When looking for the best way to calculate solid waste volumes at existing and potential landfills, some engineers and site operators have turned to digital photogrammetry.
Landfill owners and operators can use the process, which incorporates overlapping aerial photographs shot at various ranges and elevations to calculate landfill volume. Traditional aerial mapping yields a flat, one-dimensional line map that shows elevations of topographical features; digital photogrammetry, however, shows an x and y location on every point, according to Doug Isbell, director of solid waste services, Black & Veatch, Phoenix. This in turn allows computers to draw a two-dimensional model that can be compared from year to year as the landfill is filled.
The digital surface supplied by photogrammetry can be manipulated to supply contour maps and to supply volumes to determine the quantity of material put into the landfill between any two photographic dates. Photogrammetry also can supply automatic cross-sections and profiles at any interval and frequency, as well as perspective (three-dimensional) plots of sites from any vantage point, according to David Lotspeich, Analytical Photogrammetric Surveys Inc., Riverside, Calif.
"In recent years, more and more computer software has become commercially available," enabling engineering design and construction companies to use digital photogrammetry, said Isbell. Isbell most recently used photogrammetry to help design a 100-acre landfill site at Salt River, near Scottsdale, Ariz., for the Pima-Maricopa Indian Community.
Digital photogrammetry aims to reduce potential human error. When calculating landfill volume manually, you have to depend on the meticulousness of the people you've selected to do the job, said Dale Roberson, designer, BKK Landfill, West Covina, Calif. "A lot of measurements must be taken. If someone is not in the mood to [calculate] right, there can be some errors." Photogrammetry, on the other hand, according to Roberson, is more accurate, with only a five to 10 percent error rate.
Photogrammetry "is much more accurate than field surveys," said Bill Young of the Riverside County, Calif., Flood Control. Most recently, his agency used digital photogrammetry to determine which type of clay cap was necessary for the Lake Elsinore Landfills, a site with a cover ranging from two to four feet in thickness. "We did both a field grid and a photogrammetric process for the site. Photogrammetry provided the same result, but was five times cheaper," he said.
Isbell used a digital terrain model derived from photogrammetry to design a new landfill in Arizona. With photogrammetry "we found that we could design a landfill, then estimate excavations using different design configurations. It also allowed us to go through a number of different scenarios," Isbell said.
Several benefits continued to surface including: time saved; design alternatives; more choices for engineers to present information on landfill volumes; accurate calculations, from one year to the next, of fill and "borrow" material (similar to cover material); and less cost than traditional surveying (see chart).
Young estimated that for one particular site a survey crew was needed for eight to 12 days averaging $1,500 per day. Photogrammetry requires two one-day flights at $1,200 each.
With less time and money needed for projects, engineers and site operators are finding that photogrammetry is a viable alternative to calculating landfill volume.