Polygons
The requirements for polygons are the following:
- Polygons must be in GeoJSON format
- Individual polygons should be valid (Section 2 of this publication). You might get warning messages and bad results with invalid polygons (garbage in-garbage out)
- Building footprints have to form a planar partition, i.e. they should not overlap
In case your polygons do not meet the criteria, we suggest you use QGIS for preparation. It is open-source and contains most functionalities you require to prepare the data. Alternatively, you can look into prepair and pprepair for repair of non-valid polygons and planar partitions, respectively.
You might have to adjust the Coordinate Reference System (CRS) to get a better representation of your polygons. Especially in the case when a point cloud is present, you should compare whether polygons and the point cloud are using the same CRS.
Below you can find some information on handling polygons in QGIS. If you are just getting started, check some of the beginner tutorials.
Polygon splitting can be used to aid reconstruction when there are considerable height differences in a single polygon. Take for example this building, its polygon, and the LoD1.2 reconstruction of the building.
One polygon contains the whole building footprint, hence LoD1.2 reconstruction will result in a model that is not accurate. By splitting the polygon, you can get a better representation of reality.
Polygon merging can be beneficial when using flat polygons, e.g. water. Merging polygons ensures there is no abrupt jump in height at their border
QGIS contains several simplification algorithms you can use for footprint simplification. Check the figure below on where to find them.
You can use create polygon tool in QGIS in case you would like to manually define influence region/domain extent. First, you need to create a new layer by selecting Layer → Create New Layer → New Shapefile Layer and selecting Polygon as the Geometry type.
To confirm your selection, click the right mouse button. In the end, you can just export the polygon to GeoJSON format. Make sure to export influence region/domain boundary polygons in the same CRS as the buildings and the point cloud. The same exporting is used when changing to GeoJSON format from other GIS formats.
