This is the latest available contour map of the lake's bottom. This map can be used to determine where "holes" (deep spots) exist on the lake bottom. Such areas are often productive for freshwater fishing. Bathymetry is defined as the measurement of water depth at various places in a body of water.
The level of a lake changes over time, so to be meaningful a bathymetric map must indicate the lake level at the time it was made. The height of the lake's surface is specified relative to a geodetic reference point known as a "vertical datum." Common vertical datums are NGVD29 and NAVD88. NGVD29 is the older of the two and most agencies have converted, or are in the process of converting, their data to NAVD88.
NGVD29 is an abbreviation for "The National Geodetic Vertical Datum of 1929," a standard that was based on a fixed calculation of sea level computed by using data from 26 tide gauges in the US and Canada."
The North American Vertical Datum of 1988 (NAVD 88) is a vertical control datum established in 1991. It is based on mean sea level height as well, that of a single point in Quebec, Canada.
More information about vertical datums, including how they are calculated and employed, can be found on the website of the National Geodetic Survey: http://www.ngs.noaa.gov/faq.shtml#Datums