[geos-devel] GEOS returning invalid geometries

[Carl Anderson]

On 09/17/2004 03:47:35 PM, Martin Davis wrote:
>  I wonder if this is due to floating-point implementation issues?
>  Being
>  build on Java, JTS has the luxury of being able to assume that all
>  floating point is carried out in IEEE754.  GEOS on the other hand is
>  dependent on the underlying compiler (and presumably processor FPU).
>

There is more than one issue
First
   WKT in postgis uses 15 digits of precision
           %.15g in postgis_inout.c
   WKT in GEOS uses 16 digits,
           MaxSignificantDigits in PrecisionModel.cpp defaults to 16
    int decimalPlaces = precisionModel->getMaximumSignificantDigits();
        string fmt="%.";
        char buffer[255];
        sprintf(buffer,"%i",decimalPlaces);
        fmt.append(buffer);
        fmt.append("f");

The conversion can turn a valid topology into an invalid one

There is another out there as well, so I will start where you suggest.

>  I can't think of any explicit dependencies in JTS/GEOS which would
>  cause
>  this, but if another floating point model is used it's possible that
>  some computations could come out slightly differently.
>

C.

Carl Anderson