1 October 2003
Processing Digital Imagery
5-1 Introduction. Image processing in the context of remote sensing refers to the
management of digital images, usually satellite or digital aerial photographs. Image
processing includes the display, analysis, and manipulation of digital image computer
files. The derived product is typically an enhanced image or a map with accompanying
statistics and metadata. An image analyst relies on knowledge in the physical and natural
sciences for aerial view interpretation combined with the knowledge of the nature of the
digital data (see Chapter 2). This chapter will explore the basic methods employed in
image processing. Many of these processes rely on concepts included in the fields of ge-
ography, physical sciences, and analytical statistics.
5-2 Image Processing Software.
a. Imaging software facilitates the processing of digital images and allows for the
manipulation of vast amounts of data in the file. There are numerous software programs
available for image processing and image correction (atmospheric and geometric cor-
rections). A few programs are available as share-ware and can be downloaded from the
internet. Other programs are available through commercial vendors who may provide a
free trial of the software. Some vendors also provide a tutorial package for testing the
b. The various programs available have many similar processing functions. There
may be minor differences in the program interface, terminology, metadata files (see be-
low), and types of files it can read (indicated by the file extension). There can be a broad
range in cost. Be aware of the hardware requirements and limitations needed for running
such programs. An on-line search for remote sensing software is recommended to ac-
quire pertinent information concerning the individual programs.
a. Metadata is simply ancillary information about the characteristics of the data; in
other words, it is data about the data. It describes important elements concerning the ac-
quisition of the data as well as any post-processing that may have been performed on the
data. Metadata is typically a digital file that accompanies the image file or it can be a
hardcopy of information about the image. Metadata files document the source (i.e.,
Landsat, SPOT, etc.), date and time, projection, precision, accuracy, and resolution. It is
the responsibility of the vendor and the user to document any changes that have been
applied to the data. Without this information the data could be rendered useless.
b. Depending on the information needed for a project, the metadata can be an invalu-
able source of information about the scene. For example, if a project centers on change
detection, it will be critical to know the dates in which the image data were collected.
Numerous agencies have worked toward standardizing the documentation of metadata in
an effort to simplify the process for both vendors and users. The Army Corps of Engi-
neers follows the Federal Geographic Data Committee (FGDC) standards for metadata