Back to how to find the elevation for an unknown ALA....
Method 1) Take a bag of flour and whilst flying over the ALA chuck it out the window (this works best in high wing aircraft) carefully measuring the time it takes to hit the ground (some sharp manoeuvring may be needed to get good visibility of the drop zone). Using the formula d = 1/2gt2 you can subtract the distance covered by the bag of flour from the altitude indicated on the altimeter and you should have the AMSL of the airfield.