Geoscience Reference
In-Depth Information
Compared to the 1-bit and 8-bit representations of raster data, 24-bit storage
certainly requires a lot more computer memory. In the case of very large
data sets such as satellite images and digital elevation models the user should
therefore think carefully about the most suitable way to store the data. h e
default data type in MATLAB is the 64-bit array, which allows storage of
the sign of a number (bit 63), the exponent (bits 62 to 52) and roughly 16
signii cant decimal digits between approximately 10
-308
and 10
+308
(bits 51 to
0). However, MATLAB also works with other data types such as 1-bit, 8-bit
and 24-bit raster data, to save memory.
h e amount of memory required for storing a raster image depends on the
data type and the image's dimensions. h e dimensions of an image can be
described by the number of pixels, which is the number of rows multiplied
by the number of columns of the 2D array. Let us assume an image of 729-
by-713 pixels, such as the one we will use in the following section. If each
pixel needs 8 bits to store a grayscale value, the memory required by the data
is 729∙713∙8=4,158,216 bits or 4,158,216/8=519,777 bytes. h is number is
exactly what we obtain by typing
whos
in the command window. Common
prei xes for bytes are kilo-, mega-, giga- and so forth.
bit = 1 or 0 (b)
8 bits = 1 byte (B)
1024 bytes = 1 kilobyte (KB)
1024 kilobytes = 1 megabyte (MB)
1024 megabytes = 1 gigabyte (GB)
1024 gigabytes = 1 terabyte (TB)
Note that in data communication 1 kilobit=1,000 bits, while in data storage 1
kilobyte=1,024 bytes. A 24-bit or
true color image
then requires three times the
memory required to store an 8-bit image, or 1,559,331 bytes=1,559,331/1,024
kilobytes (KB)≈1,523 KB≈1,559,331/1,024
2
=1.487 megabytes (MB).
However, the dimensions of an image are ot en given, not by the
total number of pixels, but by the length and height of the image and its
resolution. h e resolution of an image is the number of
pixels per inch
(ppi)
or
dots per inch
(dpi). h e standard resolution of a computer monitor is 72
dpi although modern monitors ot en have a higher resolution such as 96
dpi. For instance, a 17 inch monitor with 72 dpi resolution displays 1,024-
by-768 pixels. If the monitor is used to display images at a dif erent (lower,
higher) resolution, the image is resampled to match the monitor's resolution.
For scanning and printing, a resolution of 300 or 600 dpi is enough in most
applications. However, scanned images are ot en scaled for large printouts
and therefore have higher resolutions such as 2,400 dpi. h e image used in
the next section has a width of 25.2 cm (or 9.92 inches) and a height of 25.7
cm (10.12 inches). h e resolution of the image is 72 dpi. h e total number of
