Chart datum

Usually, a chart datum would mean the reference level on a chart against which all other values are plotted. Without more context, it's difficult to say exactly what is meant here, but I guess it could refer to e.g. mean sea level(?).

The word 'datum' is the singular of the word 'data', but it is commonly used to describe a reference level, zero point, home position, fiducial value etc. etc.

harinathreddy: Three posts with a similar theme in 2 days makes it look like you are having problems with homework and didn't take a serious interest in Trigonometry when you had the opportunity!

The chart datum for tide tables is set a a level below which the equinox lows will not drop below for that location; from that datum the drafts of the tide are then referneced.

Try looking at wikipedia, british hydrographic office, or calcul de la maree

There are numerous datums recognized in cartography. It is the reference point from which a reference grid originates and the subsequent grid itself in most instances.

Definitely a homework question. Now, do you trust me or do you go to the books like a good little researcher and verify/dismiss the findings?