ES 555 Small Format
Digital imagery is to conventional photographs as digital sound is to conventional audio records or tape recordings. In both cases, digital file format offers great advantages for computer handling
and processing of the visual or audio data, but with lesser quality compared to traditional analog storage formats. In the case of photographic film, the visual image is captured in
minute silver crystals within the photographic emulsion. The ulimate resolution of the image is controlled by the size of these crystals. Recent improvements in digital cameras have achieved the spatial resolution equivalent to standard 35-mm film. New, high-resolution digital cameras equal 70-mm film and larger formats. Thus, digital photography has replaced film for most users.
There is a great need for digital imagery for modern information technology. This need is driving rapid developments in digital cameras as well as scanners for converting conventional film photographs into digital format. Your instructor began using digital cameras in 2001 for small-format aerial photography. In 2004, he discontinued film cameras, and he has employed digital cameras exclusively since 2005. The following table presents some advantages and disadvantages of the methods as utilized for small-format aerial photography.
Conventional film photography vs. digital imagery
for small-format aerial photography
|Method ||Advantages ||Disadvantages|
|Convenient & inexpensive|
UV, visible, & near-IR spectrum
Easy to archive & sort
Dependable under field conditions
|Delay for developing film|
Heat sensitive (color)
Deterioration in storage
|Convenient & inexpensive|
Images ready "instantly"
Easy digital image enhancement
|Near-IR possible, expensive|
Changing archive formats
Digital image scale
Scale is a fundamental property of routine aerial photographs, and is especially important for vertical airphotos used for photogrammetric purposes. Interpretability of aerial photographs is often determined by photo scale. However, digital images do not have scale in the conventional sense. For SFAP, consider a traditional 35-mm film camera. The film image size is 36 by 24 mm, which is known as full frame. Some high-end digital cameras have full-frame detector arrays, but most common digital cameras do not. The relationship between detector size and full frame is the crop factor. For example, the APS-C format has a crop factor of 1.5 (detector size = 24 by 16 mm). Many low-end digital cameras have much smaller crop factors—see sensor size comparison.
A digital image may be viewed and printed in many different scales. Thus, scale becomes a property of the device
used to display or print the image file and is not an intrinsic property of the image
file itself. In the case of digital imagery, ground sample distance (GSD)
is more appropriate as a measure for image resolution. However, GSDs can be quite different for collection, display and products of imagery from the same source. To help understand this somewhat confusing situation, we will examine different types of GSD (based
on Comer et al. 1998).
In conventional aerial photography, scale of the photo is equivalent to camera scale,
defined as follows—see photogrammetry lecture.
Consider, for example, a vertical kite aerial photograph taken with a 35 mm lens at a height
of 100 m above the ground. The scale would be 0.035 ÷ 100 = 0.00035 (or approximately
1:2860). Note: for this and subsequent calculations, all values are converted into meters.
camera scale = (lens focal length) ÷ (height above ground)
Now consider a digital camera with a charged-couple device (CCD). Collection GSD
is related to the size of each pixel element within the CCD array as follows.
Continuing with the previous example, given a pixel element size of 0.009 mm, then collection
GSD would be 0.000009 x 100 ÷ 0.035 = 0.026 meter (about 2½ cm or 1 inch). The
digital image "raw" scale is a ratio of CCD pixel element size to collection GSD, in this
case 0.000009 to 0.026, or about 1:2888. At this stage, the digital image scale closely
approximates the camera scale.
collection GSD = (pixel element size x height above ground) ÷ (focal length)
Display and print scale
Digital images rarely, if ever, are displayed at the original camera scale, however, which would be much too small for normal visual examination. Most usually, digital images are displayed on a computer monitor, in which the dot pitch controls the image size and resolution, assuming one image pixel is displayed for each monitor dot. In this case, the display scale is a ratio of the collection GSD to the monitor dot pitch.
Consider a typical monitor with a dot pitch of 0.26 mm and the previous example of collection
GSD of 0.026 meter. The display scale would be 0.00026 ÷ 0.026 = 0.01 (or 1:100). A
similar calculation can be done for printed digital images. The nominal pixel size for a
printer with 300 dpi (dots per inch) is about 0.085 mm. In this case, printer scale would
be 0.000085 ÷ 0.026 = 0.00327 (or about 1:300). These examples demonstrate that display
and printer scales are usually many times larger than is original digital image scale, because
the display pixels are many times larger than the CCD pixel elements.
display scale = (monitor dot pitch) ÷ (collection GSD)
The larger scales employed for display and printing of digital images do not imply more
information or higher interpretability, however, compared to the "raw" image data. A pixel
value is only a pixel value, regardless of the scale at which the pixel is displayed. For
visual identification of distinct objects, generally a group of 4 to 9 pixels is necessary
(Comer et al. 1998)—see image interpretation.
Scanning analog photographs
Digital cameras are rapidly replacing conventional film cameras for all types of aerial photography, as noted above. However, many older film images exist, for which digital versions would be useful. These older film images may have great historical value for documenting changing conditions. So digital and film will exist side by side for applications in aerial photography for some time to come. In order to exploit the capabilities for computer enhancement and analysis of digital images, it is necessary to convert film (or paper print) photographs into digital format. Two kinds of scanners are in widespread use today for conversion of analog photographs into digital image files. We have both kinds of scanners available in the GSA lab and earth science department.
- Flat-bed scanner – General purpose scanners that can accept various kinds of photography—prints, negatives, color-slide film, etc. Such scanners can handle original image sizes from postage stamps to legal-sized (or larger) documents.
- Film scanner – Special scanners designed exclusively for 35-mm film in either negative or positive (color-slide) modes.
- Comer, R.P., Kinn, G., Light, D. and Mondello, C. 1998. Talking digital. Photogrammetric
Engineering & Remote Sensing 64:1139-1142.
Return to SFAP schedule.
ES 555 © J.S. Aber (2016).