Two of the most confused terms in mining nomenclature are 'overall slope angle' and 'interramp slope angle.' I have put together some pictures to help illustrate the difference between the two terms and when you would use each.
Overall slope angle refers to the angle, measured from horizontal, between the lowest toe point and the highest crest point inclusive of any ramps or additional step backs. If you look at a mountain it is easy to see that the overall slope of the terrain goes from the top (crest) to the bottom (toe) (I have included a picture of a mountain above in case you have never seen one before). In some places the local slope angle will be steeper than the overall slope and in other places it will be more shallow. In fact, in some places the line between the highest crest and lowest toe may pass through high spots in the slope.
The interramp slope angle is measured from toe to toe or crest to crest exclusive of any ramps (Slope Stability in ; W.A. Hustrulid p48. By excluding ramps or other offsets the interramp angle is kept static no matter how many benches are measured. The interramp angle is similar to the face angle because it is static. The figure below illustrates the terms overall slope angle, interramp slope angle and face angle as seen in a cross cut of a pit ramp.
By definition, the overall slope angle changes depending on how many benches are measured. The figure below illustrates this relationship. When measured over two benches the overall slope angle (blue) is much steeper than the overall slope angle measured over 5 benches (magenta). The overall slope angle becomes more shallow with each bench because each bench adds a fixed offset in addition to the offset due to the face angle.
The difference between overall slope angle and interramp slope angle was the topic of one of my first posts. You can see the original post at this link: Overall Slope Angle vs. Interramp Slope Angle