After reading the question again I realized my previous post visually showed the line of sight between the two points but did not actually answer the question, which is how to determine the line of sight distance. As noted by others there is a difference. The answer to the question is how do you determine the hypotenuse of a right triangle. The formula is A squared plus B squared equals C squared.
See the example below along with the solution to the bigger problem. Keep in mind that I am pretty sure that Google Earth Pro measuring tool measures horizontal distances only which gives one of the values to plug into the formula. Google Earth Pro elevation tool determines the distance in elevation between the two points for the second value to plug into the formula. If my math is correct (and I'm sure someone will let me know about it if I'm wrong) the difference between the horizontal distance and the line of sight distance is pretty small. Let's just say small enough to even satisfy Mr. Picky Pants. (just joking!!!)
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