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Digitizing Aerial Photography - Understanding Spatial Resolution

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Digitizing Aerial Photography - Understanding Spatial Resolution Digitizing Aerial Photography - Understanding Spatial Resolution Trisalyn Nelson Mike Wulder Olaf Niemann M.Sc. Candidate Research Scientist Professor University of Victoria Pacific Forestry Centre University of Victoria (250) 721-7349 (250) 363-6090 (250) 721-7329 email: [email protected] [email protected] [email protected] Abstract Accuracy, flexibility, and cost effectiveness are advantages of using digitized aerial photographs as a data source. However, the relationship between the resolution of digital images, and the aerial photographs from which they were derived, must be addressed. In the following paper we consider issues that impact the spatial resolution of photographs and digitized images, and suggest how users can optimize spatial resolution by selecting an appropriate scanning aperture. Optimal scanning aperture can be chosen by considering the camera system resolution, the original photograph’s scale, and the desired pixel size of the digital image. There are optimal scanning resolutions to use when digitizing aerial photographs. Optimizing spatial resolution will maximize the spatial information obtained from the original photographs without generating unnecessarily large file sizes. Introduction Digitized aerial photography is increasingly the scanning aperture, thereby allowing being used as a data source for studying and important image information to be retained and managing environmental elements such as trees file sizes to be minimized. Choice of scanning (Niemann et al., 1999) and forests (Leckie et al., aperture is related to the resolution of the 1999; Niemann et al., 1999). The popularity of original photograph and/or the desired pixel size digitizing aerial photography is a result of the of the digital image. advantages of working with digitally formatted Although in this paper we only consider the data (Warner et al., 1996). For example, digital affect of scanning aperture on spatial resolution, imagery can be manipulated with relative ease it is recognized there are other impacting factors. allowing for atmospheric and geometric For example, systematic errors resulting from corrections, image enhancements, data shifts in the position of points within the pixel compression, and viewing options (Lillesand pattern reduces spatial resolution (Trinder, and Kiefer, 1994). As well, digital images 1987). As well, fitting homogenous square provide potential for semi-automated image pixels to landscapes which are heterogeneous in analysis (Barbezat and Jacot, 1999; Gougeon, shape and size will adversely affect spatial 1999). One disadvantage of converting resolution (Fisher, 1997). Target and film photographs into digital imagery is that spatial contrast also affect the resolution of imagery; the resolution is reduced during the digitizing higher the contrast the better the potential to process (Warner et al., 1996). resolve unique features (Avery and Berlin, Spatial resolution is one factor which 1992). Additionally, scanner and input image governs the information content of an image. quality impact the spatial resolution of digitized Digitizing photography with the smallest imagery. Although we do not discuss it is in available scanning aperture reduces loss of this paper, it should be noted that other types of spatial resolution, however such an approach resolution such as reflectance and colour affect produces file sizes which are unnecessarily large the usability of digital imagery. and unwieldy to use. In this paper we suggest Depending on the anticipated use of digital techniques to optimize, rather than maximize, imagery, different approaches should be used to determine the scanning aperture required to the width of the lines (Lillesand and Kiefer, optimize spatial resolution. If digital imagery is 1994). The size of the smallest lines to be used for visual interpretation, a visual distinguishable on the film is the resolution of comparison of images scanned with varying the camera system (expressed in line pairs per sized apertures should be made. The largest millimetre). scanning aperture which provides all required Camera lens and film are the key information is best used to minimize file size. components which affect camera system Recent operational work by British Columbia’s resolution. The speed of a lens, or aperture Ministry of Environment Lands and Parks setting, controls the amount of light available at (MELP) suggest that when producing digital the film’s surface. As the aperture opens to imagery for visual interpretation, a scanning increase light at the film’s surface, image aperture between 10-14 microns will optimize sharpness decreases and vice versa. Therefore, the