text: in engineering,what is chart datum?   scienceisfun is correct chart datum is referential information that describes in some cases how data on the chart was derived. a map with lat/long lines on it must always have the projection i.e. nad84, nad27, polyconic etc, to indicate how the map was computed. likewise, a map with elevations on it should indicate somewhere what they used for their 0.00 reference point, whether it be mean sea level used by ngs or some other more local vertical control point.