1. Standard Slope Functionb c d e f g h i

3. - Direction of Steepest Descent
Hydrologic Slope
- Direction of Steepest Descent 30 30 67 56 49 52 48 37 58 55 22 67 56 49 52 48 37 58 55 22
Slope:

4. (ii) The 8 direction pour point model

5. The D Algorithm 
Tarboton, D. G., (1997), "A New Method for the Determination of Flow Directions and Contributing Areas in Grid Digital Elevation Models," Water Resources Research, 33(2): ) (

6. (iii) The D method
This does not fit in the triangle so the angle chosen is along the diagonal with slope same as D8, i.e.