This file is about the NGDC Color Relief Images:
The images in this set were produced from two sets of gridded topographic/bathymetric data available from the National Geophysical Data Center:
1) Images of oceans and whole continents were generated from "ETOPO5", a set of 5-minute gridded global elevations.
2) Detailed images of continental United States areas were generated from a set of 30-second gridded elevations for the U.S.; these data are a composite of the USGS 30" elevations and 30" averaged bathymetry from the National Ocean Service coastal hydrographic surveys.
Topographic relief is represented by both color and shading. Elevation is indicated by color (green at sea level to rust and snowy white at high altitudes); ocean depth is indicated by light blue-green on the continental shelves to deep purple in the abyssal regions. Shading is a function only of the east-west slope (one-dimensional to reduce computation time), with a nonlinear preference given to low-angle slopes to enhance details.
All computations were done on a Macintosh IIx computer by Peter Sloss at NGDC. Each image took about 5 minutes to compute with 24-bit color shading. Image files are in 8- and 24-bit PICT2 format, plus a PC-compatible GIF representation of the 8-bit images. (Bit depth of each image is indicated by its name suffix, e.g., Colorado24 is 24-bit, and Colorado8 is 8-bit).
For further information on the images, their source data, and the software used to create them, contact:
