An "Altitude Dive" is defined as any dive made at 300 meters/1000 feet or greater above sea level. Lake Pleasant is at about 1700 feet above Sea Level. Since (most) decompression models, dive tables and computers were developed for use at sea level, we need to modify our dive profiles to adapt to the changes in pressure as a result of our increase in atmosphere. Decompression models control the release of nitrogen absorbed during your dive by assuming you will begin and end you dive at sea level under a full atmosphere of pressure.
When you are away from the coast, like we are in Arizona, we have to accomodate for the increase in altitude where there's less air pressure. Thus Altitude Diving requires special table or computer procedures to account for the atmospheric pressure difference. While the decompression model difference is minimal at Lake Pleasant, it should not be ignored. Always follow the proper procedures, no matter how minimal the risk differential. Diving at altitude without following the proper altitude procedures increases the risk of decompression illness.
Without getting into the whole Altitude Diver Specialty Class:
Divers with computers make altitude adjustments by reviewing the owners manual. If their computer changes automatically, then no action is needed on their part. However, it has been my experience that computer altitude preferences need to be set. In the case of Suunto computers, they have an altitude setting of (A0, A1, A2, and A3). A0 is obviously sea level and A1-3 is a range setting. For Arizona, we use the A1 Setting which is calculated up to 3000 feet or 4000 feet (I don't exactly remember).
Divers with tables make altitude adjustments by planning their dive for a theoretical depth. Following is a link to a chart that was printed by Skin Diver Magazine that looks very similar to the one I use.
http://www.sandiasnorkel.com/info/inf-g_10.php
With this, you take your actual depth, convert it to a theoretical depth depending on your altitude, and plan your dive accordingly. Please note the difference for safety stops.
Lastly, there is a lot more covered in relation to Altitude Diving such as arriving and departing altitude, physiological effects and potential safety hazards, depth gauge and computer use, etc. If it is something you are planning to do (Lake Tahoe, Colorado, New Mexico, etc.), get the proper training. While Scuba Board is a great reference tool, it doesn't replace a Scuba Instructor.
JcF
PS - Remember your "flying after diving" rules (see back of tables). That applies to driving to altitude as well. It should properly read, "reaching altitude after diving" rules.