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GTOPO30 elevation data at different resolutions

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Elevation data sets at different resolutions has been compiled from the GTOPO30 global digital elevation model for use in making several data products. Note that a resolution of 0.5 degrees means about 50 kilometer at the equator.

The data is given in three formats:

nlon nlat data files  --  use  "unzip"  or  "gunzip -S .zip"  to un-zip HDF data file
big endian little endian ascii
0.25 degrees 1440 720 dem030_big.dat.zip
zip size: 535 kB
full size: 2.1 MB
zip size: 533 kB
full size: 2.1 MB
zip size: 781 kB
full size: 7.4 MB
full size: 533 kB
[file attributes]
0.50 degrees 720 360 dem060_big.dat.zip
zip size: 142 kB
full size: 521 kB
zip size: 142 kB
full size: 521 kB
zip size: 208 kB
full size: 1.9 MB
full size: 145 kB
[file attributes]
1.00 degrees 360 180 dem120_big.dat.zip
zip size: 38 kB
full size: 131 kB
zip size: 38 kB
full size: 131 kB
zip size: 56 kB
full size: 466 kB
full size: 42 kB
[file attributes]
*) If other resolutions (multiples of 1/120-th degree) are required,
     please contact Jos van Geffen.

The data files provide the terrain height in the form of a 2-dimensional array without longitude-latitude coordinates -- these coordinates must be constructed by the user on the basis of the grid specifications. The first line of the unformatted and ascii data files provides the following information:

    nlon  nlat  nfac  nocean  nland
    nlon   = actual number of longitudes <= mlon
    nlat   = actual number of latitudes <= mlat
    nfac   = resolution specifier: resolution is nfac/120 degrees
             in latitude and longitude range
             [the number is used in the file names]
    nocean = value in the array indicating an ocean data point
             [this usually is -9999]
    nland  = part of the nfac*nfac averaged data points that
             have to be land for the average data point in the
             height array to be defined as land
These and some additional things are also mentioned in the attributes of th HDF file.

The terrain height is given in the form of nlon by nlat grid cells of the specified size with centre coordinates rlon, rlat that can be computed as follows:

   rlon(i) = -180 + D(i)    i=1,2,...,nlon
   rlat(i) =  +90 - D(i)    i=1,2,...,nlat
      D(i) = ( 2*i - 1 ) * nfac / 240
In the HDF files, the latitude range is reversed, i.e. going from south to north (rather than from north to south, as is the case for the unformatted and ascii files), so that:
    rlat(i) = -90 + D(i)     i=1,2,...,nlat

The  Fortran subroutine readheight  reads the unformatted data file. The header of the subroutine specifies the input and output of the routine. Note in particular that the evelation data are stored in an  integer*2  array and when calling the subroutine, the dimensions of the  heigth  array must be big enough to hold the data, i.e. at least  nlon  by  nlat . The given routine reads the Fortran unformatted data files; for reading the formatted files, adapt the 'open' and 'read' lines (as marked with 'cf' in the first two columns).

The following image shows the map at 0.5 degree resolution (i.e 720x360).

GTOPO30 at 0.5 degree resolution
oceans -500 m 0 m 500 m 1000 m 1500 m 2000 m 2500 m
  3000 m 3500 m 4000 m 4500 m 5000 m 5500 m 6000 m

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Page last modified: 18 February 2016