An image is a type of spatial data based on rows and columns, where a single piece of information is stored in each pixel (or grid-cell). A “BIL” image file (.bil), which means “band interleaved by line,” is an uncompressed file containing the actual pixel values of an image. It can handle black and white, grayscale, pseudocolor, true color, and multi-spectural image data. BIL data stores pixel information in separate bands within the file, so that the user can choose to display just one specific band in a multi-band image. However, the GPW and GRUMP .BIL data contains only one band.

There are four image description files (ASCII text file format) that can accompany the BIL data: a header file, a statistics file, a resolution file, and a color file.

The header file (.hdr) describes the nature of the image data, through the use of keywords and values.

The statistics file (.stx) is an optional file that describes the image statistics for each spectral band. It records the minimum and maximum pixel values, the mean, the standard deviation, and the two linear contrast stretch parameters.

The resolution file (.blw) describes the height and width of each cell and the coordinate position of the top left cell of the data. It gives the Resolution information with the following lines:

resolution in x-dimension
rotation factor (always zero for GPW)
rotation factor (always zero for GPW)
resolution in y-dimension
x-coordinate of the center of the upper-left cell
y-coordinate of the center of the upper-left cell

The color file (.clr) is an optional file that describes the image colormap (this file type is not used in the GPW data).