How to write a script to distance meassurment in a terrain

Hello,

i´m writing my beachlor thesis with unity. I have a 3D Model of a golf-place as a terrain. Now i have some problems. I´m programming in java script. My question is : ,,How i must write a code for a distance meassurment from my position at the terrain to a balk (maybe a tree or a banner)." I have a lot of trees created with the tree creator in the terrain options. SO the trees are no single objects (?) The next step is the including of live GPS-coordinates and take some meassurments in reality at the golf place.. How i should do this? Can somebody help me please?

Greetings Filip