very flat 15 mile ptp link.. .how do you measure how high the tallest tree is?

I have access to an antenna on one end, right now I have mikrotik dynadish 6's and I still can't seem to get a signal. (I'm about 85 ft up on one end and 40ft up on the other, According to the ubiquity and mikrotik link calculators about 85FT up should do it, but I'm pretty sure there are trees taller than that in the middle, Is there any map type that will show the tallest trees/things for a given area? If the frezel zone calc is say 60ft and there is a 60ft tall tree in the middle of a 20 mile ptp would you go up another 60ft on both ends? I've done 8 or 9 ptp links with no issues, I"ve never done one that I couldn't see with the naked eye and its sorta breaking my brain.... tips?

Thanks in advance.