2.1
Analog (Hard-Copy) Image Digitization
John R. Jensen
Steven R. Schill
Department of Geography
University of South Carolina
Columbia, South Carolina 29208
Direct Comments to: jrjensen@sc.edu

Introduction

Remotely sensed data must be in a digital format before it can be digitally processed. Before beginning a project, first check to see if the information you need is already in digital format. A variety of digital databases are now available and can be accessed over the internet. If the data you are processing is not in a digital format, it must be converted through digitization.


Analog (Hard-Copy) Image Digitization

Data collected in hard-copy pictorial records are termed photographs. Photographs are recorded two-dimensionally on photosensitive emulsions that are either reflective (such as in a print) or transmissive (such as a slide). In remote sensing, the term photograph is reserved exclusively for images that were detected as well as recorded on film. The more generic term image is used for any pictorial representation of image data. Because the term image relates to any pictorial product, all photographs are images, but not all images are photographs (Lillesand et al., 1994).
Figure 2-1.1 
Digital Image Components
 

A pixel is defined as a two-dimensional picture element that is the smallest nondivisible element of a digital image (Fegas et al.,1992) (see Fig. 2-1.1). In applications where spectral patterns are highly informative, it is preferable to analyze images in a digital, not analog format. When a photograph is converted into a digital format, the density of the light-absorbing silver or dye deposited within a user-defined pixel is measured and assigned a positive integer brightness value. This process is called analog-to-digital (A-to-D) conversion and creates a matrix of brightness values corresponding to the average radiance measured from individual pixels that have been scanned (see Figure 2-1.2). The relationship between the digitizer Instantaneous Field of View (IFOV) measured in dots-per-inch or micrometers and the pixel ground resolution at various scales of photography can be seen in Table 2-1.

Figure 2-1.2
 
1 meter and 30 meter data. As data is scanned and converted into a digital format, 
a matrix of brightness values is collected that corresponds to the average radiance 
measured from individual pixels (Courtesy Space Imaging, Inc.).
 

Three major methods exist for converting hard-copy aerial photography, radar imagery, thermal infrared imagery, etc. into a format suitable for digital image processing:

 


Go to Section 2.2 - Remotely Sensed Data Already in Digital Format 

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References

Fegas, R. G., J. L. Cascio, and R. A. Lazar, 1992, "An Overview of FIPS 173, the Spatial Data Transfer Standard," Cartography and Geographic Information Systems, 19(5):278-293.

Jensen, J. R., 1996, Introductory Digital Image Processing: A remote sensing perspective, 2nd Edition. NJ: Prentice-Hall, pp. 17-24.

Lillesand, T. M., R. W. Kiefer, 1994, Remote Sensing and Image Interpretation, 3rd Edition. John Wiley & Sons, Inc., 23 p.

Robinson, A. H., J. Morrison, P. Muehrcke, A. Kimerling, S. Guptill, 1995, Elements of Cartography, 6th Edition, John Wiley & Sons, Inc., pp. 190-192.