QGIS » QGIS Application

Hi

I have noticed that short circular arc/curve geometries in QGIS gets wrong center and radius and so they are also drawn incorrectly.
As a result there can be seen big circles in many places in map data.

I made one example for you to see incorrect result.

I attached three CSV (WKT) files.
You can add them in QGIS separately to three layers with following parameters:

Files:
arc_endpoints_line.txt
arc_points_linestring.txt
arc_wrong_circle.txt

Separator: ";"
First row has attribute names
Geometry format: Well known text (WKT)
Geometry attribute: wkt_geom
Geometry type: lookup
Coordinate system: EPSG:3879 (ETRS89 / GK25FIN)

In levels window, put off level "arc_wrong_circle"
Put on levels "arc_endpoints_line" and "arc_points_linestring"
Zoom to full.
Put different colors for levels to identify them and zoom more near line centers.
See that "arc_endpoints_line" is on right side of "arc_points_linestring" which means that correct circle center should be on right side too.
Geometry "arc_wrong_circle" shows that QGIS calculates circle center on wrong side and it's radius is also wrong

I find one place in your code, that I would like to make better (I don't know if there are more functions of same kind).

void QgsGeometryUtils::circleCenterRadius
Definition at line 269 of file qgsgeometryutils.cpp

My replacement code is in attachment file QgsGeometryUtils_circleCenterRadius.txt

I have tested my code with Python in Safe Software FME environment and also in PostGIS environment with 17000 arcs inside different kind of geometries and it has looked good.

Our staff in City of Helsinki is using QGIS more and more, so it would be nice if someone has time to make this correction to code.

Sincerely yours

Timo Iipponen
GIS expert
City of Helsinki, Finland