| AIRBORNE: Laser Terrain Mapping for Cut and Fill Solutions
By Mark Romano Late one Friday afternoon John Lohr, Plant Manager of Southdown Inc., in Lyons, Colo., was faced with a dilemma. Southdown had obtained approval of a reclamation plan by the State Mined Land Reclamation Board (MLRB). The plan encompassed flooding Southdown's limestone mine pit with water from the Saint Vrain River during the spring runoff period, so it could be used as an irrigation as a reservoir for the arid Colorado summer months. Southdown had also secured all proper permits with the Boulder County Commissioners office, State Engineer's office and Water Court. Everything was going smoothly until the time came to open the head gate. When Southdown notified the water commissioner at the State Engineer's office to begin the flooding process a problem emerged. They wanted to know accurate pit volumes. With no current topographic or volumetric information available, and spring runoff beginning, Lohr needed a solution fast. The Solution
Lohr contacted EagleScan Inc. in Boulder, Colo., which formulated a plan to fly and collect data the following Monday with its Digital Airborne Topographic Imaging System (DATIS) and use the data to produce a high resolution terrain map and volumetric assessment of the area.
Monday brought overcast skies and high winds which would normally complicate the data collection using standard aerial photography. Due to the timeliness of the situation the project was flown. Provided there is no cloud cover below the aircraft operational altitude (6000ft AGL nominal), DATIS projects can be flown under these types of adverse weather situations. Flight altitudes could be adjusted on the fly as needed although Monday's conditions did not require adjustment. Even under the adverse conditions data collection took less than two hours.
On Tuesday, EagleScan completed processing of Southdown's data, and a volumetric study was delivered with acre feet volumes calculated at 1-foot increments, This one day turn-around enabled Southdown to immediately respond to the water commissioner's request and gain approval for the water diversions. Bill Gambrell, water commissioner from the Division of Water Resources, reported this was the "fastest project turn around I had ever seen." Gambrell also stated, "Under the best circumstances, it typically takes weeks to produce this type of study." Additionally, a contour map and images of the project site were delivered to Southdown the next day.
Since LIDAR systems directly measure topography no film development, scanning, or stereo plotting is required to generate Digital Elevation Models (DEMs), and resulting value added products such as contour maps or volumetric assessments. This in turn enables rapid project turnaround since data is digital to begin with and the need for film and stereo plotting is eliminated. Projects that once took months to years can now be accomplished in days to weeks. In mining and other industries regulated by government agencies, permitting and other requirements often require rapid response. Down time is a significant disadvantage in this highly competitive industry. Accuracy
The DATIS system contains a high-accuracy scanning laser topographic mapping LIDAR system coupled with an on-board GPS, inertial measurement unit, and high-resolution digital camera. System parameters such as laser pulse rate, swath width, and image size are adjustable to meet end-user needs.
The high sample density of the LIDAR system provided an extremely accurate volume assessment and contour map of the Southdown mine site. The DATIS system measured over 60,000 elevation data points in the 65-acre mine site. Traditional survey approaches do not cost-effectively yield these types of sample densities. For extremely undulating terrain such as the pit mine, greater sample density means a more accurate representation of the exact terrain or surface model, and hence a more accurate volumetric analysis. Figure 1 shows an example of a pit mine terrain model. This dramatic model is the direct result of combining high resolution LIDAR topographic data and digital camera techniques.
With the immediate problem solved, Southdown and EagleScan, in conjunction with the NASA Earth Observations Commercial Application Program (EOCAP) (contract # NCC13-12), decided to put the new process to the test. Measuring smaller volumes such as stock pile inventories is a major part of mine companies yearly inventory process. Depending on the mine type and company, volume studies are performed quarterly, bi-annually, or annually for inventory reporting. Errors can mean the difference between yearly profit or loss in this highly competitive market where jobs are won and lost over just a few cents per ton. Typically, stock piles are measured with traditional aerial photogrammetric or ground survey techniques. In both cases a minimal amount of points are measured using the traditional techniques. For comparison, Southdown chose a stockpile that had been measured by ground survey and was also very accurately weighed before being deposited. The pile size approximately 91 x 118 feet had already been measured with a single fly over of the DATIS to produce over 200 measured points. Table 1 compares the results.
This independent accuracy validation provides a more than adequate solution for stockpile measurement. Additional Benefits
Additionally, LIDAR systems function with fewer constraints due to weather and seasonal constraints such as snow and vegetation. LIDAR systems can provide data collection even at night if imagery is not required. Shadowing effects due to platform angular displacement can reduce or prevent acquiring accurate ground observations with traditional photogrammetric approaches. High-density urban and metropolitan areas are extremely difficult to map due to obstructions and shadowing effects. The DATIS addresses this problem directly with high sample densities and steep (+/- 10 degree scan angle) laser trajectory. True ground heights can be obtained where traditional mapping techniques would fall short.
Although not required for this particular job, excellent images were acquired from the digital camera in spite of the severe weather. The DATIS differs from traditional film systems, which would not have been flown under such severe conditions because of flat light and aircraft turbulence which result in extreme pitch and roll attitudes. Imagery acquired under poor conditions can be post-processed to provide extremely usable imagery, even under mixed cloud cover varying from direct sunlight to dark overcast terrain.
Unlike other current LIDAR platforms, the DATIS has a critically aligned digital camera system with optics calibrated to photogrammetric standards. This enables the DATIS to go a step beyond basic contour mapping. Orthophotos can be rapidly produced from the digital imagery without the use of ground control paneling. Proprietary processes enable automated orthophoto generation and mosaics. This translates directly to significant schedule time and cost savings. Additionally, planimetric features such as roads, hydrology, and other historical data can be extracted and provided on the base map or as separate layers as customer needs warrant. The Future
The independent accuracy validations shown with the Southdown project, coupled with rapid turn around, reduced seasonal or weather constraints, and competitive cost give new laser mapping technology high marks for application in mining and other land based applications. As laser and digital camera technologies mature, and GIS users requirements for high resolution data increase, systems such as the DATIS will be used for many new applications. About the Author:
Mark Romano is manager, applications engineering at EagleScan, Inc. in Boulder, Colo. Back |
