Surface plates are flat and plane surface used as a horizontal reference space for dimensional measurement, it’s a base for inspection, toolmaking, gauging, spotting, marking, and layout. They can be made from granite, metal, cast iron, or glass.
A surface plate provides a precisely accurate plain surface that is wear-resistant, and in the case of the granite surface, it has a very low thermal coefficient of expansion, so this helps its flatness resilience against temperature excursions. Heights may be measured and surfaces marked accurately.