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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.