If you are d metres away from the base of an object, and the top of the object is at an angle x from the horizontal, then the height of that object is given by:h = d*tan(x).
If you want to be more accurate, or if x is small, you should add the height of your eyes from the ground to the answer obtained from the above formula..
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By means of trigonometry if you know the angle of elevation or by comparing it with a nearby object if you know its height and shadow length.
Trigonometry
Neither, it is a mathematical discipline.
It is the reciprocal of the sine ratio.
If the triangle is right-angled then you should be able to use Pytharoras Theorem to calculate the missing side otherwise you need an angle to be able to use trigonometry