Digital Shorelines for Boston Harbor
A traditional and still common method of preparing digital files is with a digitizing tablet using a "hockey puck" digitizer to encode data points. The source map or orthophoto is registered geographically, and the data points can then be defined relative to this system. Manual digitization is slow and is being widely supplanted by autodigitization and vectorization techniques. A range of representative systems is given in Table 1. These are commonly referred to as raster-to-vector conversion systems, or intelligent document conversion systems.
We have utilized a newly-released system, ABICAS, that runs in the DOS environment and is designed for use by individual scientists. It operates on raster images. If the source material is in multiple parts (e.g. adjacent aerial photographs or maps too large to be scanned in one piece.), images are georeferenced and then tiled together. All processing is done on a desktop computer.
|ArcScan||works within ArcInfo system||ESRI Corp.|
|Vectorizer||part of Microstation||Intergraph Corp.|
|Infotec, LT4X||free-standing unit||Pace Inc.|
|Vtrak||Unix-based b/w or color system||Laserscan Inc.|
|MapFlex 2000||free-standing unit||Audre Corp.|
|AutoCAD Map||works within Autocad system||AutoDesk Inc.|
|ABICAS||free-standing system||Innovative Technologies of America Inc.|
The scanned images are saved as 24-bit Targa files and then converted to the ABICAS format. They are processed to reduce the amount of dithering and pixilation of colors. The user then selects a color to follow and clicks two points on a line of that color. If Auto-Trace-Full is used ABICAS follows the line until it comes to a dead end or the edge of the screen, at which point the user may accept or reject the line. If Auto-Trace-Semi is used ABICAS will prompt the user to accept or reject each point as it is added. This can be quite useful if the line being digitized crosses other lines of the same color or has many tight bends. When using Auto-Trace-Semi the user can interrupt the tracing to add points manually, e.g. to take a coastline past a bridge or to close off a river mouth in a coastline.
The orthophotos used as the background for comparing shoreline files had a ground resolution of 0.5 x 0.5 meter, obtained from aerial photography at a (negative) scale of 1:30,000. The files we downloaded from the MIT/MassGIS web site had been resampled to reduce the number of pixels by a factor of 4, resulting in a file with ground resolution of 2.0 x 2.0 meters. The images had been rectified to Massachusetts State Plane, North American Datum 1983 (NAD83) by the cooperative program. In order to tile the images together we needed to convert the state plane coordinates to latitudes and longitudes. This was accomplished with CorpsCon software released by the U.S. Army Corps of Engineers (TEC, 1994). All sources of information (except MCOAST) were based on NAD83. The original topographic map data upon which the digital data are based were derived from 1978 overflights.
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