Where is a Datum set from?

A datum is a line that is an imaginary perfectly level line. it's height does not matter as it is used to measure height as a comparative measurement to more than one place on a structure or structures. Basically, if you wanted to build a tree house for an example you could make a datum line out of strings in a square shape the same size as your tree house, they could be attached to sticks driven into the ground, using a spirit level you make sure the strings were perfectly level, not necessarily the same distance from the ground along their length as the ground may have a slope to it or high and low spots. Then using this level line you can measure up to the floor of the tree house at all the corners to make sure they were all the same distance from the line this would insure that the tree house was as level as the string. You also could extend the lines using the spirit level to build tree houses at the same level as your first one. A rotating laser level is an electronic datum line that cabinet installers use to make sure that the upper cabinets are even all around the room regardless of the unevenness of the floor. Wow that got longer than I thought it would, Hope it helps