Thanks for the reply. Unless I have misunderstood, your method would result in an interpolated slope that fills the gap. However, what I would like to fill the gap with a surface of specific parameters
The dataset is a section of a cliff, with a relatively straight section of road cut into it. The road surface is the area with no data. I would like to add in the road surface as a flat plane, with a slight slope. Therefore the resulting elevation will have a step at this point, rather than an interpolated slope.