Military utility tools / Intervisibility and dead ground
Intervisibility and dead ground
Observer, crest and target heights in metres above mean sea level. Visibility, clearance and dead ground out.
Set the inputs, then work it out.
Result--over the crest
Clearance--m at the crest
Dead ground--m beyond the crest
Dead ground ends--m from observer
Minimum target height--m to be seen
Curvature allowance--m at the crest
Depression to target--mils
Grazing angle--mils over crest
What is dead ground?
The stretch of ground beyond a crest that the observer cannot see, because the crest masks it. It runs from the crest out to where the grazing line of sight meets the ground again, and it is where anything can move unobserved.
Why does earth curvature appear at short ranges?
It is small but not zero. At two kilometres the allowance is a few centimetres; by ten it is metres. The four thirds earth model also accounts for atmospheric refraction, which bends the sightline slightly downward and partly offsets the curve.