Slope angle, be it an incline or a decline. Three typical ways to measure it: (1) The difference between the ground or horizontal, the ‘grade line’ and the incline or decline stated in degrees, or grads. (2) Ratio from dividing the vertical distance by the actual distance along the slope. This is the sine of the slope angle. A gradient of 1 in 4 is a vertical distance of 1 unit over 4 units slope distance. It is expressed as 1/4 grade or 25 percent grade. (3) Ratio of vertical distance, incline or decline, over horizontal distance. This is the slope angle’s tangent.