The horizontal line at the foot of the slope refers to

The figure corresponding to the edge of the dihedral angle is set from the toe point a as follows.

Follow the slope 100 meters to point B.

Then AB= 100 crosses point B.

Dihedral dihedral

vertical line

The vertical foot is D.

because

Angle error = 30

Then BD= 100/2=50 passes through point B as the vertical line of the horizontal plane.

Look down on O.

Then OB is the required high connection OD.

The BDO angle is a plane with dihedral angle =60 degrees.

So OB=50*sin60=25* root number 3.