ABSTRACT: An evaluation of two field survey technologies for geomorphological mapping: real-time kinematic GPS versus a robotic total station.

CGRG Bibliography of Canadian Geomorphology
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Abstract

Field survey technologies for geomorphological mapping have advanced at a remarkable rate over the last few decades. In particular, improvements in Global Positioning Systems (GPS) and total stations make it now possible for one person to collect high-resolution topographic data. While many developments in these technologies have been considered from the viewpoint of geodetic applications, their advantages and disadvantages in geomorphological mapping situations has been given limited attention. This case study evaluates the precision and versatility of a RTK GPS (Real-Time Kinematic Global Positioning System) and a RTS (Robotic Total Station) for topographic mapping under different terrain conditions. Surveys were conducted in February at an active landslide along a grassland hillslope and at a forested floodplain site near Lethbridge, AB. By using two Trimble® products (R7 GPS and S6 total station) we were able to combine the technologies for this investigation with Trimble’s Integrated SurveyingTM system. This provides both physical integration, whereby the RTK rover antenna is secured directly onto the RTS prism, and seamless data integration using Trimble’s Bluetooth®-enabled survey controller and software. Results from the case studies illustrate several advantages and disadvantages of the two survey technologies depending on terrain conditions. At the landslide both technologies provided mm- to cm-scale horizontal and vertical precision; however, the RTK GPS provided a clear advantage in terms of versatility. In particular, the RTK GPS does not require line of sight between the base station and rover in order acquire precise measurements. Furthermore, under ideal conditions the RTK rover can survey topographic points with cm-scale precision up to 10 km from the base station, whereas the RTS requires line of sight between the station and prism in order to acquire measurements, and is limited to a maximum separation distance of 1 km for automatic tracking of the prism. At the floodplain site the vertical precision of measurements acquired with the RTK GPS decreased substantially, ranging from 0.005 m in open areas to 1.95 m in heavily forested areas. We expect the precision of the RTK to decrease further when tree canopy is present. Conversely, the RTS provided mm-scale precision but failed to acquire 7% of the measurements due to obstructions (branches) between the prism and station. Overall, this study demonstrates that RTK GPS is well-suited for cm-scale geomorphological mapping in open areas. In forested settings, satellite signal multipath can substantially reduce the precision of measurements, limiting the ability to develop accurate digital terrain models. Although the RTS is generally less versatile in both settings because the station must be moved to maintain line of sight, it is more reliable for obtaining precise measurements.

Bibliography of Canadian Geomorphology