Image Integration and On-Screen Digitizing Method of Geographic Information System Update and Maintenance Applied to the Hofmann Forest

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Title: Image Integration and On-Screen Digitizing Method of Geographic Information System Update and Maintenance Applied to the Hofmann Forest
Author: Moore, Jennifer Anne
Advisors: Dr. Hugh A. Devine, Chair
Dr. Heather M. Cheshire, Member
Dr. E. Carlyle Franklin, Member
Abstract: The Hofmann Forest is a self-sustaining forest that provides the North Carolina State University College of Natural Resources with support for research, education, and extension service. The management of the Hofmann Forest requires data concerning historical records, complete and current resource inventory, and the ability to model future forest conditions. A geographic information system (GIS) database was created for the Hofmann Forest in 1992 to facilitate achievement of these data objectives. The database was not maintained or used regularly. Ongoing forestry research and silvicultural activities are constantly changing the resource conditions on the forest. This research examined a practical and accurate method for maintaining currency in the GIS database. Digital imagery was integrated into the original GIS database, and silvicultural records were used to update the existing data layers. Digital orthophotography, in the form of USGS Digital Orthophoto Quarter-Quads (DOQQs), was the primary source of imagery, but where the imagery was unavailable or contained insufficient spatial detail, unrectified aerial photographs were scanned, registered, and substituted. For the vegetation data layer of the GIS, spatial and attribute updates were completed and evaluated for silvicultural operations covering over three thousand acres. Some updates involved only changes in attributes. Spatial updates were completed with the digital orthophotography or digital aerial photographs; of these, some updates involved fairly simple spatial editing and others involved more complex spatial editing. The updates required the digital aerial photographs were all spatially complex edits. Acreage estimates accompanied the silvicultural records. GIS-derived area measurements were compared with those on the silvicultural records. There was not a significant difference between the two measures of area, however some discrepancies were present. A series of comparison tests were designed and performed to identify the potential elements of the area discrepancies. Spatial complexity of the editing procedure, different sources of digital imagery, and size of updated vegetation polygons were all examined. Degree of spatial complexity in the updates did not significantly contribute to area discrepancies. There was no significant difference in area discrepancies when either the DOQQs or digital aerial photographs were used. Size of the updated vegetation polygons was significantly negatively correlated with the discrepancies, showing that small absolute differences in area in small polygons result in large relative discrepancy values. Differentially corrected global positioning system (GPS) data were used to assess the horizontal positional accuracy of the GIS data layers. Following National Map Accuracy Standard (NMAS) guidelines, a sample of 25 'well-defined' locations were collected using a Trimble GPS Pathfinder ProXR receiver with real-time differential correction capabilities. These same locations were identified on the Roads layer of the GIS database, the DOQQs, and the digital aerial photography. Root mean-square error (RMSE) was calculated for each of the different data layers, using the GPS data as reference locations. Only the DOQQ-derived points met the NMAS Class 2 horizontal positional accuracy standard. RMSE for the aerial photography and Roads layer were greater than the limiting RMSE for the NMSE Class 3 standard. Based on these results, it can be concluded that DOQQs possess greater horizontal accuracy than the digital aerial photography and are the preferred imagery source for the on-screen digitizing. Should greater resolution be required for a database update, orthorectification of the digital aerial photography could be used to correct horizontal positional errors. Recently, software packages have become available to orthorectify aerial photographs effectively and affordably.The presence of extraneous features in the vegetation layer of the GIS database almost certainly contributes to the area discrepancies. Features such as windrows, fire ponds, and logging decks are included in vegetation polygon area but not silvicultural record area estimates. Future database improvements should consider subtracting these features (and their associated areas) from the vegetation layer and creating separate database layers for each type of feature. A methodology report was developed to accompany the GIS database as a reference for future updating. Continuous maintenance of the Hofmann Forest GIS database is necessary to provide timely information for on-site forest managers and research activities, and to preserve an account of forest conditions that may be useful in present and future management decisions. On-screen digitizing with integrated digital imagery proved to be a feasible method for updating and maintaining the Hofmann Forest GIS database.
Date: 2002-04-22
Degree: MS
Discipline: Forestry

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