drgclip

                                USGS DRGs have been accuracy tested. The accuracy test was performed by comparing the positions of UTM grid intersections on the source graphic against the corresponding location on the digital image. A visual inspection will determine whether those coordinates fall within the pixels that define a grid tick. USGS DRGs duplicate the horizontal accuracy of the source maps.
                              

                                National Geodetic Vertical Datum of 1929. USGS topographic maps utilize the vertical positional accuracy of the source. This accuracy is retained in USGS DRGs; however, the DRGCLIPs for Maine have been projected to NAD83 and vertical positional accuracy of elevation data may be affected by this conversion. The National Map Accuracy Standard applies to USGS topographic maps. At all map scales, the maximum allowable vertical tolerance is one half the published contour interval.
                              

                                The collar information was removed from USGS Digital Raster Graphic (DRGs) to create DRGCLIP images for Maine GIS.
                              

                                Originally clipped DRGs were created from standard USGS 1:24000 Digital Raster Graphics (DRGs) for use by the Maine Forest Service, as part of an Arcview application. USGS DRGs contain the white margin and legend areas found on paper USGS quadrangles. This is inconvenient for those operations needing to mosaic several adjacent quads, so the following procedure was established. The Maine Forest Service has statewide jurisdiction, the project area covered the entire state of Maine. MFS has nine ranger districts, and the finished clipped DRG products were grouped onto CDs corresponding to those districts. The work environment was established in the following steps. The base data was 1:24000 scale USGS DRG quads on CD, supplied by USGS EROS data center, dated April 1998. The DRGs were in a UTM NAD27 georeferenced TIFF format. The data was supplied on CDs conforming to 1:250000-scale tile structure. Each tile was identified by alphanumeric format, not by tile name. In order to conform to MEGIS tile structure, and for operator convenience, the finished products would need to have the appropriate quadrangle names. Each USGS-supplied CD had a text file containing the DRG identification code and the corresponding quad name. For each CD, this data was imported to a Microsoft Access file, edited, then exported to an Info file called Quad.dat. The purpose of this Info file was to select the appropriate TIFF file for each tile when processing, and to name the output file by its quadrangle names. DRGs were processed in batches corresponding to MFS districts. An Arc/Info coverage was created which represents the MFS ranger districts. The getdist.aml was used to establish a workspace for a given district. The AML then created a polygon coverage of the district boundaries by reselecting from the MFS district coverage.
                              

                                In October 2000, The Maine Department of Environmental Protection used the following process to create clipped DRGs projected to NAD83 and these were used to replace and upgrade original DRGCLIPS at MEGIS. All processes were completed using ArcInfo and ArcInfo GRID.  The DRG TIFF image was converted into an ArcInfo GRID using the "imagegrid" command.  To remove the collars from the DRG, the DRG grid was then clipped to the extent of the quadrangle using the MEGIS 7.5 minute map series index, 24KINDEX.  The collarless, DRG grids were then projected from NAD27 to NAD83 using the ArcInfo "project" command with the "grid" option.  The DRG grids were buffered by 100 meters and edges of adjacent grids were merged to fill in the "null" values providing data to complete "tiles" , and finally grids were converted back to TIFF images using the ArcInfo "gridimage" command.