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Digital Mapping Techniques '02 -- Workshop Proceedings
U.S. Geological Survey Open-File Report 02-370

Raster to Vector Conversion of Geologic Maps: Using R2V from Able Software Corporation

By Kent D. Brown

Utah Geological Survey
1594 W. North Temple
Salt Lake City, Utah 84116
Telephone: (801) 537-3300
Fax: (801) 537-3400
e-mail: kentbrown@utah.gov

INTRODUCTION

Raster to vector conversion of geologic maps has proven to be a valuable asset in the creation of a GIS database. However, the ability of applications to vectorize scanned (raster) maps varies widely. Most vectorizing software is optimized for the archival of old engineering drawings, architectural plans, and historical documents, where most lines meet at right angles. Geologic maps do not vectorize well with many of these programs.

The Mapping Program of the Utah Geological Survey (UGS) has attempted to scan and vectorize geologic maps of various types with mixed results. The software we used made workers frustrated with the amount of time required to vectorize a map, and the editing tools were inefficient and not user friendly.

In an effort to minimize our table digitizing and increase efficiency and positional accuracy, we evaluated five commercially available vectorizing programs. There was no real scientific method to our evaluation. We downloaded trial versions of the software and vectorized the same map using each application. Although these applications are also capable of heads-up digitizing and interactive vectorizing (line following), the focus of this test was to autovectorize the entire map. This paper summarizes the autovectorization process using R2V from Able Software Corporation.

OVERVIEW OF METHODS

Recently the UGS vectorized 32 7.5-minute geologic quadrangles to compile a 30 x 60 minute quadrangle. The source maps were paper topographic base maps on which the author had inked lines and labels and polygons were hand colored in pencil. The maps were scanned and saved as 300-dpi, 24-bit, color JPEG files.

The JPEG files were opened in R2V (Figure 1), and a process was used to eliminate the colors, preserving just the black line features from the author's original mapping. Because the base-map features were colored over with pencil, the software did not interpret them as black and they could be eliminated almost entirely, leaving the inked contacts, faults, labels, and leader lines (Figure 2). Further image editing was done (Figure 3) to eliminate the leader lines, labels, and any trace of the base map that remained.

JPEG of original map opened in R2V       Figure 1. JPEG of original map opened in R2V.

Colors from the original jpg have been removed       Figure 2. Colors from the original have been removed.

Only lines to be used for autovectorizing remain in the jpg       Figure 3. Only lines to be used for autovectorizing remain.

Next, the autovectorizing tool was used. This tool traces the pixels on the raster, so the resulting vectors have a stair-step appearance with too many vertices. The "smooth lines" command was used to eliminate many of the vertices that were redundant, making the lines look better. Because R2V supports layer creation, we chose to separate geologic contacts and faults onto different layers for easier data management. After manually editing the whole map, the vectors were suitable for use in our GIS (Figure 4).

 Vectors ready for export and use in GIS

Figure 4. Vectors ready for export and use in GIS. Pixelation seen here is an artifact of the computer screen capture.

The vectors were then georeferenced by placing control points on the corners of the quadrangle and assigning correct UTM coordinates to them. Georeferencing is the final step before the lines are exported as a DXF file. We chose DXF because the layers are preserved, saving much time in GIS by importing layers from the DXF and attributing the whole layer at once. If exported as a shapefile, all layers are combined into one and we find this to be less useful.

CONCLUSIONS

R2V is designed for GIS users who are making maps and includes many useful and easy-to-use tools for that purpose. Other programs were designed with engineering drawings as the focus and are less useful for maps. Using R2V, georeferencing is easy to perform and image warping and world file creation tools are included. R2V, a Windows 98/NT/2000/XP program, is designed for exporting vectorized data directly to ArcInfo generate (.gen), ArcView shapefile (.shp), DXF (.dxf), MapInfo (.mif), IGES (.igs), and a few other common formats. At the Utah Geological Survey, we found R2V to be a very useful and efficient tool for vectorizing geologic maps in preparation for GIS map production.


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