Laser Ranging on Turtle Mountain (Electronic Distance Measurement)

What is an Electronic Distance Measurement Device?

Electronic Distance Measurement (EDM) technology uses a ground-based, high-precision survey instrument (a theodolite) that emits a laser pulse to measure the time the laser beam travels from the theodolite to a mirror prism installed on the moving rock face. This is also called laser ranging. In theory, as the rock face moves downslope (toward the survey instrument), the distance between the survey instrument and the prism will decrease. Using this time, the distance can be precisely determined 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 applied EDM technology on Turtle Mountain. They 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 Geomatics Engineering Department (formerly Survey Engineering) at the University of Calgary took new readings - the first in 20 years. They compared the prism coordinates to the original 1982 EDM measurements to see if there was any movement. According to this analysis, only the distance from 'a' to 'c' had significant movement, which was a decrease of 15.7 mm. This movement corresponds to an average displacement rate of 0.65 mm/year over the 24-year period.

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, approximately three kilometres away, inside a small municipal building to obtain high-precision distance measurements to each prism each hour. These measurements are accurate to approximately 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 are 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.