Image Interpretation

ES 555 Small Format
Aerial Photography

Table of Contents
Basic features Nature of resolution
Rating criteria Analog vs. digital
Related sites References

Basic features

Visual interpretation of aerial photographs is based on recognition of objects, as seen from above, in pictorial format. This visual recognition often takes place without any conscious effort by the interpreter. Nonetheless, there are several basic features that can aid in visual examination and interpretation of airphotos.

Nature of resolution

The term resolution is used in different ways in remote sensing. In general, it refers to the smallest identifiable feature within an image. For a digital image, this means an individual cell or pixel (picture element). For conventional film, the number of resolvable lines per inch (or mm) is a measure of resolution. However, these quantitative approachs ignore the fact that size is not the only element necessary for identifying objects in an image. As noted above shape, pattern, color, contrast, texture, shadow, and context are other elements that aid in feature recognition.

Rule of thumb: Positive recognition of objects in aerial photographs requires a ground resolution three to five times smaller than the object size (Hall 1997).

For example, to positively identify an automobile (width 2 m), the camera/image system needs to achieve a ground resolution between 0.67 and 0.4 meters. With these limitations in mind, a perceptual measure of image quality or interpretability has been developed. This measure is the National Imagery Interpretability Rating Scale--NIIRS (Leachtenauer et al. 1997). An image rating depends on the most difficult interpretation task that can be performed, which indicates the level of interpretability that can be achieved. The following listing provides the civilian NIIRS criteria.

Civilian NIIRS criteria

Based on (Leachtenauer et al. 1997).

Rating Level 0

Rating Level 1

Rating Level 2

Rating Level 3

Rating Level 4

Rating Level 5

Rating Level 6

Rating Level 7

Rating Level 8

Rating Level 9

Manned-space photography, as practiced on space-shuttle missions, provides images of the Earth's surface for rating levels 1-3. Rating levels 4-6 are typically attained by conventional aerial photography, and sometimes level 7 is possible. However, the highest rating levels (7-9) are generally not available for civilian use. These highest NIIRS rating levels (7-8) may be acquired, in many cases, with SFAP from low-height platforms.

Analog vs. digital imagery

Conventional photography with film is called analog imagery, as the photographic emulsion records continuous variations in light intensity. Digital cameras, on the other hand, record light intensity in discrete intervals. High-resolution digital cameras now achieve the same spatial resolution as conventional 35-mm films, although medium- and large-format films still have a spatial resolution advantage. This situation is likely to change quickly, as digital resolution continues to improve rapidly.

The conversion of analog (film) photographs into digital format by scanning carries a potential disadvantage--resolution of the digital image may be degraded both spatially and in terms of tone/color range. The amount of degradation can be estimated by comparison of NIIRS rating levels for the original photograph and scanned digital image. On this basis, the scanning technique may be adjusted in order to preserve a necessary level of interpretability for a given project or application of the digital imagery.

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ES 555 © J.S. Aber (2016).