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There is no clear height requirement for such a type of easement. Every easement is different.
The first thing that the parties would do, would be to look to the easement's verbiage. If the easement's verbiage is clear as to the height, it controls. If it is not and the parties cannot agree, then either party can file a QUIET TITLE suit to get the Court to weigh in. The Court would then enter a ruling based on what the Judge/Jury believes (1) the intent of the parties, and, (2) what is reasonable necessary to fulfill the reason for the easement (as you can tell, highly subjective).
The servient and dominant party (servient - owner of land, dominant - owner of easement) are also free to enter into a voluntary agreement to better define the easement's parameters, if needed. The quiet title option is only if the parties cannot agree.
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