READ.ME file for the sediment thickness files accompanying the MGG-09 sea 
floor topography prediction.


Units, reference surfaces, storage conventions:

There is one data set supplied:  sediment thickness (sedthick).

The file "sedthick.grd" contains floating-point information and so is stored
in units of meters.  The file "sedthick.bin" has values of meters
rounded to the nearest integer.  

Latitudes are geodetic latitudes according to WGS-84.


File formats:

	"sedthick.grd" --

The file "sedthick.grd" is in the hardware-independent,
NetCDF format used by the GMT graphics software.  GMT is a free package
of C language graphics tools designed for use in the UNIX environment.
GMT tools make graytone and color PostScript output for scientific
visualization and publication-quality maps and graphs.  GMT is distributed
free of charge over the Internet by anonymous ftp from kiawe.soest.hawaii.
edu (128.171.151.16) in the /pub/gmt directory.  NetCDF is a package of
I/O routines that allow hardware-independent data storage using XDR plus
a description and retrieval layer; it is available by ftp from unidata.
ucar.edu (128.117.140.3).

If you are using GMT the file sedthick.grd will be readable
directly, and you will not need to write your own code to read them.


	"sedthick.bin" --

The file "sedthick.bin" is two-byte signed integer values in
the Sun byte order (twos-complement, big-endian, or most significant byte
first).  Vaxen and Intel chips use little-endian, so IBM PCs and VAXes will
have to swap the bytes to use these files; Macs, Suns, SGIs, HPs, newer
DECs, etc. should be able to use them as-is.

This description should enable you to write your own code to read the .bin
file.  The file describes an array of 480 rows and 4320 columns.  The
sequence of the array elements is in "scan-line order", which is written
by rows, from the top row to the bottom row, and within each row is
written from left to right.  The rows span latitudes from -30 degrees
south to -70 degrees south with 12 rows per degree.  The columns span 360
degrees of longitude from 0 degrees east to 360 degrees east, with 12
columns per degree.  The array elements are "pixel registered", meaning
that the latitude and longitude of the center of a pixel can be described 
this way in C:

	short int	data[2073600];
	FILE		*filepointer;
	int		ij, jrow, icolumn;
	double		xlongitude[4320], ylatitude[480];

	for (jrow = 0; jrow < 480; jrow++)
		ylatitude[j] = -30.0 - 0.0833333 * (jrow + 0.833333);

	for (icolumn = 0; icolumn < 4320; icolumn++)
		xlongitude[icolumn] = 0.0833333 * (icolumn + 0.833333);

	if ( (filepointer = fopen("sedthick.bin", "r")) ++ NULL) {
		/* Error condition  */
	}

	for (ij = 0, jrow = 0; jrow < 480; jrow++) {
	  for (icolumn = 0; icolumn < 4320; icolumn++, ij++) {
	    if ( (fread((char *)&data[ij], 2, 1, filepointer)) != 1) {
		/* Error condition  */
	    }
	    /* Here xlongitude[icolumn], ylatitude[jrow], and data[ij]
		correspond to one another.  */
	  }
	}

	fclose(filepointer);



Other things to know about the sediment thickness data:

   Sediment thickness data were compiled from three principle sources; 
previously published isopach maps (see list at the end of this document); 
ocean drilling results, both DSDP and ODP; and seismic reflection and 
refraction profiles archived at NGDC as well as seismic data available 
as part of the IOC's Geological/Geophysical Atlas of the Pacific (GAPA) 
project. Kenji Nemoto of Tokai University, Japan, compiled the sediment
thickness information for the western Pacific, the eastern Pacific 
data were compiled at NGDC.

   The gridded data estimate the total sediment thickness, in meters, 
from the sea floor to the top of acoustic basement.  Basement is presumed 
to be oceanic basaltic crust created at mid-ocean ridges.  In some areas, 
however, strongly reflective chert layers and mid plate volcanics mask 
the underlying oceanic crust.  Therefore the deepest observable reflector 
in the seismic reflection profiles is defined as acoustic basement.  It 
should be noted that acoustic basement may not actually represent the base 
of the sediments.  These data are intended to provide a minimum value for 
the thickness of the sediment in a particular geographic region.  Velocity 
regression equations were used to convert two-way reflection time, obtained 
from the seismic profiles, to thickness in meters.


Technical people to contact:

David Divins
National Geophysical Data Center
NOAA mail code E/GC-3
325 Broadway
Boulder CO 80303
303-497-6505   (phone)
303-497-6513   (fax)
ddivins@ngdc.noaa.gov  (Internet)


References

Hayes, D.E., and J.L. LaBrecque, Sediment Isopachs: Circum-Antarctic
to 30S, in Marine Geological and Geophysical Atlas of the 
Circum-Antarctic to 30S, edited by D.E. Hayes, pp. 29-33, American 
Geophys. Union, Washington, D.C., 1991.

Divins, D.L., and P.D. Rabinowitz, Total sediment thickness map for 
the South Atlantic Ocean, in International Geological and Geophysical 
Atlas of the Atlantic and Pacific Oceans (GAPA), edited by G.B. Udintsev, 
pp. 147-148, Intergovernmental Oceanographic Comission, 1991.

Matthias, P.K., P.D. Rabinowtiz, and N. Dipiazza, Sediment thickness map 
of the Indian Ocean, Map 505, Am. Assoc. Petrol. Geol., Tulsa, Ok, 1988.

Ludwig, W.J., and R.E. Houtz, Isopach map of sediments in the Pacific 
Ocean basin, color map with text, Am. Assoc. Petrol. Geol., Tulsa, Ok, 
1979.