Find slowest possible height decay to specific value from the point of maximum height in a 3D surface

Dear matlab professionals,

I have a problem with which I'm fighting for quite some time. I have given a 3D surface in coordinates which has a height maximum in the center. From this location the height decays in variable rates depending on the direction.

What I'm searching for, is the location which is farthest away from the center, which is still part of the center height plateau and has the value of 20% of the maximum height.

So it must be possible to draw a line (must not be straight) from the center to that outer point without intersecting any points that possess a height below 20% of maximum height. If I just look for the farthest point with the height of 20% then I get a point which is part of a plateau of another local maximum. I want the point to be connected to the center plateau (height>20%maximum around center) though.

Is there any algorithm that can do this?? I am very grateful for any help.

kind regards, Mark