Page 105 - Digital Analysis of Remotely Sensed Imagery

76 Cha pte r T w o

acquisition, such as specification of scanning resolution. Commonly
adopted scanning intervals are 150, 300, and 600 dots per inch (DPI).
Since the original data are recorded in the graphic format, scanning at
any resolution, no matter how fine it is, inevitably causes loss of
information, even though this loss can be minimized by adopting a
finer scanning resolution. A large DPI enables a great deal of details
in the original photograph to be preserved in the captured image, but
also leads to an enormous amount of data to maintain and process
subsequently. This huge data volume can be very problematic and
troublesome to handle. For instance, a black-and-white aerial
2
photograph of 23 23 cm in dimension requires 7.29 Mb (see Sec. 3.1)
of storage space if scanned at 300 DPI. This figure rises to 29.16 Mb
if the scanning resolution doubles to 600 DPI, or to 87.48 Mb if
the photograph happens to be color. Therefore, it is important to
determine the optimal resolution of scanning. Which scanning
resolution is the most appropriate depends on the desired smallest
objects on the ground to be resolved on the scanned photograph, or
ground resolving distance (GRD). It is calculated using Eq. (2.1).
GRD (m) 25.4 SF/(1000 R) (2.1)
where SF the scale factor of the photograph being scanned, R the
scanning resolution expressed as DPI.
This equation can be inversed to determine the optimal scanning
resolution R. Below is an example of how to determine the required
scanning resolution using Eq. (2.1) above:

Example If the smallest ground feature to be preserved is 0.5 m in the digital
image scanned from a 1:12,500 aerial photograph, what scanning resolution
should be adopted?

Solution R 25.4SF/(1000 GRD) 25.4 12500/(1000 0.5) 635 DPI

The following four points need to be borne in mind with the
above calculation:

• First, the user may specify the scanning resolution as calculated
above prior to scanning. However, the extent to which the
scanner can be sensitive to this resolution is affected by its quality
(e.g., there may be no difference between 634.5 and 635.4).
• Second, the calculated resolution is only theoretical in that the
spatial resolution of the source photograph is not taken into
account. So long as the grain size of the photograph is sufficiently
fine (e.g., a few micrometers), it should be abundantly detailed.
This detail level should not affect discerning the smallest feature
in the scanned image if the scanning resolution is not overly
high (e.g., in excess of 1200 DPI or 21 μm).