Photographic Scale and Resolution

The scale of a vertical aerial photograph can be calculated simply in two ways. The scale (S) depends on the average height above the ground (Hg) and the lens focal length (f) of the camera. In either case, the units of measurement must be the same.

S = photo distance(d)/ground distance(D)

(Equation 2-1)

In cases where objects of known size appear in the vertical photograph, the first method may be utilized for scale calculation (Fig. 2-3). If no objects of known size are visible in the photograph and the flying height above ground is known, the second method is employed. Scale is usually expressed as a fraction or ratio, such as 1/1000 or 1:1000, meaning one linear unit of measurement on the photograph equals 1000 units on the ground. In rugged terrain, however, photo scale varies because of large height differences within the photograph. Likewise oblique photos also display large variations in scale.

Scale is a fundamental property of routine aerial photographs, and is especially important for vertical airphotos used for measurements and photogrammetric purposes. Interpretability of aerial photographs is often determined by photo scale. For analog aerial photographs, the original film scale is the most commonly used characteristic for describing the amount of detail identifiable in the image. The actual photographic resolution, determining the size of the smallest identifiable feature within an image, is

FIGURE 2-2 Three views of Gammelsogn Kirke (old parish church), near Ringk0bing, western Denmark. (A) High-oblique view showing the horizon. (B) Low-oblique view in which the horizon is not visible. (C) Near-vertical view. The church dates from the 1170s when the Roman nave and choir were built; the tower and entry house were added later. Kite aerial photographs by JSA, SWA, and IM, September 2005.

measured as the number of resolvable lines per inch (or mm) and depends on film emulsion and image contrast.

For digital images, the original scale on the image sensor is not of much interest, as the scale of a digital image is easily changed when viewing it on a display device and becomes a property of this device. However, the original image resolution does not change with varying display scale, and the size of the smallest visible object depends directly on the size of the sensor cells or pixels in the electronic detector. In the case of digital imagery, ground sample distance (GSD) is, therefore, more appropriate as a measure for image scale (Comer et al., 1998).

Consider a digital camera with a charged-couple device (CCD): collection GSD is related to the size of each pixel element within the detector array. Using the scale calculations noted above, GSD can be determined as follows:

GSDs = (pixel element size)x Hg/f (Equation 2-3)

However, a single pixel usually cannot be identified as a unique object by itself. For visual identification of distinct

objects, generally a group of 4-9 pixels is the minimum necessary (Comer et al., 1998). This leads to a general rule of thumb (Hall, 1997).

• Positive recognition of objects in aerial photographs requires a GSD 3-5 times smaller than the object size.

Digital images as well as small-format analog photographs are rarely, if ever, displayed at the original camera scale, which would be much too small for normal visual examination. Most usually, digital images are enlarged substantially for display on a computer monitor, in which the dot pitch controls the image size and scale, assuming that 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.

Display scale = (monitor dot pitch) /GSD

(Equation 2-4)

As an example, take a digital vertical photograph acquired at a height of 100 m using a camera with a 35-mm lens focal length and CCD pixel element size of 0.009 mm.

FIGURE 2-3 Biological study site near Pueblo, Colorado, United States. The north arrow is 4 m long by 1 m wide; it provides both a scale bar and directional indicator for this vertical kite aerial photograph. Note two people standing next to the survey arrow. Photo date August 2003 (Aber et al., 2005).

FIGURE 2-3 Biological study site near Pueblo, Colorado, United States. The north arrow is 4 m long by 1 m wide; it provides both a scale bar and directional indicator for this vertical kite aerial photograph. Note two people standing next to the survey arrow. Photo date August 2003 (Aber et al., 2005).

Collection GSD would be (0.000009 x 100) o 0.035 = 0.026 m (~ 2.5 cm or 1 inch). Now, displaying this image at full size on a monitor with dot pitch of 0.26 mm, the display scale would be 0.00026 o 0.026 = 0.01 (or 1:100). A similar calculation can be done for printed digital images. The nominal pixel size for standard printing at 300 dpi (dots per inch) is about 0.085 mm. In this case, printed scale would be 0.000085 o 0.026 = 0.00327 (or about 1:300). Displaying or printing the image at smaller scales would mean losing some of its information content when viewing it on the screen or printout.

This example demonstrates that display and printed scales are usually many times greater than original digital image scale, because the display/print pixels are many times larger than the electronic detector pixel elements. The larger scales employed for display and printing of digital images do not imply more information or better interpretability, however, compared to the "raw" image data (Fig. 2-4). A digital number is simply a color value for a single pixel, regardless of the size at which the pixel is displayed.

So far, we have used the term resolution, which is employed in different ways in remote sensing, for describing the spatial aspects of small-format aerial photography. Other aspects of resolution include spectral and temporal dimensions, which may be equally or even more important than is spatial resolution for identification of certain objects. For example, color-infrared photography was developed originally for camouflage detection and is widely employed now for vegetation, soil, and water studies (Finney, 2007). The combination of green, red, and near-infrared radiation reveals objects that may appear similar in visible light (Fig. 2-5).

FIGURE 2-4 Enlargement of the arrow and people in the previous figure. This image contains no more spatial or spectral information than the previous image; each pixel represents exactly the same ground area and color as before. All the details visible in the enlarged image are present in the original image.

FIGURE 2-4 Enlargement of the arrow and people in the previous figure. This image contains no more spatial or spectral information than the previous image; each pixel represents exactly the same ground area and color as before. All the details visible in the enlarged image are present in the original image.

Temporal resolution refers to how objects change through time on scales ranging from diurnal to decadal. Deciduous vegetation, as an example, is strongly seasonal in character, and this situation may be exploited for identification of plant types (Fig. 2-6). Finally, the term radiometric resolution refers to the number of digital levels, also called precision, that the sensor uses for recording different intensities of radiation—usually 0-255 or 2 per image band for SFAP cameras.

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