================================================ MAPS-L ** MAPS-L ** MAPS-L ** MAPS-L ** MAPS-L ================================================ Subject: GNIS Elevations Date: Tue, 13 Sep 2005 From: John C Fouke To: maps-l --- Forwarded by John C Fouke/RGIO/USGS/DOI on 09/12/2005 The following was written by Roger Payne: We have changed the method by which we display elevation information. We now display elevation figures directly from the National Elevation Dataset (NED) of the U.S. Geological Survey. We made the decision to adopt this method because it allows us to display elevation data for every entry in the database, and we no longer have to devote valuable resources to a highly specialized activity for maintaining a specific data field. Also, elevation in GNIS has always been ancillary albeit relevant data. Still, elevations in GNIS never have been official; only the name and location (latitude and longitude) are considered official. The difference in elevations occur because even the steadiest hand or most precise measurement can be a few seconds off, which is well within tolerance yet can yield elevation difference of many feet or meters especially in highly varied terrain with much local relief. If the elevation figure is of note such as the highest point in the State, for example, then the actual highest point elevation is recorded in the description field of the entry. The problem is further complicated, and altered by the recent conversion in the database of the coordinates from the NAD 27 datum to that of the more accurate NAD 83, but which caused a shift in location that is mostly negligible, but more exaggerated in Alaska and the Pacific Basin. We apologize if this decision causes any problems in the user community, but after much consideration, it was determined that it was best overall considering the mission and purpose of the Geographic Names information System (GNIS), and its enhancement. The original elevation figures were not always precise as the method of compilation formerly used indicates. If the elevation of a feature was actually published on the topographic map, it means that the elevation data represented an actual, precisely surveyed point, and there is often a benchmark or triangle at that location on the map, and also for these there is actually a benchmark seal physically located and anchored into the ground at the site. Elevation data was also from other surveyed points, which might or might not be marked by an ?x.? If such a number was present, it was recorded as the elevation. If there was no surveyed point, then formerly for use in GNIS, the elevation was determined by interpolation of contour lines. These lines are the brown lines on topographic maps that are drawn to connect points of equal elevation thereby providing a general schematic of landforms or the ?lay of the land.? In the GNIS database, the Primary coordinate values for communities are taken at the center of the "original" community meaning the city hall, main post office, main intersection, etc. For other areal features, elevation was measured at the approximate center, and for reservoirs at the dam. The elevation for features classified as summit (all uplifted features), was recorded at the highest point. The elevation figure was determined by taking one-half the distance of the contour interval as stated on the map between the last or highest contour line and where the next contour line would be located if present. For example, if the last contour line measures 1,000 feet and the contour interval is 100 feet (the distance between each contour line), then the elevation recorded for entry into the GNIS database is 1,050 feet which is an approximation as the exact elevation could be anywhere from 1,001 feet through 1,099 feet.