Describe in your own words (no equation) how a computing algorithm derives the slope of the center cell by using elevations of its eight immediate neighbors.
As an example, Horn's algorithm applies a weight of 2 to the 4 immediate neighbors and a weight of 1 to the 4 corner cells. In the 3-by-3 window, the 3 cells in the first column of 3 cells are added together, with the middle cell multiplied by 2; from this are subtracted cells in the third column, similarly summed; to this are added the cells of the bottom row, summed in the same way, minus the cells in the top row, summed in the same way; a square root is taken of the result and divided by 8 times the cell size.