Don’t confuse the photo scale with the final mapping scale. For the typical project, the photo scale will be several times that of the map scale. Most times the multiplier will vary between six and 10 times. For example, photography captured at a scale of 800 feet per inch is often used to produce mapping at a scale of 100 feet per inch.
With such a wide range of photographic scales available to the mapping professional, the challenge comes from selecting the right scale for each project. And of course, cost plays a significant role in the decision. Projects become cheaper as the photo scale becomes smaller. The reasoning is simple—fewer photos are required to cover the project area when the photo scale is decreased. Fewer photos mean fewer control points, reduced control costs in the office and fewer hours in the mapping process.
If the wrong scale is chosen, you will either pay too much for the mapping or lose out on the accuracy, features or resolution you require.
Three Main ConsiderationsTo minimize the project cost, the challenge exists to select the smallest photo scale that can meet the requirements of the project. Not too long ago the photogrammetrist was only concerned with the final map scale, the features to be collected and the contour interval. But in today’s digital world, the mix is a little more complex. When digital imagery is produced, the resolution of the imagery also becomes an important consideration.
We know the project cost decreases as the photo scale decreases. But this comes at a price. Smaller photo scales also result in less accuracy for the planimetric features captured in the mapping, less accuracy in the elevation data (and therefore a larger contour interval) and a limitation on the resolution of the digital imagery that can be produced from the photography.
The mapping professional will carefully consider all three factors when planning a project. And it is extremely important for you to maintain open conversation when you are participating in the planning process.
Map ScaleThe first consideration in the planning of aerial photography is the intended final map scale for the project. A mapping firm is typically required to meet the National Map Accuracy Standards (NMAS) as published by the United States Geological Survey. This bible for accuracy in a completed project places requirements for the absolute accuracy in any planimetric features captured in the mapping. So, the photogrammetrist must select a photographic scale in which they feel comfortable meeting the accuracy requirements.
The ability to meet these map accuracy specifications from smaller scale photography has increased significantly over the last 10 years or so. We have technology to thank for this increased ability. GPS surveys can produce much better accuracy for ground control points. The camera technology in use today is far superior to older technology. The film used to capture the photography is much better. And the office tools have also greatly improved. All of these factors directly affect the accuracy of mapping that can be realized from aerial photography.
But accuracy isn’t the only factor. The number of features that can be clearly identified in the project photography is reduced as the photographic scale is reduced. The aircraft flies higher to acquire smaller scale photography. When this happens, the features become smaller. And contrast in the photography is lost due to the increased flying height. As the aircraft flies higher, the photography is shot through more of the atmosphere, and therefore more haze and dust particles appear. Features that can be clearly identified at one photographic scale may be lost as this scale is reduced. For example, utility poles, fire hydrants, manholes and private fences may be easily identified in photography captured at a scale of 600 feet per inch. Conversely, they may not be visible at all in photography captured at 1,500 feet per inch.
Contour IntervalThe second consideration in planning a project is the contour interval required for the project. Once again, the NMAS impose a requirement on the photogrammetrist to meet an accuracy target in the mapping. A mapping professional has a number of mathematical formulas they can rely on when equating the photographic scale to the accuracy of the elevation data, and therefore the contour interval produced from the photography. In reality, most photogrammetrists know their craft well enough that they can quickly determine an appropriate photographic scale without relying on these formulas when given the required contour interval.
There are a number of natural combinations of map scale and contour interval. These combinations generally arose from the ability to produce the accuracies required for the map scale and contour interval from common photography. For example, mapping produced at a scale of 100 feet per inch will typically include a 2-foot contour interval while mapping with a scale of 50 feet per inch will include 1-foot contours.
Image ResolutionIn today’s digital world, the resolution of digital imagery produced from the photography is often an important consideration. More features are visible in the digital imagery at higher resolutions. And today’s desktop computers have more processing power, better video cards, increased RAM and large hard drives. The result is the ability to manage higher resolution images.
Traditional film imagery is converted to a digital format by scanning the aerial negatives or film diapositives (a positive transparency made directly from the negative) on a precision photogrammetric scanner. These scanners work on the same principles as a typical desktop scanner you might purchase for a few hundred dollars. The photogrammetric scanner, however, has a much higher geometric accuracy (the pixels fall in the exact same location in the digital image as they do in the film original) and produce imagery of superior radiometric quality. Unlike the typical desktop scanner, the photogrammetric scanner can weigh in excess of 100 pounds and cost more than $100,000.
The resolution of these scanners is typically listed in the units of microns (1/1000 of a millimeter) instead of the more traditional measure of dots per inch (dpi). You can convert between the two units by dividing the constant 25,400 by the resolution in microns. The highest resolution on these scanners can be 7.5 microns or better—the equivalent of 3387 dpi. However, the practical limit is somewhat less. Scans made at such a high resolution will appear grainy—picking up the individual silver halide grains on the film.
While the highest practical resolution is somewhat debatable, an affective argument could be made for a limit in the range of 15 microns, or 1,693 dpi. At this resolution, a 0.5-foot ground pixel can be produced from 800 scale photography or a 1.0-foot ground pixel can be produced from 1,600 scale photography. Carefully consider the options when digital imagery is to be produced.