The development of the coverages in the NRGIS Library represents a major investment of staff time and effort. As a professional responsibility, the Iowa DNR expects to receive proper credit when these GIS coverages are utilized. The Department also expects that the original documentation will remain with any redistribution of these coverages.
Two methods were used in georeferencing the township maps largely due to a change in software versions during the project. The first method used ArcInfo 9.0, with control points set at every section corner and using 3rd order polynomial transformation. The area using the first method is primarily the eastern third of the state where the greatest concentration of anthropological features occur on the maps and alignment was the most critical. The area includes the following counties and all of the counties east: Mitchell, Chickasaw, Bremer, Black Hawk, Benton, Iowa, Dallas, Warren, Monroe, eastern half of Wayne. Western counties' townships georeferenced with this method include: Fremont, Mills, Pottawattamie and Harrison.
The second method used ArcInfo 9.2, control points at the section corners of the township boundary only and Spline for the transformation. The area produced from this method is primarily the south central and western counties where map features were much less dense. These counties include: west half of Wayne, Lucas, Madison, Decatur, Ringgold, Taylor, Adams, Cass, Montgomery, Page, Monona, Woodbury and Plymouth.
Images were rectified as tif's with cell size of 4 m. The rectified images were then uploaded to the Iowa Geological Survey ftp site.
Due to a bug in ArcGIS 9.2 that has not been fixed yet with service pack 3, a work around had to be implemented that adds a few more process steps than what is required for normal georeferencing. The work around steps are posted on the ESRI forum at ArcGIS Desktop - Raster display and management <http://forums.esri.com/Thread.asp?c=93&f=1740&t=209585> under the posting by Melanie Riley.