- SPECIAL REPORTS
- THE MAGAZINE
In the span of 30 miles as the crow flies, the Larimer County Canal network zigzags for 100 linear miles across northern Colorado. Part of an irrigation ditch network that supplies water to farmers and municipalities, the canal is a vital asset to the region.
In 2011, with the demand for water constantly increasing, the company that manages the network needed to enhance its ability to model equitable and legal distribution of the precious resource at various points throughout the system. In addition, repairs of the decades-old earthen structure had to be planned, budgeted and contracted.
With only a 30-year-old paper sketch map known to exist, the irrigation company opted to develop a GIS for better management of the canal. Having no previous experience with geospatial technologies, the company looked to Ayres Associates, a national engineering design firm with an office in Fort Collins, Colo., to provide a highly accurate digital basemap of the network. Ayres asked Western Research & Development (Western) to submit a proposal for an airborne LiDAR survey of one 30-mile segment of the corridor.
Ayres showed the data to its client, and they were impressed with the detail found in both the elevation points and imagery. Western won the contract, which was immediately expanded to include the entire 100-mile length of the canal network.
As a licensed surveyor and engineer, Grigsby founded Western to offer a full line of civil engineering and ground surveying services. With the advent of airborne LiDAR, he recognized the laser scanner could provide elevation datasets that were superior to ground surveys in certain types of applications–like the Larimer Canal–thanks to the saturation of point collection. A ground survey of the canal would have captured cross-section profile points at 100-foot intervals, leaving the rest to be interpolated and possibly missing important features in between.
“The LiDAR doesn’t miss any points, and the contours generated from the point cloud yield extremely accurate results in hydraulic flow models,” says Grigsby. “The civil engineer can sit in his office and cut cross sections of the canal anywhere he wants [using visualization software].”
Looking for a market niche, he knew that a helicopter could provide a higher density of points-per-meter due to its slower airspeed compared to a fixed-wing aircraft. He purchased the Bell 206L Long Ranger, which sports a rear horizontal stabilizer, making it a steady remote sensing platform even at low speeds. Western placed the chopper in service to offer aerial surveys of pipelines, roads, power lines, drainages and other corridors where high point density is preferable and the more maneuverable helicopter bests an airplane in efficiency.
“In terms of cost-competitiveness, we have found the helicopter is comparable to fixed-wing aircraft for projects up to about 200 square miles in total size,” Grigsby says. “But the helicopter [LiDAR] puts down four times as many points per square meter on a single pass.”
For LiDAR, Western chose the Leica ALS60 laser scanner with multi-pulse option, primarily because it was the most powerful available at the time and therefore could provide the highest point density, even in vegetated environments. The firm decided on the RCD105 medium-format camera due to its large 39-megapixel capacity in an eight-pound body. At the 300-meter average collection altitude of the helicopter, the camera would easily achieve 3 to 4 centimeter spatial resolution, as detailed as needed for any corridor project.
“The digital camera adds so much information content to every application,” Grigsby says. “We have never had a client turn down the offer to buy the imagery along with the LiDAR.”
If the client sees an anomaly in the point cloud, they just look at the high-resolution imagery to identify what it is, he explained. That’s a huge cost savings compared against the alternative of sending a crew back into the field to investigate.
Western initiated the acquisition portion of the project in March 2011. Flying at an average speed of 60 knots per hour and 300 meters above ground level, the helicopter collected a 200-meter-wide swath of elevation points and image data covering the 10- to 20-meter-wide canal and adjoining service road with plenty of room to spare. Due to low sun angle in March, the flights were conducted in mid-afternoon to ensure the bottom of the 1.5- to 3-meter-deep structure was illuminated. This limited each day’s collection window to less than three hours.
With the canal twisting and turning along its path across the county, flight planning software was used to divide it into 207 linear tracks. Just as an airplane would fly the mission, the helicopter flew each segment in a straight line, quickly looping back to start the next. Since planned flight lines could be flown from either direction, Grigsby chose the heading each day that took the chopper into the wind, which allowed him to fly at a lower relative ground speed and hit the surface with a greater density of LiDAR points. “The ALS60 has a built-in low-resolution video camera that films each day’s route,” he says. “Every afternoon, we posted these small video files on our FTP site for Ayres to follow the progress.”
Ayres used the video to determine if there were any areas that should be surveyed by a ground crew. At that time of year, most of the canal was dry, but the video revealed several spots where water had pooled in the bottom. Knowing the LiDAR couldn’t penetrate water and using the video as a guide, Ayres dispatched crews to those areas for onsite collection of elevation points at the floor of the canal.
As delivered to Ayres, the LiDAR dataset contained 25 to 30 points per square meter with accuracies of 4 centimeters horizontal and 4 centimeters vertical. The spatial resolution of the color imagery was about 6 centimeters. Ayres’ Fort Collins and Madison, Wis., offices collaborated on generating high-quality bare-earth digital elevation models and surface contours from the point cloud in a variety of file formats for use in hydraulic and hydrologic modeling simulations. “With the high density of the LiDAR data, we could view it at half-foot contours,” Alvarado says.
Ayres has assisted the irrigation company with building an ArcGIS database representing the canal, and integrating the LiDAR and image data into it. The company has already begun running hydraulic models with the data to simulate different operating scenarios for the irrigation network. One of the primary objectives is to determine how delivery of specific amounts of water at various outlet points impacts flow and volumes elsewhere in the system.
As time goes on, the LiDAR and digital images can be updated with additional flights, and the irrigation company can analyze how the flows effect changes in the structure of the canal. This will allow the engineer to adjust the design of the canal to increase the life expectancy and minimize possible failure points.
“The combination of highly accurate LiDAR data and imagery gives them all the information they need to efficiently operate and maintain the canal network,” Grigsby says.
For more information about Ayres Associates, visit www.ayresassociates.com. For more information about Western Research & Development, visit www.wrd-ltd.com. More details about Leica LiDAR and camera systems can be found at www.leica-geosystems.us.