3D VIRTUAL OUTCROP: CONCEPTION, CONSTRUCTION AND APPLICATION

A 3D virtual outcrop was created with precision digital mapping instruments and techniques, GIS, and outcrop photo mosaics of NW Harbor Island, East Muscongus Bay, Maine, to assess its potential as an educational and analytical resource. Electronic Total Stations (ETS) (+/- 0.5 cm) were used to collect ~1.07 points/m2 over the 2343 m2 outcrop (a total of 2513 points) and imported into GIS to create a TIN (Triangular Irregular Network) , and ultimately to generate a raster digital elevation model (DEM), representing the outcrop's fine scale topography. Elevations range from -1.42 to +4.09 meters above mean sea-level and are best classified at 0.25 m intervals with contours displayed every 0.5 m, with no vertical exaggeration. Low-elevation aerial photos were taken of the outcrop surface from a height of ~6 m with an Olympus 4.1 megapixel digital camera and the aid of a telescoping, aluminum camera-pole. Adobe Photoshop CS was used to create a seamless mosaic from 214 original photos. Each frame measured ~3.5 x 5 m with ~1 m overlap between adjacent photos. The full outcrop photo mosaic was georeferenced using RTK GPS (+/- 1 cm) surveyed control points and then draped over the DEM using ArcScene. The final resolution of the full outcrop mosaic image is a product of the original digital photography and pixel degradation occurring during the rendering process. The original images have a resolution of 2272 x 1704 pixels per frame; each pixel covering 0.23 x 0.23 cm of ground surface. The images maintained a 1:1 pixel to raster cell resolution during mosaicking and importation to ArcGIS as a raster dataset. Georeferencing the raster image warped cells, creating non-orthodeclinated parallelograms. The images were then rectified using the default cell size in ArcMap (0.54 x 0.30 cm or 0.49 x 0.49 cm) creating orthodeclinated raster cells which were then resampled using a bilinear interpolation for a smoother image. When the photo mosaic was draped over the DEM, the cells were stretched over sloped surfaces, further degrading the image. The virtual outcrop serves as an excellent visualization tool, and the photo mosaic is an effective base for onscreen digitizing of smaller geologic features in outcrop. However, the construction process is very time consuming, using large and cumbersome image files that are difficult to process.