AGL Above Ground Level. Height above common ground to the the mid-point of the radiating antenna.
AMSL Above Mean Sea Level. Height referenced above sea level, or zero elevation.
The K factor accounts for refraction of radio waves close to the surface of the earth. For antenna towers less than a couple of thousand feet above the surface, a K factor of 1.33 is usually adequate for most line-of-sight calculations over average terrain.
Typical Values of K
Dry mountains (above 1500 meters) 1.20 1.20
Mountains (to 1500 meters) 1.25 1.25
Midwest and Northeast 1.50 1.30
South and West Coast 1.55 1.35
Southern Coast 1.60 1.50