PHOTO INTERPRETATION- Although most field locations are pre-selected with some consideration to accessibility, a number of sites are randomly chosen. Depending on project scale, accuracy requirements, and budget, Sewall also performs aerial reconnaissance. The amount of checking and choice of method depend upon the complexity of the classification system and accuracy requirements. On some projects, Sewall performs interpretation in conjunction with an inventory, in which field sampling observations are made independently of the photo interpretation process. Sewall performs photo interpretation directly from roll film on a Bausch and Lomb SIS 95 stereoscope. Coding frame-by-frame, The compant pay careful attention to completeness of line work, labeling, and edgematching between photos and flight lines.
FEATURE CONVERSION- Following photo interpretation, Sewall converts the photo-interpreted features, both lines and points, to an accurate, horizontally controlled digital base map. During this phase, Sewall clients can select one or a combination of several conversion options, including orthophoto creation, orthorectification using existing orthophotography, and monoscopic transfer.
DIGITAL MAP CREATION- A carefully prepared manuscript streamlines the conversion to a clean final product. To process scanned features into final GIS coverages, GIS analysts use ArcInfo editing software and standardized procedures to ensure completeness and correctness of polygon topology. Labeling is performed on ArcInfo workstations under the control of a software program custom written specifically for the project’s classification system in ESRI’s Arc Macro Language. Sewall analysts use this program to check that all coded values are valid and that all polygons have labels. To ensure that each label matches the corresponding photo-interpreted class, Sewall photo interpreters create a checkplot, which highlights any stands with incomplete or missing labels. The mapping process is complete when the final map is faithful to the photo interpretation.