system
Context: Height above average terrain (HAAT) (or less popularly, EHAAT, Effective Height Above Average Terrain) is a measure of how high an antenna site is above the surrounding landscape. HAAT is used extensively in FM radio and television, as it is more important than effective radiated power (ERP) in determining the range of broadcasts (VHF and UHF in particular, as they are line of sight transmissions). For international coordination, it is officially measured in meters, even by the Federal Communications Commission in the United States, as Canada and Mexico have extensive border zones where stations can be received on either side of the international boundaries. Stations that want to increase above a certain HAAT must reduce their power accordingly, based on the maximum distance their station class is allowed to cover (see List of North American broadcast station classes for more information on this). 

The FCC procedure to calculate HAAT is: from the proposed or actual antenna site, either 12 or 16 radials were drawn, and points at 2, 4, 6, 8, and radius along each radial were used. The entire radial graph could be rotated to achieve the best effect for the station. The altitude of the antenna site, minus the average altitude of all the specified points, is the HAAT. This can create some unusual cases, particularly in mountainous regions—it is possible to have a negative number for HAAT (the transmitter would not be located underground, but rather in a valley, with hills on both sides taller than the transmitter itself, for example).
human
What does HAAT stand for?
gpt
It stands for Height about average terrain
human
What is a similar acronym that means the same thing?
gpt
The less popular version, EHAAT, Effective Height Above Average Terrain
human
Can it be a negative number?
gpt
Yes
human
Where does that happen?
gpt
Usually in the mountainous regions
human
What industries is it used in?
gpt
More commonly, FM radio and television
human
What transmission types?
gpt
Usually VHF and UHF
human
Is it given in feet?
gpt
NO
human
What unit is used?
gpt
Meters
human
Why?
gpt
So that stations can be recieved on either side of the international boundaries.
human
What is subtracted from what to find it?
gpt
average altitude