A surface plate is a solid, flat table that can be confidently used as a datum for marking out, measuring workpieces, or setting up tooling. Considering that it is the baseline for all future measurements and that tolerances as low as +/- 1 micrometre may be in use, its precision is critical.

There are varying grades that are applied to the quality of metrology equipment, A, B and Workshop grade come to mind. While workshop grade is the lowest it is still required to be greater than the name workshop may suggest.

The table may be made from ribbed cast iron, the ribbing increases strength while reducing the weight. It will be aged and stress relieved to decrease the likelyhood of it moving (twisting or warping) over time.

Glass is an alternative material and was used during the second world war when material and production dead lines were either in short supply or over stretched. Glass can be suitably ground and has the benefit that it chips rather than raises a burr like cast iron.

Granite is the best material of all as not only does it chip instead of burr, it is inherently stable. It is aged to the point where any further movement in the material will not affect any measurements based from it.