A datum line is a line that is perfectly flat and level, like the top of still water. You make one with a string line and a spirit level, or with a laser.
You stretch it across the gate opening. Then you measure from that line: down to the ground, or up to where the top of the gate goes. Because every number comes from the same flat line, your measurements stay right even when the ground is bumpy or sloping.
It is the simple trick behind measuring a gate on sloping ground. In CAD60, the single rake and double rake numbers are all taken from this line.