Geoscience Reference
In-Depth Information
pixels is therefore 72∙9.92≈713 in a horizontal direction, and 72∙10.12≈729
in a vertical direction.
Numerous formats are available for saving vector and raster data into a
i le, each with their own particular advantages and disadvantages. Choosing
one format over another in an application depends on the way the images
are to be used in a project and whether or not the images are to be analyzed
quantitatively. h e most popular formats for storing vector and raster data
are:
•
Compuserve Graphics Interchange Format
(GIF) - h is format was
developed in 1987 for raster images using a i xed 8-bit colormap of 256
colors. h e GIF format uses compression without loss of data. It was
designed for fast transfer rates over the Internet. h e limited number of
colors means that it is not the right format for the smooth color transitions
that occur in aerial photos or satellite images. It is, however, ot en used for
line art, maps, cartoons and logos (http://www.compuserve.com).
•
Portable Network Graphics
(PNG) - h is is an image format developed in
1994 that is used as an alternative to the GIF. It is similar to the GIF in that
it also uses a i xed 8-bit colormap of 256 colors. Alternatively, grayscale
images of 1 to 16 bits can be stored, as well as 24 and 48 bit color images.
h e PNG format uses compression without loss of data, with the method
employed being better than that used for GIF images.
•
Microsot Windows Bitmap Format
(BMP) - h is is the default image
format for computers running Microsot Windows as the operating
system. However, numerous converters also exist to read and write BMP
i les on other platforms. Various modii cations of the BMP format are
available, some of them without compression and others with ef ective
and fast compression (http://www.microsot .com).
•
Tagged Image File Format
(TIFF) - h is format was designed by the Aldus
Corporation and Microsot in 1986 to become an industry standard for
image-i le exchange. A TIFF i le includes an image i le header, a directory,
and the data in all available graphics and image i le formats. Some TIFF
i les even contain vector and raster versions of the same picture, as well
as images at dif erent resolutions and with dif erent colormaps. h e main
advantage of TIFF i les was originally their portability. A TIFF should
perform on all computer platforms; unfortunately, however, numerous
modii cations of the TIFF have evolved in subsequent years, resulting in
incompatibilities. h e TIFF is therefore now ot en called the
h ousands of
Incompatible File Formats
.
