2021 at 9:01 AM Paul Ramsey <pramsey at cleverelephant.ca>
wrote:

> For interests sake here's a little program that shows the difference
> between std::fixed and the default aka std::defaultfloat. Note that
> defaultfloat (which sort of does "what we want" from a trailing zero point
> of view), also does sigfigs when restricted to a particular precision and
> scientific notation.
>
> I think if changes are to be made, I like the idea of doing
> - default, take the C++ defaults, don't apply anything. this is a change
> to current behaviour, which applies a default precision
> - if precision is specified, try to do a trimmed, fixed number of decimals
> output, which is kind of what my PR does
> P.
>
>
>
> > On Jan 6, 2021, at 7:54 AM, Martin Davis <mtnclimb at gmail.com> wrote:
> >
> > Well, yes.  The current default behaviour seems really unpleasant:
> >
> > POINT (-0.4225977 46.3406448). ==>. POINT (-0.4225977000000000
> 46.3406447999999997)
> >
>
> > bin/geosop -a "Point (-0.4225977 46.3406448)" -f wkt reducePrecision 100
> > POINT (-0.4200000000000000 46.3400000000000034)
> >
> > I agree with Andrew Bell - there is no way GEOS should be trying to
> outsmart the C++ language.  And add to that, that setting output precision
> is a perilous hack, since rounding/truncating data pointwise can result in
> invalid topology.
> >
> > Not saying get rid of the setRoundingPrecision, since it's the user's
> decision. But the default should be to just output "full" precision (as
> decided by the standard floating-point output routines, which know about
> weird things like IEEE-754 guard digits). And forget about trimming, since
> the standard output seems to do that just fine.
> >
> >
> >
> >
> >
> > On Wed, Jan 6, 2021 at 7:43 AM Paul Ramsey <pramsey at cleverelephant.ca>
> wrote:
> > For all these reasons and the fact that the current behaviour has
> existed for a long time and is now baked into downstream (those tests in
> GeoSwift!!) I'm inclined to just do nothing.
> >
> > Any objections?
> >
> > P
> >
> > > On Jan 6, 2021, at 7:41 AM, Andrew Bell <andrew.bell.ia at gmail.com>
> wrote:
> > >
> > >
> > > 1) This fight really can't be won without implementing all the various
> things already provided for by a language like C and allowing users to make
> these choices for themselves.  GDAL, for example, has its own strange logic
> to do this kind of thing. It's ugly and it's not obvious to a user what's
> going to happen as it's not well-defined by any documentation.  Some users
> may want the full precision, and spending a bunch of time figuring out if
> .999997 is significant or not isn't really the role of a library like GEOS,
> IMO.  And for some values, scientific notation is what you need. This is
> why %g exists for printf in C.
> > >
> > > 2) If you're using a text file for your output, you really don't care
> about size, even if you say you do. Seems like time could be better spent
> elsewhere unless someone is paying for this functionality.  Someone could
> certainly reprocess any WKT file to remove digits if they so chose.
> > >
> > > On Wed, Jan 6, 2021 at 10:25 AM Martin Davis <mtnclimb at gmail.com>
> wrote:
> > > Is it possible the problem is the use of std:fixed ?  (Which is
> invoked if the trim option = FALSE, which is the default).
> > >
> > > Currently in WKTWriter.writeNumber there is this code (and the
> defaults invoke fixed precision):
> > >
> > > if(! trim) {
> > >         ss << std::fixed;
> > >     }
> > >     ss << std::setprecision(decimalPlaces >= 0 ? decimalPlaces : 0) <<
> d;
> > >
> > > This results in the following (as noted on the GeoSwift issue)
> > >
> > > POINT (-0.4225977 46.3406448). ==>. POINT (-0.4225977000000000
> 46.3406447999999997)
> > >
> > > This carries too much precision, obviously.  I think it might be
> exposing the IEEE-754 guard digits unnecessarily.  FP output is notoriously
> tricky, and I suspect it's better to let C++ just do the right thing.
> > >
> > > Also, running reducePrecision causes problems, again I suspect due to
> to imprecise FP representation:
> > >
> > > bin/geosop -a "Point (-0.4225977 46.3406448)" -f wkt reducePrecision
> 100
> > > POINT (-0.4200000000000000 46.3400000000000034)
> > >
> > > If the std::fixed setting is dropped, the output looks more reasonable:
> > >
> > > POINT (-0.4225977 46.3406448). ==>. POINT (-0.4225977234 46.3406448)
> > >
> > > Check that all input sig digits are shown:
> > >
> > > POINT (-0.4225977234 46.3406448) ==> POINT (-0.4225977234 46.3406448)
> > >
> > > Reduced precision displays as expected:
> > > bin/geosop -a "Point (-0.4225977 46.3406448)" -f wkt reducePrecision
> 100
> > > POINT (-0.42 46.34)
> > >
> > >
> > > Is the "trim" option needed at all?
> > >
> > > On Tue, Jan 5, 2021 at 3:41 PM Paul Ramsey <pramsey at cleverelephant.ca>
> wrote:
> > >
> > > What do people think is the best practice for outputing WKT precision?
> > > _______________________________________________
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> > > geos-devel at lists.osgeo.org
> > > https://lists.osgeo.org/mailman/listinfo/geos-devel
> > >
> > >
> > > --
> > > Andrew Bell
> > > andrew.bell.ia at gmail.com
> > > _______________________________________________
> > > geos-devel mailing list
> > > geos-devel at lists.osgeo.org
> > > https://lists.osgeo.org/mailman/listinfo/geos-devel
> >
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