GML: gml:CircleByCenterPoint is converted to WKT which is no longer compliant

[heidivanparys]

What is the bug?

When reading a GML file containing a gml:CircleByCenterPoint geometry, the geometry is reported as a CURVEPOLYGON with a CIRCULARSTRING, consisting of three points, the first point being equal to the third point.

This is compliant with earlier editions of the SQL/MM standard (ISO/IEC 13249-3), as e.g. documented in the PostGIS documentation, https://postgis.net/docs/manual-3.5/using_postgis_dbmanagement.html#CircularString.

In ISO/IEC 13249-3:2016, the current version of the standard, an ST_CircularCurve with equal segment start and end point is no longer permitted, Instead, type ST_Circle is introduced to deal with circles. This is described in ISO/IEC 13249-3:2016 Annex D Incompatibilities with ISO/IEC 13249-3:2011.

The WKT reported when converting a gml:CircleByCenterPoint geometry, will fail in e.g.

• Oracle, with ORA-13346: the coordinates defining an arc are collinear
• SQL server see https://learn.microsoft.com/en-us/sql/relational-databases/spatial/circularstring?view=sql-server-ver16

It would be better if a gml:CircleByCenterPoint would be converted to a CURVEPOLYGON with an CIRCULARSTRING defined by 5 points. That encoding will be recognized by more databases and other tools, hence increasing interoperability.

I noticed this issue with the WKT encoding of circles when reading GML. The underlying problem as described above possibly occurs in other drivers, I did not investigate that.

Steps to reproduce the issue

The following geometry:

<gml:Polygon srsName="http://www.opengis.net/def/crs/EPSG/0/25832"
             srsDimension="2">
    <gml:exterior>
        <gml:Ring>
            <gml:curveMember>
                <gml:Curve>
                    <gml:segments>
                        <gml:CircleByCenterPoint numArc="1">
                            <gml:pos>1 2</gml:pos>
                            <gml:radius uom="m">2</gml:radius>
                        </gml:CircleByCenterPoint>
                    </gml:segments>
                </gml:Curve>
            </gml:curveMember>
        </gml:Ring>
    </gml:exterior>
</gml:Polygon>

is reported as CURVEPOLYGON (CIRCULARSTRING (-1 2,3 2,-1 2)).

SELECT sdo_geom.VALIDATE_GEOMETRY_WITH_CONTEXT(sdo_util.FROM_WKTGEOMETRY('CURVEPOLYGON (CIRCULARSTRING (-1 2,3 2,-1 2))') , 0,005) FROM dual; fails with ORA-13346.

A better WKT representation would e.g. be CURVEPOLYGON (CIRCULARSTRING (-1 2,1 0,3 2,1 4,-1 2)).

Relevant code (I think):

gdal/ogr/gml2ogrgeometry.cpp

Lines 2042 to 2057 in abaccaa

	auto poCC = std::make_unique<OGRCircularString>();
	p.setX(dfCenterX - dfRadius);
	p.setY(dfCenterY);
	poCC->addPoint(&p);
	p.setX(dfCenterX + dfRadius);
	p.setY(dfCenterY);
	poCC->addPoint(&p);
	p.setX(dfCenterX - dfRadius);
	p.setY(dfCenterY);
	poCC->addPoint(&p);
	if (bInvertedAxisOrder)
	poCC->swapXY();
	return poCC;
	}

Versions and provenance

GDAL 3.10.1, released 2025/01/08, installed from OSGeo4W.

Additional context

No response