Surfer can import United States Geological Survey (USGS) ETOPO5 files.
ETOPO5 is a product of a digital data base of land and sea- floor elevations on a 5-minute latitude/longitude grid.
The ETOPO5 data files are commonly available in two different formats.
An ETOPO5 file with the .DAT file name extension is typically stored in native or “ big endian” byte order, as used by Macintosh, Sun, and some other workstations.
An ETOPO5 file with the .DOS file name extension is typically stored in converted IBM-PC/DEC-VAX (or " little endian") byte order.
Data sources are as follows: Ocean Areas: US Naval Oceanographic Office; USA., W. Europe, Japan/Korea: US Defense Mapping Agency; Australia: Bureau of Mineral Resources, Australia; New Zealand: Department of Industrial and Scientific Research, New Zealand; balance of world land masses: US Navy Fleet Numerical Oceanographic Center. These various data bases were originally assembled in 1988 into the worldwide 5-minute grid by Margo Edwards, then at Washington University, St. Louis, MO.
The data file is formatted as 16-bit BINARY INTEGERS in two byte orders: ETOPO5.DOS is in IBM-PC/DEC-VAX "swapped," lo-byte-first order. The file ETOPO5.DAT is in "normal," or hi-byte-first order, as used by Macintosh, Sun, and some other workstations. In both files, there are 2160x4320 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
The resolution of the gridded data varies from true 5-minute for the ocean floors, the USA., Europe, Japan,and Australia to 1 degree in data-deficient parts of Asia, South America, northern Canada, and Africa.
Data values are in whole meters, representing the elevation of the CENTER of each cell.
Accuracy of the data set 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 fet), depending on the available source data. Very little detail is contained in the oceanic data shallower than 200m; 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 -1 m; 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 = 4320 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.)
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