Page 124 - Digital Analysis of Remotely Sensed Imagery

Storage of Remotely Sensed Data 95

the header, the image file, and the tag (Davenport and Vellon, 1987).
The 8-byte image header contains information vital for the correct
interpretation of the remainder of the TIFF file. Namely, it points to
one or more image file directories. Also contained in the header is the
information on the TIFF version number and the byte offset of the
first image file directory. An image file directory consists of a 2-byte
count of the number of fields, followed by a sequence of 12-byte field
entries, and a 4-byte offset of the next image file directory, if present.
The image file directories contain information about the image, and
pointers to the actual image data. Multiple image file directories are
needed to store multispectral imagery, each image file directory reserved
for a single band.
The flexibility of the TIFF format derives from its tags that describe
the image. In total, there are 35 commonly used tags in the TIFF
specification (version 5.0). Four classes of images can be contained in a
TIFF file, B (Bilevel or bitmap), G (Gray level), P (Palette or pseudocolor),
and R [RGB (Red-Blue-Green)]. B class images are stored with 1 bit per
pixel. Grayscale images require 2 to 8 bits per pixel, color images need
up to 24 bits per pixel. This added complexity slows access to the image
files. Their size may be reduced through data compression, with the
compression method stored in the “compression” tag.
The GeoTIFF interchange standard is an extension of the popular
TIFF format, to support georeferenced remote sensing data (Ritter and
Ruth, 1997). This standard unifies various internally represented
transformations between raster data and the reference coordinate
frame. It guarantees accessibility to images stored in the conventional
TIFF format, as well as all additional data needed for georeferencing
or geocoding independent of the TIFF image data. This limitation in
the number of available TIFF tags is overcome with the addition of a
new level of abstraction called GeoKeys (Hild and Fritsch, 1998). In
this format geospatial tags are imbedded within the TIFF file. With
this metatag concept, only six TIFF tags suffice to carry all georeferencing
information, namely, cartographic projection, geodetic datum, pixel
size, image spatial coordinates, and any additional information such
as projected coordinate systems, without destroying the data structure
of files saved in the standard TIFF format. GeoTIFF is especially
suitable for processed remote sensing data. In fact, it is the only generic
image format that enables geospatial information of a geometrically
rectified image to be preserved. In general, this format is also platform
independent, just like TIFF. Any digital image analysis systems for
analyzing remotely sensed and GIS data are able to read GeoTIFF data
correctly. When the image file is read, all the georeferencing information
is automatically loaded up into the computer. Any generic graphics
software packages that do not utilize spatial information, such as
Photoshop or CorelDraw, will still be able to read in GeoTIFF files as
regular TIFF files (in some cases support for TIFF 6.0 is required).
However, all the spatial information contained in the tag will be lost.