National Ecological Framework (7 of 23)

Elevation Data

The following attributes are included:

NAME DESCRIPTION
MIN_ELEV Minimum Elevation
MAX_ELEV Maximum Elevation
MEAN_ELEV Mean Elevation
ELEV_DIFF Difference in Elevation

The elevation data presented in the database comes from the ETOPO5 5-minute gridded elevation data of the United States NOAA National Geophysical Data Center/World Data Center for Marine Geology and Geophysics (MGG). The complete description of the original data source can be found at their Internet web site: http://www.ngdc.noaa.gov/mgg/global/etopo5.html.

In Canada, a new digital elevation model based on the 1:250 000 National Topographic Base map series is being developed under the direction of Geomatics Canada, Natural Resources Canada. This more comprehensive and accurate digital elevation model (DEM) for Canada is expected to be completed in 1999. When the new DEM is available, it will be used to replace and upgrade the existing NOAA elevation data in this database.

Summary information from the NOAA web site about the ETOP05 5-Minute Gridded Elevation Data has been reproduced below for quick reference. The official web site should be consulted for more complete technical information and official contacts.

Sources for ETOP05:

ETOP05 was generated from a digital database of land and sea- floor elevations on a 5-minute latitude/longitude grid. The resolution of the gridded data varies from true 5-minute for the ocean floors, the U.S.A., Europe, Japan, and Australia to 1 degree in data-deficient parts of Asia, South America, northern Canada, and Africa. Data sources are as follows: Ocean Areas: U.S. Naval Oceanographic Office; U.S.A., Western Europe, Japan/Korea: U.S. Defense mapping Agency; Australia: Bureau of Mineral Resources, Australia; New Zealand: Department of Industrial and Scientific Research, New Zealand; balance of world landmasses: U.S. Navy Fleet Numerical Oceanographic Center. Margo Edwards originally assembled these various databases in 1988 into the worldwide 5-minute grid, then at Washington University, St. Louis, MO.

Data Formats for ETOP05:

The data file is formatted as 16-bit BINARY INTEGERS in two byte orders: ETOP05.DOS is in IBM-PC/DEC-VAX" swapped," lo-byte-first order. The file ETOP05.DAT is in "normal,", or hi-byte-first order, as used by Macintosh, Sun, and some other workstations. In both files, there are 2160 x 4320 data values, one for each five minutes of latitude and longitude, for a total of 9,331,200 points or 18,662,400 bytes.

Resolution and Accuracy:

Data values are in whole meters, representing the elevation of the CENTER of each cell. Accuracy of the data is hard to define, due to the disparate sources of the data. In general, the data sets for the USA, Western Europe, Korea/Japan, Australia and New Zealand are the most precise, having a horizontal resolution of five minutes of latitude and longitude, and vertical resolution of 1 meter. Data for Africa, Asia, and South America vary in resolution from +/- a few meters to only representing every 150m (500 feet), depending on the available source data. Very little detail is contained in the oceanic data shallower than 200 m; the interpolation algorithm used by the US Navy to create the oceanic grid from contour charts was set to an arbitrary cutoff of -10m wherever the algorithm would have "overshot" and marked points as above sea level. An example of such an area is off Argentina, near 45S, 60W. All oceanic data are coded at least -1m; land data are at 0 or greater, except where lake bottoms or other landlocked features go below sea level (Dead Sea, Death Valley, and in central Australia).

Data Order in the Files:

The file may be thought of as having a logical record size of 8640 bytes. The data start at the North Pole (90 deg N, 0 deg 0’E) and are arranged in bands of 360 degrees x 12 points/degree = 5320 values (8640 bytes) ranging eastward from 0 deg 0’ East longitude to 359 deg 55’ East longitude (since it represents the North Pole, all possible longitudes still refer to a single point, thus the first band has 4320 identical values of -4290 m). The 8641st starts the latitude band for 89 deg 55’ N, and so on. There is NO record for the South Pole (elevation 2810 m).

Referencing the Data:

The ETOP05 data may be credited in publications by reference to "Data Announcement 88-MGG-02, Digital relief of the Surface of the Earth. NOAA, National Geophysical Data Center, Boulder, Colorado, 1988." The version of the data making up ETOP05 is from May, 1988, with the exception of a small area in Canada (120-130 W, 65-70 N), which was regridded in 1990.