INTEGRATED PRECISION DIGITAL MAPPING TECHNIQUES FOR STRUCTURAL GEOLOGY IN CASCO BAY, MAINE

Integrated precision digital mapping techniques were used to study syntectonic granite intrusions at three locations in Casco Bay, Maine. Sea kayaks enabled the team to transport gear with minimal environmental impact and land on coastal outcrops not otherwise accessible. These techniques were able to capture fine-scale detail over large areas, providing insight into structural processes by mapping features too small to see on traditional air photos but too large for conventional outcrop mapping. Trimble 5700 dual frequency GPS receivers gathered static data for post processing to establish a series of datums. Two hour static observations were uploaded to the National Geodetic Survey's Online Positioning User Service (OPUS) web site where positions were processed relative to three nearby Continually Operating Reference Stations (CORS base stations), yielding results accurate to <1cm. Real-time kinematic GPS, with onboard corrections broadcast from a field base station on established datum, was used for topographic survey, structural data collection and mapping of large intrusions with best results accurate to 2.0 cm. Spectra Precision Geodimeter 608 series total stations yielding best results accurate to 1.0 cm were used to survey intrusion geometry. Total stations were mounted on GPS datums so all map data would be in a single georeferenced coordinate system. Surveying strategies were employed to reduce mapping errors, such as mounting total station prisms at the base of range poles or using a stationary bipod mount when collecting survey data points. Total station and GPS data was transferred to laptop computers in the field using PCMCIA cards, converted to ASCII format in Excel and imported into ArcView GIS. Laptops in shoulder harnesses enabled direct field editing of mapped features as shapefiles, further enhancing mapping accuracy. The resulting map images are sufficiently precise for structural analysis and are compatible with larger features such as soil and tide lines, large granite bodies or foliation trends that can be digitized on-screen from georeferenced digital air-photos.