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How did you figure the head location is 100?
If the above is true, when you find the feet elevation on the right picture, you can add 100 to it to find the head elevation.
Maybe you can use a flying unit and set it's fly height to 0, and when it gets to the top of the terrain like in the right picture, you can use some sort of trigger to detect the current fly height.

xD this is for anachron's system, is not that simple =)

Create some random location like:

location LOCATION_Z = Location(0.0, 0.0)

Then use this custom function (c'n'p it into your map custom script or some library you use for these custom functions):

function GetUnitZ takes unit u returns real
call MoveLocation(LOCATION_Z, GetUnitX(u), GetUnitY(u))
return GetLocationZ(LOCATION_Z)
endfunction

This will return the height of the terrain (or the target unit's feet if you prefer).
To calculate the height of it's head, just add 100.0 as you did.

if this works you can make projectiles that ignore terrain height.

This is what I use to make chain spells, when calculating a unit's chest height.
Haven't tested it on anything else, but it should work well.