Image Scale and Resolution

One of the following questions is usually the first to be addressed in this context:

• What is the size of ground details to be distinguished, i.e., what GSD is desired?

• What is the size (width and length) of the area to be covered?

The answers to these questions determine at the least the target flying height and minimum number of images required, but they might also affect the choice of camera lens or even camera model and the platform to be used.

If the image GSD is of primary concern, the flying height above ground that is necessary for the target GSD can be calculated for a given lens focal length by transformation of Equation 3-4. This flying height and focal length in turn determine the area covered by the camera image format, either film or electronic detector (Eq. 3-3). If the calculated area turns out smaller than desired, there are now three possibilities:

• Change lens to a shorter focal length or increase flying height. Both would result in a larger area being covered, but also would proportionally increase GSD.

• Cover study area with multiple overlapping and adjacent photographs rather than a single image. This would result in additional field effort and expenses during image processing as the individual images have to be georeferenced, mosaicked, and color-balanced (see Chapter 11.2).

• Change image pixel size by choosing a camera with smaller CCD element size. For analog photography, use film with higher photographic resolution and scan the photograph with higher spatial resolution.

If the size of the area covered by a single photograph is of primary concern, the width and length of the area are usually of greater interest than its extension in units of area. Consider a study site or object with a certain shape: its width and length would have to fit onto the image format completely if it is to be captured by a single image. Unless the width/length ratio is identical with that of the camera sensor or film negative, care has to be taken that the target flying height is calculated using the side of the area that is longer in comparison to the image. This would result in the image covering more than the required area in one direction, rather than cropping the area in the other direction (see below). Remember that all these calculations only apply for the ideal case of the camera being in exactly the right position at exactly the right height; depending on the controllability of the SFAP platform and camera alignment, a generous amount of error margin should always be allowed.

When deciding on the area to be covered by the photographs for a monitoring project, the expected changes—e.g. the upslope movement of gully headcuts, the shifting of river meanders, or the retreat of glaciers—have to be taken into account from the beginning. The image area should be chosen to provide enough room for the expected changes, so the time series would in the earlier images also show the yet unchanged areas and allow for comparison and interpretation of the changes documented in the later images.

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