Use this guide for CircleByCenterPoint workflows.
What a true GML circle is
A true GML circle is stored as a centre coordinate and radius rather than dozens of perimeter coordinates. EditPolygon represents it as CircleByCenterPoint. This keeps the curve exact while it remains a circle and is exported to GML.
Centre longitude/latitude, radius in metres and CRS metadata.
A temporary ring of vertices used only by formats or tools that cannot represent a curve.
1. Draw a true circle
- Choose Draw → Circle.
The Circle tool does not ask for fallback vertices during normal drawing.
- Click the centre.
This becomes the stored centre coordinate.
- Click the radius point.
The geographic distance from the centre becomes the radius in metres.
2. Select and edit the circle
Click the circle directly on the map, then choose Edit circle. Drag the blue centre handle to move it without changing radius. Drag the perimeter handle to resize it without changing centre. The Inspector also allows exact centre and radius entry.
3. Export as GML to preserve the curve
Choose GML in the export format. The output uses a curved GML representation with one centre position and one radius. It should not contain a LinearRing or perimeter posList for that circle.
<gml:CircleByCenterPoint numArc="1">
<gml:pos>153.02 -27.45</gml:pos>
<gml:radius uom="m">15000</gml:radius>
</gml:CircleByCenterPoint>4. Export to KML, KMZ, GeoJSON or another non-GML format
These formats do not store EditPolygon's true-circle model. The Export window displays an informational notice and creates a polygon approximation only in the downloaded copy. The circle in the project remains a true curve.
Choosing the vertex count
- Lower values create smaller, more visibly angular polygons.
- 72 is the standard default and is suitable for many map workflows.
- Higher values create a smoother approximation but increase file size and processing cost.
- The best value depends on circle radius, map scale and required positional tolerance.
5. Use geometry tools
Merge, split, cut, repair, simplify and other polygon-only tools require real boundary coordinates. Before running one of these operations, EditPolygon warns that the true circle will be permanently converted to an ordinary polygon for that project operation.
The conversion dialog defaults to 72 vertices and provides Advanced options. After conversion, the feature can use normal polygon tools and vertex editing, but it can no longer export as a true CircleByCenterPoint. Use Undo when the conversion or operation was not intended.
6. Test a GML round trip
- Export the circle as GML.
- Open the exported GML in a new or cleared project.
- Confirm the Inspector reports Circle and CircleByCenterPoint.
- Compare centre and radius.
Allow only normal floating-point differences.
Troubleshooting
The Inspector says Polygon with 72 vertices
That feature has already been polygonised, or it was created by an older build. Draw or import a new true circle after updating the application.
A GML export contains LinearRing and posList
The circle was converted to a polygon before export, or the export path did not retain the original circle model. Confirm the Inspector still reports CircleByCenterPoint.
I cannot merge a true circle immediately
Accept the explicit polygon conversion warning and choose the required vertex count, or cancel to preserve the true curve.
The non-GML circle looks angular
Increase Vertices per circle under Advanced export options and export again.
- Inspector reports CircleByCenterPoint.
- Centre and radius were reviewed.
- GML was chosen when the curve must be preserved.
- Non-GML approximation settings suit the map scale.
- Polygon conversion was accepted only when a geometry tool required it.
- Original true-circle project state was saved when needed.