Global Quality Services, Inc.

Scan Rate, Ground Pixel Size & Scale

The scan rate is directly related to the scale of the photography and the desired ground pixel size (GPS). Dividing the scale of the photography by the scan rate derives the GPS, which is usually the driving factor in any orthophoto project. In metric units this yields pixel size in meters and in English units this yields pixel size in feet.

There are many considerations when deciding what scale to fly a project. For instance:

Below we have scanned two sets of imagery from the same camera. The scale of the first is at 1:3000 and the other is at 1:6000. The scan rates were chosen to correspond to whole pixel sizes in feet. This imagery table will show the level of detail at certain pixel sizes and also the loss of detail that occurs when scanning at very high resolutions. Some films hold up better to high resolution scanning than others. T his film is AGFA HX100, which tends to hold up rather well even below 10um.

1:3000		1:6000
2 foot pixels	Too low resolution for this scale.		2 foot pixels
1 foot pixels			1 foot pixels
0.5 foot pixels			0.5 foot pixels
0.25 foot pixels			0.25 foot pixels
0.1476 foot pixels			0.1476 foot pixels
0.125 foot pixels		Exceeds maximum resolution for this scale.	0.125 foot pixels
0.0738 foot pixels		Exceeds maximum resolution for this scale.	0.0738 foot pixels
1:3000 Scale Full Range			1:6000 Scale Full Range

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