PRECISION DIGITAL TECHNIQUES FOR MAPPING MULTI-SCALE OUTCROP STRUCTURE: AN INTEGRATED GIS-BASED APPROACH

GPS, total stations and GIS are used in an integrated system of precision digital mapping of structural features in coastal Maine outcrops to better understand basic crustal processes such as dike intrusion, strike-slip faulting, and shearing deformation. GIS allows the display, coordination and analysis of digital maps and georeferenced photos over a wide range of scales. Precision digital instruments including dual frequency RTK GPS (2 cm precision) and electronic total stations (<1cm precision) are used to map intricate structural detail digitizing features directly on the outcrop surface. Handheld WAAS-enabled GPS (~1m precision) is used to map outcrop outlines, record sample locations and to position and log structural orientation data. All RTK GPS and total station survey data are collected in metric units in a UTM coordinate system tied to 2-hour static GPS datum points post-processed through NOAA's Online Positioning Users Service web site (www.ngs.noaa.gov/OPUS). Points and point sets for lines and polygons are exported and converted to layers in ArcGIS. Shoulder harnessed laptops allow field editing at the outcrop. Topo maps, geologic maps, orthoquad photos and standard aerial photos in georeferenced digital formats provide a backdrop for detailed mapping and contribute additional larger scale data. New hi-resolution (15 cm) digital aerial images are useful in mapping high contrast outcrop scale features through on-screen digitizing. Hand-held digital cameras create outcrop photos and mosaics that can be assembled and georeferenced to mapped controls. Thin section-scale features from oriented samples can also be incorporated into the database using digital images from a flat bed film scanner and on-screen digitizing. Strain analysis in GIS includes the determination of line lengths, angles and surface areas used to calculate elongations and gamma shear strains for deformation of syntectonic intrusions as well as average dike thickness, dike intensity and % extension for dike swarm intrusions. Surface models of alternate z-values from an attribute database can be used to show the spatial relationships of selected descriptive parameters such as strike or dip of foliation in relation to regional shearing with intriguing analytical potential.