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The scanned source maps were used as a reference for attributing.
Most geologic descriptions follow naming conventions found on the original source map. Some descriptions may be abbreviated. Several polygons were of an undetermined age because of printing problems or lack of clarity on original source map. There was no grouping of ages in the attributing process.
1) The original source map was provided in two plates at a scale of 1:2,500,000. Each plate was scanned at 300 dpi in RGB color mode. Adobe Photoshop 6.0 was used to merge east and west images together at edges. Some discrepancies were noticed on edge match. Edge mismatch was no greater than 500 m in a few areas, while good matches were found elsewhere. Discrepancies were averaged while digitizing.
2) A vector dataset originating from the National Imagery and Mapping Agency (NIMA) Digital Chart of the World (DCW) series was available for georeferencing. This dataset is known to have a nominal scale of 1:1,000,000 and was available as a shapefile from the United States Geological Survey, Digital Data Series DDS-62-A Global GIS Database: Digital Atlas of Central and South America.
3) The ArcMap 8.3 georeferencing tool was used to establish control points and reference merged image. The original maps were provided in Mercator projection with true scale at 19 degrees north latitude. Attempts were made to transform the reference shapefile to the stated projection for georeferencing, but major errors existed along southern edges of map. A geographic coordinate system (WGS_1984) was used with more success. 57 control points were established to perform a 3rd order polynomial transformation. Other transformation methods were attempted along with other control point placement, but resulted in more error. A root mean square (RMS) error of 1611.8 m was recorded.
4) Discrepancies were anticipated between source map shorelines and country boundaries because of scale difference. Other distortions may have been inherent in the source maps. Overall dimensions of map fit well at desired display scale of 1:2,500,000. Discrepancies in shoreline were random and variable, rarely exceeding 3,500 m.
Although all data and software published on this CD-ROM are used by the U.S. Geological Survey, no warranty, expressed or implied, is made by the U.S. Geological Survey as to the accuracy of the data and related materials and (or) the functioning of the software. The act of distribution shall not constitute any such warranty, and no responsibility is assumed by the U.S. Geological Survey in the use of these data, software, or related materials.