|Control points are positions on a map (and/or on the Earth's surface) from which accurate measurements can be made. A horizontal control point is a location where the coordinates (latitude/longitude) are certain. Elevation may or may not be determined. Vertical control is the accuracy standard for elevation. For vertical (elevation) accuracy, no more than 10 percent of a map's contour invervals may be in error beyond half a contour interval. Maps that fail to comply are candidates for complete revision.
A benchmark, abbreviated "BM," is a location whose elevation and horizontal position has been surveyed as accurately as possible. Benchmarks are designed for use as reference points, and are usually marked by small brass plates.
In the definitions below, "third order or better" is an indication of position accuracy. Third order is the most generous standard acceptable on a USGS topo map. In the third order classification, surveys must be permanently marked and adequately described.
The Provisional Edition map series was established to expedite the completion of the large-scale topographic quadrangle coverage of the conterminous United States. They contain essentially the same level of information as the standard series maps. (One important difference is that roads on Provisional Edition maps are not classified as primary, secondary, or light duty. They are all symbolized as light duty roads.) This series is identified by the title "Provisional Edition" in the lower right hand corner of the maps.