| Topographic Mapping From Aerial Photographs
Scale vs Precision & Detail
By Rick Hanson, CP. Topographic mapping from aerial photographs is the most widely practiced application of photogrammetry. The most accurate and comprehensive Geographic Information Systems (GIS) are built on photogrammetric base maps. While today's buyers and users of GIS are becoming increasingly more sophisticated in their knowledge of the photogrammetric process, there are still those who rely on boilerplates in their requests for proposals to define their mapping accuracy requirements. Boilerplates are often copied from a document that was probably written for an entirely different purpose and, most likely, does not allow for the most recent advancements in technology. The purpose of this article is to arm prospective buyers of photogrammetric services with some basic knowledge about the precision and detail that may be extracted from a given aerial photo scale. This will hopefully allow someone to design their own aerial mapping specifications and throw away the boilerplate.
First, the buyer must define a set of standards by which the quality of the photogrammetric products will be measured. In 1941, the U. S. Bureau of the Budget issued the United States National Map Accuracy Standards (NMAS), a copy of which can be obtained on the Internet at http://mapping.usgs.gov/standards/. These standards were revised in 1943, and again in 1947. Though NMAS is more than fifty years old, it is the most well known and widely used and, best of all, it fits on one page. The USGS still applies these standards to their mapping products. More recently, the American Society for Photogrammetry and Remote Sensing (ASPRS) has developed a slightly more rigid set of standards, intended to apply more specifically to modern digital/analytical technology. Information about ASPRS standards can be found at http://www.asprs.org/resources.html.
Whichever standard the buyer chooses to specify is less important than ensuring that some specific criteria that set forth the spatial accuracy is to be expected for a given map scale and contour interval. Once the standard is established, the buyer and the photogrammetric consultant have a baseline for making the important decisions about scale, precision and detail.
For purposes of this discussion I will refer to NMAS for Large Scale Mapping wherever applicable. NMAS for large-scale maps (1"=40' to 1"=1600') basically says: Horizontally, not more than 10 percent of the points tested shall be in error by more than 1/50 of an inch, as measured at map scale. This means that, at a map scale of 1"=100', 90 percent of well defined features will be plotted within two feet of their true geographic positions. Vertically, as applied to contour maps of all scales, not more than 10 percent of elevations tested shall be in error by more than one-half the contour interval. This means that on a map with a two-foot contour interval, 90 percent of elevations not obscured by shadows or vegetation will be within one foot of the true elevation.
Having set the standards, the buyer must decide how accurate the map must be, based on need and budget. One way to do this is to query the prospective end-users as to their needs, but be careful. Some users will say they want the map to be as accurate as possible. It should be need, not want, that drives the final decision. Let's say that a 1"=40' map with 1/2-foot contours to NMAS is as accurate as one may realistically hope to achieve-this may be true because the FAA specifically forbids fixed-wing aircraft to fly lower than 1000 feet above people and structures, and a 1/2-foot contour map would require a flight height in the vicinity of 1000 feet above ground. Budget is an inescapable factor in planning a GIS project, and the reality is that it takes four times the number of aerial photographs to cover an area for 1/2-foot contours as it does to cover the same area for one-foot contour mapping. Consequently the 1/2-foot contours will cost more than three times the price of the one-foot contours. To strike a balance between cost and need, a municipality might choose to have the central business district mapped at 1"=50' with one-foot contours, the surrounding urban areas at 1"=100' with two-foot contours, and the less densely populated or rural areas at 1"=200' with five-foot contours.
Once the buyer has established the desired accuracy, the rest is simple arithmetic. Map scale, photo scale, contour interval and spatial precision are all mathematically interrelated. The photo scale is controlled by flight height above the terrain and the focal length of the aerial mapping camera (six inches). Flight height (h) divided by the focal length (f) equals photo scale (S), or S = h/f. Flight height is determined by multiplying the desired contour interval (c) by a C-factor (C), or h = c x C. C-factor is a variable applied by photogrammetrists that combines a number of elements including available technology, camera and film characteristics, intrinsic accuracy of the mapping instruments to be used, and the skill of the technicians who will perform the data collection. Because different photogrammetrists and mapping companies have varying access to these elements in varying degrees, C-factors may range from 1200 for companies with antiquated equipment and inexperienced technicians to 3000 for companies using state-of-the-art equipment and highly skilled technicians. Assuming a medium C-factor of 1800, the flight height required for a map with a contour interval of two feet will be 2 x 1800, or 3600 feet above ground. The scale of photography flown 3600 feet above ground will be 3600/6, or 1"=600'. Table 1 suggests some standard photo scales with associated flight heights, map scales, contour intervals and NMAS accuracy.
Because C-factor is so subjective, the buyer may choose not to specify the photo scale and/or flight height, so as not to restrict the photogrammetrist's ability to minimize cost by maximizing C-factor. The RFP might be phrased as follows, "The prospective vendor shall propose a photo scale and flight height sufficient to support National Map Accuracy Standards for two-foot contours at a map scale of 1"=100'." Compare the proposed flight heights and verify that the vendor of choice will employ the appropriate technology, equipment and staff to achieve the required accuracy. Require and check references.
Now the proposed buyer needs to decide which planimetric features will be required and determine whether or not the desired map scale will support the collection and depiction of those features. The smaller the map scale, the less planimetric detail the photogrammetrist is able to depict. For example, it is not reasonable to expect manholes to be collected at a map scale of 1"=200' or smaller. Manholes are too small to be visible from a height of 9000 feet above the ground unless they were targeted prior to the flight, and the targets would need to be at least five feet in diameter. It is also not practical to attempt to depict the actual size and shape of single family dwellings at a map scale of 1"=400', because most houses would be smaller than one-tenth of an inch on a side at map scale. Table 2 suggests the smallest map scales appropriate for some of the more frequently requested planimetric features.
Finally, if the buyer is still not sufficiently confident in determining the desired map scale with regard to accuracy and detail, there is no shortage of qualified consultants and vendors who would be happy to help assess a potential buyers' needs and present options and recomendations. About the Author:
Rick Hanson has worked in the aerial mapping industry since 1972. He is a project manager and certified photogrammetrist with the GIS Services team at Merrick & Company in Aurora, Colo. Questions and comments may be e-mailed to him at [email protected]. Back |
