Measure the distance to the object on the map. You also may estimate the distance. However, for accurate results, use a map to find the precise size of the object.
Use a compass that has a clinometer to determine the size angle of the object. The clinometer looks like a viewport with a string and markings for degrees on the side. Look through the clinometer at the object to see how many degrees are taken up by it.
Find the object's size using the formula as follows, object size equals angular size by 2 multiplied by 3.14 multiplied by distance on the map to the object. So, if the distance to tree is 500 feet and the angular size is 6 degrees, you would have:
Six divided by 360 times 2 times 3.14 times 500 feet to get an object size of 52 feet in height.