Slope results in values close to 0 where 0 is expected

[OliverSchmitz]

areas at borders of a map with values of e.g. 0.1225, 0.17, 0.055 or 0.0925 result in slope values such as 7.45e-10 instead of 0

[kordejong]

This only occurs when the focus cell with the above value is located in the first or last row or column, and is surrounded by cells with the same value.