spatial resolution when digitizing film; and a the aperture setting is a compromise between scanning aperture of 36 microns is optimal when image sharpness and the amount of light digitizing prints (Fish, 2000). required for exposure. Not all digital imagery is used for visual The film’s emulsion is composed of silver interpretation. Often it is used as input for halide grains, the size of which affects the image processing. Particularly when imagery is amount of light required to expose the film and used with semi-automated techniques, visual the graininess of the image. As silver halide interpretability becomes secondary to grains decrease in size image resolution maintaining the digital information required for improves, but more light is required for processing. In this paper our goal is to present exposure. Resolution may be sacrificed in order methods for selecting a scanning aperture which to achieve appropriate exposures (Avery and will optimize spatial resolution when converting Berlin, 1992 ). aerial photography film into digital imagery that is to be used for image processing. In the following paper we define spatial resolution, present methods for optimizing spatial resolution, and discuss example results of the spatial resolution optimization techniques used when scanning aerial photographs. Spatial Resolution Aerial Photography and Spatial Resolution Figure 1. Tri-bar resolution test chart (Avery and Berlin, 1992). Camera System Resolution There are many ways to define the spatial Measurements of camera system resolution resolution of a photograph or digital image. do not take into account the effects of converting Table 1 is a summary of the spatial resolution the image from a negative to photographic measurements discussed in this paper. The paper. Generally, the spatial resolution is spatial resolution of aerial photography is decreased during printing as the emulsion of the defined as the capability of a camera system to photographic paper has a lower spatial image spatial detail (Avery and Berlin, 1992 ). resolution than the emulsion of the film. In this paper we use Avery and Berlin’s (1992) Therefore, the camera system resolution is only definition of spatial resolution to define camera an accurate measure of the film, not the print, system resolution which is measured using a tri- resolution. bar resolution test chart (Figure 1). The tri-bar chart consists of consecutively smaller groups of three parallel lines separated by spaces equal to Table 1. Summary of the resolution measurements discussed in this paper. Adjacent resolution definitions for aerial photography and digital imagery are comparable. For example, the closest digital resolution measurement to camera system resolution is scanning aperture. Aerial Photography Digital Imagery Resolution Definition Units Resolution Definition Units Camera The capability of the camera line Scanning The pixel size used to sample microns System and film system to image pairs Aperture an image during digitizing. Resolution spatial detail (Avery and per mm As the size of the scanning Berlin, 1992). aperture decreases the spatial resolution theoretically increases. Ground The number of line pairs line Resolution which represent 1 m on the pairs ground (Avery and Berlin, per m 1992). Minimum The size of the smallest object cm or Pixel Size The ground area represented cm or m Detectable which can be detected. m by a specific size of scanning Object MDOS is equivalent to the aperture, when used in Size ground distance represented conjunction with a certain by one line of a line per mm scale of imagery. (MDOS) pair (Avery and Berlin, 1992). Effective The smallest object that can cm or Effective The smallest object that can cm or m Resolution be separated from the m Resolution be separated from the background. Similar to background. Similar to pixel MDOS, but also considers the size, but also considers the effects of contrast, effects of contrast, atmosphere, etc, (Hyppanen, atmosphere, etc. (Hyppanen, 1996). 1996). Ground Resolution Ground resolution considers the effect of camera system resolution decreases, the ground scale and camera system resolution on the ability resolution also decreases. to identify ground objects (Avery and Berlin, 1992). Ground resolution is equivalent to the Minimum Detectable Object Size number of line pairs it takes to represent one Once the ground resolution is calculated, the metre on the ground and is calculated with the minimum detectable object size (MDOS) can be following equation: determined. The MDOS is the equivalent to the (Rs)(F) ground distance represented by one line of a line Rg = (1) per millimetre pair as measured by the tri-bar H chart (Avery and Berlin, 1992 ). To determine where: Rg = ground resolution in line pairs per the MDOS first calculate the ground resolution, metre then determine the
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