A Mountain Rescue Team Organizes to Map Trails
By Gary Manzer

This is the story of how in areas where GPS receivers regularly lose lock, the volunteer North Shore Rescue (NSR) Team of Vancouver, British Columbia used a series of interlocking technologies to accurately map mountainous recreation trails.
      NSR had just received digital 1:20,000 contour maps through the generosity of the Provincial Ministry of Environment Lands and Parks to assist them in the management of search operations. However, these maps did not show any of the hundreds of trails which are widely used by the urban population of Vancouver because the trails could not be mapped using aerial photography.
      This rugged mountainous terrain has some of the largest coniferous trees in Canada. Without accurate trail maps, Search and Rescue (SAR) managers have to rely on shared knowledge of the local terrain common to NSR members. SAR managers must have confidence that prime search areas are adequately searched before the search radius is expanded. With poor maps this confidence is difficult to achieve. Sections of prime areas could be searched many times, while other prime sections could be unintentionally neglected. The NSR executive committee determined that trail mapping was a team priority.
      GPS could not be used to map the trails because of the extremely tall trees and the limited horizon in the mountains. Moreover, real time differential GPS corrections cannot be easily received by FM signal in mountainous areas, due to poor signal reception.
      A method of ground survey had to be selected that was easy to manage using volunteer labor, yet could be relied upon to provide 10 meter absolute accuracy. This level of accuracy would ensure that the trails would be mapped to the same accuracy as the 1:20,000 digital basemap.
      The method selected used the following items: a GPS receiver capable of logging pseudorange 1 second observations, a digital laser rangefinder mounted on a mono pole equipped with a bubble level, a survey grade prism reflector on another mono pole also equipped with a bubble level, software for downloading digital observations to a computer, the British Columbia active control network, differential GPS software, a traversing software package, and software for translation to the mapping software on the SAR manager's laptop computer.
      As the number of trails on the mountains were too many to map during this first phase of the project, the NSR executive committee selected the priority trails to map. Pairs of NSR volunteers committed to spend most all the daylight hours for 21 consecutive days, walking and surveying these key trails. The volunteers were shown how to collect a 10 minute GPS pseudorange 1 second rate static observations marking the beginning and end of every trail using the Magellan Pro-Mark V GPS receiver. If no GPS point could be gathered at the trail's start, it was necessary to find an area in unobstructed satellite view and start the survey from that point. A manual log was kept to record the point number at the start and at the end of each trail. All trails had to start and end with a 10 minute GPS 1 second pseudorange observation.
      The Criterion 400 digital laser rangefinder made by Laser Technology of Englewood, Colo., was used to survey the trails. Two volunteers worked their way down each trail, one person with the rangefinder and the lead person carrying the survey grade reflector prism. Point by point the mono pole carrying the Criterion 400 would occupy the hole in the ground left by the mono pole carrying the reflector prism. At each position the Criterion 400 recorded and stored each point digitally as survey number, point number, azimuth, inclination, and horizontal distance. A new survey number was assigned to each trail surveyed.
      Each night both the beginning and end GPS points from the Magellan Pro-Mark V and the surveys stored as digital offsets in the Criterion 400 rangefinder were downloaded onto a PC using Traverse PC and Proprietary Magellan software. The Criterion's batteries were recharged and the GPS and digital laser rangefinder were prepared for the next day's work.
      Differential corrections necessary to bring the GPS start and end points were performed by post processing with the help of a government operated active control network. The British Columbia Ministry of Environment Lands and Parks maintains an active control network (ACN), consisting of first order GPS satellite receiving stations. One second data consisting of satellite data is recorded to files in one hour blocks of time. These ACN data files were downloaded from the government bulletin boards in Victoria, B.C. and Williams Lake, B.C.
      These ACN data in one hour time blocks corresponded to the same times as the pseudorange observations. These one second data provided two baselines, each less than 350 kilometers. Post processing was performed on the Magellan pseudorange 10 minute observations using the ACN information, thus differentially rectifying the start and end point of each trail. Final GPS positions accurate to within 5 meters after differential correction were calculated.
      These differentially corrected coordinates in NAD '83 UTM coordinates, provided the method of tagging the start and finish of the point by point traverse offsets recorded digitally with the Criterion 400.
     Using Traverse PC, a compass rule adjustment holding the two ends fixed was then performed on each trail. This adjustment spread any accumulating observation errors collected with the Criterion 400 laser rangefinder over the entire trail and coordinates were reliably calculated to within the 10 meter accuracy required in the project design.
      A DXF file showing the trail coordinates was then exported from the Traverse PC software and imported into the 1:20,000 digital MicroStation map supplied by the government, adding accurate trails for use by NSR.
      SAR managers have Intergraph's MicroStation Review software on a laptop computer to view, zoom and plot the sections of the maps necessary for conducting searches. This software from Intergraph does not allow search managers to change the MicroStation files but does allow the SAR manager to redline any area of the map requiring edits. In this way the integrity of the digital map can be supervised and edited separate from daily operations. Editing is carried out by an NSR volunteer using a full blown version of MicroStation CAD software.
      The volunteers of the NSR team found the digital laser rangefinder and GPS instruments easy to learn to use. The traverses using the Criterion 400 digital laser rangefinder were done in the rain and snow, over extremely rugged terrain and in freezing conditions. Typically, closed loop traverses closed to within a few meters, and one 400 point traverse of over 7,000 meters closed within 3 meters.
      In this manner, 28 key trails were accurately mapped in a 21 day period. Ten NSR volunteers are now trained in the procedures and are able to collect accurate trail data as the mapping project continues over the coming years.

About the Author:
Gary Manzer is marketing manager for Triathlon Mapping in Burnaby, British Columbia. He may be reached at 604-294-8861.

Back