surveying a concrete floor, by levelling, it is important to mark out a
reference grid on the floor before any measurements are taken. The points of
the grid should then be marked on the plan of the floor to make it easy to see
which measured value corresponds to which point of the floor.
should be rectangular and the points should be spaced at regular intervals from
each other (usually 3m).
It is typically marked in chalk or something
else which is easy to clean off the floor surface once the survey is complete.