Name:   Osms - Subject:   msl vs md Date:   4/21/2017 9:22:32 AM

The answer is, when the lake was built in 1928, the level was established as Martin Datum (md) and all was fine except the rest of the world uses Mean Sea Level (msl) and there is a one foot difference between the two.  During relicensing FERC required APCo to standardize with the rest of the world so we now use msl as measurement.  490 md equals 491 msl.  Official use of 490 md has disappeared forever.

My guess is a mistake of one foot was made in 1928 when the height of the lake was established.  Maybe their GPS wasn't working well that day.