On an ordinary city lot, the most common type of marker is an iron pipe or rod driven in the ground. A newer marker will include a plastic cap or a metal disk bearing the license number of the surveyor under whose supervision it was set. These are usually four- or five-digit numbers, with the letters "LS", "PLS", or "RLS" above or before the number. Unfortunately, the cap or disk sometimes is knocked off or damaged by grading or excavation work. These markers are set at lot corners and at points where property lines change direction. They are usually driven level with the ground when installed. Over the years, a few inches of dirt and sod can cover them. If the lot was graded after the markers were installed, they can be buried more deeply or may be missing entirely. Section monuments may be stone, concrete, cast iron, cast aluminum, or large pieces of pipe. Section monuments set by a County Surveyor are usually marked as such. Many other types of survey markers exist.