## Description

This tool implements a modified version of the Delta Surface Fill method of Grohman et al. (2006). It can fill voids (i.e., data holes) contained within a digital elevation model (`dem`) by fusing the data with a second DEM (`fill`) that defines the topographic surface within the void areas. The two surfaces are fused seamlessly so that the transition from the source and fill surfaces is undetectable. The fill surface need not have the same resolution as the source DEM.

The algorithm works by computing a DEM-of-difference (DoD) for each valid grid cell in the source DEM that also has a valid elevation in the corresponding location within the fill DEM. This difference surface is then used to define offsets within the near void-edge locations. The fill surface elevations are then combined with interpolated offsets, with the interpolation based on near-edge offsets, and used to define a new surface within the void areas of the source DEM in such a way that the data transitions seamlessly from the source data source to the fill data. The image below provides an example of this method.

The user must specify the `mean_plane_dist` parameter, which defines the distance (measured in grid cells) within a void area from the void's edge. Grid cells within larger voids that are beyond this distance from their edges have their vertical offsets, needed during the fusion of the DEMs, set to the mean offset for all grid cells that have both valid source and fill elevations. Void cells that are nearer their void edges have vertical offsets that are interpolated based on nearby offset values (i.e., the DEM of difference). The interpolation uses inverse-distance weighted (IDW) scheme, with a user-specified weight parameter (`weight_value`).

The `edge_treatment` parameter describes how the data fusion operates at the edges of voids, i.e., the first line of grid cells for which there are both source and fill elevation values. This parameter has values of "use DEM", "use Fill", and "average". Grohman et al. (2006) state that sometimes, due to a weakened signal within these marginal locations between the area of valid data and voids, the estimated elevation values are inaccurate. When this is the case, it is best to use fill elevations in the transitional areas. If this isn't the case, the "use DEM" is the better option. A compromise between the two options is to average the two elevation sources.

## References

Grohman, G., Kroenung, G. and Strebeck, J., 2006. Filling SRTM voids: The delta surface fill method. Photogrammetric Engineering and Remote Sensing, 72(3), pp.213-216.

## Function Signature

`def dem_void_filling(self, dem: Raster, fill: Raster, mean_plane_dist: int = 20, edge_treatment: str = "dem", weight_value: float = 2.0) -> Raster: ...`

## Project Links

WbW Homepage User Manual Support WbW