Determine the specific situation that you are calculating for the altimeter height. The FAA prescribes different methods for each situation. For instance, if the threshold elevation is the highest point in the touchdown zone, then the calculation is different than if it is not the highest elevation in the zone.
Measure the horizontal distance from the ground point intercept, or GPI, to the point directly below the aircraft. To calculate the altimeter height, this is one of the variables you must know. You also need to determine how far away the aircraft is from the intercept point.
Multiply the horizontal distance times the tangent of the angle of contact. The angle of contact is the estimated angle of the line between the aircraft and the GPI. Therefore, if the angle is 2.5 degrees and the tangent of 2.5 is .04366, this number is multiplied times the horizontal distance. If the horizontal distance is 5000 feet, then the altimeter height is approximately 218 feet.
Adjust your calculations if the threshold elevation is not the highest point. In other words, if the terrain dips below the runway, then the calculation must also take into account the highest point on the runway behind the GPI. If this point is 50 feet beyond the GPI with the altimeter calculation of 218 feet, the 50 feet must be added, for a total calculation of 268 feet.