What angle of depression means
On Omni's construction page, the angle of depression is defined as the angle between the horizontal and a line of sight directed downward. In practice, it is the same triangle relationship used for terrain grades and right-triangle surveying.
Formula used by the fetched source
angle of depression = arctan(vertical distance / horizontal distance)
line distance = vertical / sin(angle)
line distance = horizontal / cos(angle)
Omni's sample check
The source article uses a 1.5 m vertical difference and a 3.0 m horizontal distance. Plugging those into arctan gives an angle of about 26.565 degrees.