A THREE DIMENSIONAL APPROACH: DIGITAL TOPOGRAPHY OF SEGUIN AND SALTER ISLANDS, MAINE

Detailed topographic surveys were completed for Seguin and Salter Islands, mid-coast Maine to enhance interpretation of geological and cultural data collected as part of a broader study. Trimble 5700 RTK GPS instruments set on 10m continuous topo were preferred in data collection for their mobility despite poor or no signals in tall vegetation and behind rocky high points. Data was acquired in inaccessible areas with Trimble Geodimeter 608 Series Total Stations established over RTK referenced datums and corresponding reference objects. All elevation data was exported from the instruments as .csv files and imported into ArcGIS where it was converted to a raster file with a 0.5m cell size by way of a TIN. This process proved more accurate than converting directly from a shapefile which limited interpolation. Base level was established by placing points assigned elevations of 0m above mean sea level along the shoreline on a high resolution georeferenced and orthorectified aerial image. Contours were created from the raster, with best results for Seguin with 47m relief at a 5m interval and for Salter with 16m relief at a 2.5m interval. Contour aesthetics were improved by removing extraneous topo lines with the editor function and by smoothing, utilizing the PAEK algorithm set to a tolerance of 15m. High resolution digital island images were imported into ESRI's ArcScene software and draped over corresponding TIN files to produce an accurate and interactive 3D topographic rendering. Cultural and geologic features were extruded from the land surface and made partially transparent. This manner of data visualization was particularly useful in fold analysis on the south end of Seguin Island, in interpreting the location of granite contacts via changes in elevation on the north end of Salter Island and in considering effects of resistant granites and metamorphic layers to yield glacially-streamlined ridges in the island interiors and steep shoreline exposures where facing incoming wave erosion. Additional attempts were made to correlate island topography with regional topography to define overall fold structure of the Robinhood Cove Syncline. The end product of this topographic survey is an interactive, georeferenced, 3D island rendering which enhances the spatial thinking process required by complex analytical studies.