Methods Outline and Discussion |
Preparing the GIS Surface Effects Maps |
Outline:
Discussion: The primary objectives of this work were to preserve the original surface effects maps and to make these maps available in easily accessible formats. In its final form, the archive contains GIS surface effects maps that are exact digital reproductions of the original, hand-drawn maps. Each of the original, post-detonation surface effects maps were reproduced using digital scanning, georeferencing, and manual digitizing techniques as outlined above. These digital reproductions were subsequently assembled and organized into the GIS maps and databases contained in the archive. The original maps were scanned at resolutions of 300 to 600 dots per inch to preserve map detail and accuracy. One-by-one, each scanned map was georeferenced to a common map coordinate system (Nevada State Plane, Central, in feet, North American Datum 1927) using 4-12 ground control (reference) points placed at known grid coordinates. A new raster-based, georeferenced map was produced by resampling the map, pixel-by-pixel using a bilinear algorithm applied to the ground control points. Pixel sizes of 0.5 and 1.0 meter were used for large-scale (1:6,000) and small-scale (1:12,000) maps. The final georeferenced maps were inserted into the GIS mapping program as a background layer from which each detonation’s surface effects could be digitized.
Digitizing refers to the drawing or tracing of points, lines, and polygons from a reference map. While often done using a digitizing tablet, the operator has little control over the original scale of the map, and therefore, the resolution and precision of the digitizing process. Another method of digitizing, which was used for this project, is to digitize on-screen from a georeferenced, raster-based original map. This approach allows the operator to zoom in to very large map scales and control not only the resolution and precision of digitizing, but also the placement of the line segments and nodes that form the various surface effects and symbols shown on the maps. The surface effects maps contained in the archive were digitized on-screen at very large scales. For most areas, a digitizing scale of 1:500 (1 inch equals 42 feet) to 1:1,200 (1 inch equals 100 feet) was used. For areas containing very complex surface effects, however, a digitizing scale as large as 1:50 (1 inch equals 4.2 feet) was necessary. As part of the digitizing process, each map element (surface effects feature) was identified (attributed) with such unique information as NTS Operational Area, borehole (siteid) number, feature type, feature length, and source map information. These spatial and attribute data were then combined into a comprehensive set of GIS map databases for each of the main testing areas of the Nevada Test Site.
Using the attributes of the GIS surface effects maps and the sites database the user can easily query and retrieve maps and specific surface effects information. The query and retrieval process can be used to prepare individual surface effects maps or composite surface effects maps of multiple detonation sites. This attribute information provides the real power behind the GIS archive. Using these database, various types of maps can be prepared. The GIS surface effects map of Site U-4d is an example of an individual site map that was produced from the archive. Similarly, the GIS surface effects map of Site U-4d and Surrounding Sites is an example of a composite map of multiple detonation sites. These maps illustrate not only the complex pattern of surface effects that were produced by underground testing in the area, but also the abundance of detailed map information that was prepared for underground nuclear tests conducted in this part of Yucca Flat. Map Reproduction Accuracy The accuracy of the digital surface effects maps is equivalent to that of the originals. That is to say, the methods used to transform the original analog maps to digital format yielded very precise results. One way to quantify this precision is to compare the locations of control points on the digital maps with their actual, on-the-ground locations. The horizontal distance between these locations is referred to as residual error. Measurements taken of residual error at ground control points during the production of these maps revealed that the scanned and georeferenced maps often had maximum residual errors of approximately one meter. In some cases this error was as large as three meters for some of the oldest and most poorly preserved maps.
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