With the aerial photographs digitized and recorded on CD-ROMS, they were brought into the field for the next step in the construction of the GIS---the collection of ground control points (GCPs). GCPs are features that can be identified in the aerial photograph, such as a house, dock or distinct topographic feature, in which their locations are established in the field using a GPS receiver. A total of 66 GCPs were collected in an even distribution over the entire study area during the field season (see Appendix 1). The GPS equipment used was a Trimble Geo Explorer 3, which has a accuracy of 1 to 5 meters. For each GCP at least 50 positional "fixes" were taken with the GPS receiver and then later using the GIS software the positions were averaged to obtain a highly accurate coordinate location. The reason for collecting GCPs is so that geometric rectification can be performed on the aerial photographs. This is a process using GIS software where the images, in this case the aerial photographs, are projected into an accurate coordinate system so that area, direction, and distance measurements can be performed.        Once the GCPs were collected we then georectified the 47 images in the field on a laptop computer using ESRI Arc/GIS and Arc/View software. Each of the GCPs were carefully collected so that they were evenly distributed throughout each of the images in the Universal Transverse Mercator coordinate system for zone 52 South (the appropriate zone for the study area). Once the images were georectified they can then be displayed in the GIS software in their precise geographical position on the earth's surface. For one part of the study area, the southern half of Vonvavona lagoon, the images were placed together and merged into one large file. This composite image is a called a mosaic, and we intend to create one large mosaic for the entire study area.       Dr. Aswani's Home Page  Back to Project Page UCSB Athropology Web Site