Cheers, Hannes
Am 28.09.22 um 14:42 schrieb Johannes Kröger (WhereGroup) via QGIS-Developer:
Hi! I am in QGIS 3.22 in an empty project set to EPSG:3035. Considering the following Python code run via the script editor: >>> point_a = QgsPointXY(7.824, 48) >>> point_b = QgsPointXY(7.829, 47.98) >>> da = QgsDistanceArea() >>> print(da.willUseEllipsoid()) False >>> print(da.ellipsoid()) NONE >>> measure = da.measureLine(point_a, point_b) >>> print(measure) 0.02061552812809131>>> meters = da.convertLengthMeasurement(measure, QgsUnitTypes.DistanceMeters)>>> print(meters) 2294.910093653533I was wondering how it was able to turn the seemingly unit-less 0.0206... into a distance in meters and checked the docs. The documentation https://qgis.org/pyqgis/master/core/QgsDistanceArea.html#qgis.core.QgsDistanceArea.lengthUnits says:> Usually, the measurements returned by QgsDistanceArea are in meters. If no valid ellipsoid is set, then the units may not be meters. The units can be retrieved by calling lengthUnits() and areaUnits().As it said "usually" and I didn't do anything special on purpose, I expected this to return a distance measurement in meters but instead I apparently got a distance in degrees (well, some cartesian measure between my points as a float) which I could *convert* to meters. I have no valid ellipsoid set so as the docs say, this could have happened ("may not be meters").Checking the .lengthUnits() as suggested confirms that my QgsDistanceArea somehow decided to consider my coordinate values, and thus the measurement, to be in degrees:>>> da.lengthUnits() == QgsUnitTypes.DistanceDegrees True But why? I find the language very unclear and confusing there.Do "usually" and "may" mean that this is undefined, random behaviour unless one explicitly sets an ellipsoid? If so, should the docs strongly suggest to always use an ellipsoid so that there is no "may"? Do those words express the doc writer's vision of how the class what usually be used?If so, should the docs suggest a specific setup of the class? Why did it consider my point coordinates to be in degrees?How did it convert the degrees to meters if it supposedly does not use an ellipsoid?8) Cheers, Hannes _______________________________________________ QGIS-Developer mailing list QGIS-Developer@lists.osgeo.org List info: https://lists.osgeo.org/mailman/listinfo/qgis-developer Unsubscribe: https://lists.osgeo.org/mailman/listinfo/qgis-developer
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