MAPPING A CONTACT IN THE FIELD AND THEN FINDING WHERE IT ACTUALLY IS

During the past 15 years, Appalachian State University undergraduates have traced and mapped a well exposed contact across a grassy pasture in order to practice geologic mapping, gain experience using the Brunton compass, and learn to accurately make Brunton compass traverses of contacts across undulating terrain. The product was a map of the contact and a plot of the "surveyed" traverse. As each student individually processed group-collected traverse data, there were differences in the final maps. The project helped most students produce more accurate future maps and improve field techniques. In fall 2000, a new group of students returned to the same area to map the contact. This time the objective was to first mark contact exposures with flags and then map contact locations on a 1:6,000 topographic base map by walking it. Next, the class recorded time-averaged flagged contact locations using a hand-held Garmin GPS-12 unit. Then, contact locations were surveyed using a hand-held Atlanta Advantage Laser Rangefinder to record azimuth, slope distance, and slope angles. The electronic data were manually recorded by class members who processed the data without software and submitted maps that compared the three differently mapped contacts. Raw GPS data plotted the contact over 1000 m outside the location of the area as shown on the 7.5’ quadrangle map. When the GPS traverse overlay was moved and tied to a known location (the contact at a road intersection) and superimposed on the Laser Advantage traverse, there was excellent agreement. Conventionally mapped contact accuracy was up to 30 m in error for several flagged contact locations. The electronic data show the contact as previously mapped by a 50 m tape and Brunton had contact location errors of up to 15 m. Laser rangefinder combined with the GPS data provide an excellent standard that serves as a means for learning how to be better mappers who normally have to map conventionally.