national aerial resources::topo revision info

Quad Map Revision/Exchange Update

When we speak of a quadrangle map being revised, we are talking about updating, improving, and correcting it as needed. An existing map is revised by first determining which features are no longer current. New information is then used to make the map content and collar information more up-to-date. Current USGS map revision practices include the following:

Complete revision.

All planimetric and hypsographic (contour) information is revised. All data are revised to meet current vertical and horizontal accuracy standards. Field checking is performed to verify map updates.

Basic revision.

Planimetric information is revised using rectified orthoimages derived from current aerial photographs and ancillary sources, if available. In addition, boundary and name information is updated. The accuracy of the original map is maintained. No hypsographic information is revised. No field checking is performed to verify the updates.

Photorevision.

Before 1995, locally registered aerial photographs, rather than orthoimages, were used to update planimetric information. The photorevised information was portrayed in a distinctive purple color. There are still many maps updated by photorevision in the USGS inventory, but basic revision has superseded this earlier technique.

Minor revision.

Boundary, name, and collar information are updated on maps found, through photoinspection, to contain minimal planimetric changes; that is, map content is considered current.

Photoinspection.

The published map is compared with recent aerial photographs and defined change criteria are used to determine if the map qualifies for revision.