Laser Ranging on Turtle Mountain (Electronic Distance Measurement)

What is Electronic Distance Measurement?

Electronic distance measurement (EDM) (also called laser ranging) uses lasers and mirrors to measure the mountain's movement. A theodolite emits a laser pulse to a mirror prism that's installed on the moving rock face. The instrument measures how long it takes the laser beam to travel from the theodolite.

In theory, as the rock face moves down the mountain slope (toward the survey instrument), the distance between the instrument and the prism will decrease. Using this time, we can precisely measured the distance to one part per million (i.e., one millimetre over a distance of one kilometre).

EDM Monitoring on Turtle Mountain

In the early 1980s, the Survey Engineering Department at the University of Calgary first used EDM technology on Turtle Mountain. It installed a series of remotely monitored reflective prisms on the west side of the south peak of Turtle Mountain. We measure these prisms from four locations on the valley floor, and they range between three and seven kilometres from the mountain peak.

In 2006, the University of Calgary team took new readings - the first in 20 years. It compared the prism co-ordinates to the original 1982 EDM measurements to see if there was any movement. According to this analysis, the average movement is 0.65 mm per year.

Between 2005 and 2007, an array of 20 prisms were installed on the eastern face of Turtle Mountain. We mounted a computer-controlled, laser-ranging theodolite in a protected area on the valley bottom, about three kilometres away, inside a small building to obtain high-precision distance measurements to each prism every hour. These measurements are accurate to about three millimetres. McElhanney Engineering Services of Vancouver, British Columbia, designed and installed this system.

Four prisms are mounted on cement pillars, with GPS stations, to keep them above the snow. The other 15 prisms sit on the east face of South Peak and lower down the mountain.

Results

In summer 2007, the prism and theodolite installation for the modern EDM system was completed. Since then, we have collected data hourly for the 19 prisms on the eastern face of the mountain. One of the unique aspects of the Turtle Mountain EDM system is that it uses co-located prisms with GPS antennas to counteract errors associated with shooting the laser at long ranges through variable atmospheric conditions. Below is a sample of the data from this system.

EDM target installed on Turtle Mountain in 1982. Reflective prism inside metal canister. Click on images to enlarge.