Photogrammetry’s Place in the Geospatial Toolset
Photogrammetry is the science of making measurements from photographs — especially for recovering the exact positions of surface points.
“The problem is that it is hard to find a qualified photogrammetrist under the age of 50,” says Darryl Murdock, vice president of professional development for the U.S. Geospatial Intelligence Foundation. “Instead, what we have are many individuals who have geographic information systems (GIS) background, but very little analytics experience or exposure to other geospatial disciplines.”
Why does this matter?
“Several years ago, I received a phone call from an attorney on the east coast who was asking me about photogrammetry,” he says. “I was curious since so few individuals knew about the field, let alone persons who were outside of the industry.”
As it turned out, the attorney had a client who was suing a business establishment for negligence. The attorney’s client had slipped on a crack in the sidewalk directly in front of the business and was claiming that his resulting injury was the fault of the business because the business had a duty to maintain and to make safe the sidewalk in front of its entry so that patrons would not be exposed to the risk of tripping over the crack and injuring themselves.
The business argued that it was the city and not the business that was responsible for performing maintenance on the sidewalk, and that because of this the business should not be held liable. It cited a local ordinance on sidewalk repair that specified how large cracks in the pavement had to be for the city to assume responsibility for sidewalk maintenance and argued that the crack in the sidewalk was sufficiently large so that it met the requirements of this ordinance and was rightfully the city’s — not the business’s — responsibility.
“The law firm for the business hired an engineer who used an Internet-based mapping function to map the street and to measure the size of the crack,” Murdock says. “But the problem with this approach was that the map had very little information that accompanied it. It had the date that the photos were taken, but not the time of day, the point above the grade of the sidewalk that the photo was taken from, or the orientation of the camera. Without this information, it is hard to make an exact determination about the nature or the size of the crack.”
In contrast, when you use photogrammetry you can correct distortion in satellite images and adjust for the tilt of the plane as photos are taken. You can also make corrections that account for photos that are not taken at “straight down” angles, or for areas that are being photographed and that are not perfectly flat in their own right. “When you make these adjustments to photos and bring them into a truer perspective, you can sometimes find that there is a several thousand foot difference between how you thought an element you are photographing was positioned, and how it actually is positioned,” Murdock says.
What the outcome of the sidewalk negligence case was is unknown, but it is certain that the greater refinements of images and situations that photogrammetry could have provided was employed as a legal argument.
There is also a message to those in geospatial disciplines that every possible tool should be utilized if we are to arrive at the most exact expressions of geospatial truth.
“With the proper use of geospatial data, you can incorporate all kinds of data about a photo to maximize you accuracy,” Murdock says. “This is what we strive for as we advocate and provide for expanded geospatial training that can enrich the GIS and mapping skills that many people in the profession already have.”
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