Green roof detection

Extraction of green roofs based on digital orthophotos.


On behalf of the Ruhr Regional Association, mundialis developed a remote sensing-based detection method to automatically detect and classify green roofs based on Digital Orthophotos (DOPs) and 2.5D point clouds. The process should identify both the buildings with green roofs and the specific m² area and percentage of the roof that is green.


First, an NDVI raster was generated from digital orthophotos (DOP) (target resolution 0.5 m). Analogously, a Digital Surface Model (DOM) was first generated from the point clouds and then an nDSM was generated using the NRW-DEM (target resolution 0.5 m). The study area was limited to house perimeters (the source of the house perimeters is variable in principle, here the OpenNRW perimeters were used as a starting point, the buildings newly detected in the previous project can also be used).

Based on this, a region growing segmentation was calculated based on the NDVI and nDSM grids. Statistics (mean, range, standard deviation, etc.) of these grids were assigned to the created objects.
An NDVI threshold was then determined, which can be used to extract candidate vegetation objects on roof surfaces. This threshold can be set manually or selected automatically from vegetation covered areas (from land use mapping – FNK). The vegetation objects were then compared to the NDVI / nDSM statistics of the surrounding roof area to minimize false detections. The remaining vegetation objects were intersected with the house perimeters and the latter were assigned the binary attribute greened (yes / no).

Optionally, a user-defined threshold can be used to set a minimum percentage of green roof area per building. The analysis was conducted for both spring and summer aerial surveys, as well as the combination of both time points, to derive recommendations for appropriate input data.

Ruhr Regional Association


A green roof creates a more comfortable building climate and helps save energy costs for heating or air conditioning. The vegetation also protects the roof waterproofing from the weather, so green roofs last up to twice as long as conventional flat roofs. In addition, green roofs store rainwater, which then evaporates more slowly, and support biodiversity by providing habitat for insects, which is scarce in urban areas. For these and other ecological reasons, green roofs are all the rage these days.