COMPARISON OF TABLET AND ONSCREEN DIGITIZATION TECHNIQUES OF SHORELINES FROM AERIAL PHOTOGRAPHS: A CASE STUDY OF COASTAL GEORGIA

A recent study focused on delineating coastal erosion of Georgia’s barrier islands has prompted the evaluation of techniques used to extract shoreline position data. Such investigations are critical to the accuracy which erosion rates and shoreline position forecasting are calculated. The two primary techniques evaluated were (1) digitizing the wet/dry shoreline from registered aerial photos using a digitizing tablet and (2) onscreen digitization of rectified aerial photos. In the “tablet” technique, aerial photographs were registered in ArcView 3.2 on a Calcomp digitizing tablet. Four to twenty ground control points (GCPs) were selected and accepted if the root-mean-squared (RMS) value was 0.004 inches or better. The “onscreen” technique involved scanning aerial photos using a Microtek ScanMaker9600xl at 1200 dpi. The scanned images were then georeferenced using ERDAS Imagine 8.5. Root Mean Square (RMS) errors in registering the digital images were comparable to those derived from digitizing tablet techniques in ArcView 3.2. Shorelines were digitized into ArcView, saved as shapefiles, and projected to a common datum and spheroid.

A comparison of shorelines derived from both techniques was performed using ArcView by measuring deviations of shoreline position from a reference USGS Digital Orthophotoquad. The study shows evidence that the “onscreen” method reduces both human and technical error introduced from the “tablet” method. One clear advantage to the “onscreen” method is that it allows the georegistered images to be archived for later analysis without the need for re-registering. Hence, onscreen digitizing improves upon the spatial accuracy of previously digitized shoreline data collected using a digitizing tablet. Future efforts continue to concentrate on improving both digitizing techniques and change-analysis methods to better assess accretion and erosion along the Georgia coast.