How to measure facade heights and roof dimensions in oblique imagery, how accurate that is and when you need another source.
To measure a facade height you determine two points: the bottom of the facade and the top, for example the roof edge, the gutter or the ridge. Because the imagery is georeferenced and linked to an elevation model or point cloud, every selected point is resolved to a coordinate rather than a pixel position. The difference between the two points gives the height. The same principle applies to widths and roof planes. If you have more than one image of the same situation, from different directions, you can cross-check the result.
Accuracy depends on the resolution of the image, which in an angled view is not equal everywhere: close to the camera a pixel is smaller than far away. The viewing angle, the quality of the image georeferencing and the elevation model also play a part. So does what you select: the foot of a facade is often hidden by a fence, cars or vegetation, and a gutter or roof edge is not always sharp. Ask the supplier for the specifications of the imagery and, when in doubt, check a measurement against a reference on site.
For facade heights and roof dimensions used for screening, pre-selection and assessment, an angled aerial photo gives usable values. Where millimetres count, or where a statutory measuring rule applies, for example for building height or eaves height in a permit, a ground survey or a point cloud remains the better source. In GEORIZON those sit next to the angled image, so you choose the right source per question.
