Images of 7-1/2 minute series topographic quadrangle maps (1:24,000 scale) created from U.S. Geological Survey Digital Raster Graphics (DRG) data do not line up correctly with vector data when displayed together in ArcMap, even when the coordinate systems are defined correctly for the raster and vector data.This issue affects the DRGs downloaded directly from the U.S. Geological Survey website, as well as TOPO! data obtained from the National Geographic Society, and other images derived from the USGS source data.
Originally, DRGs were georeferenced to the North American Datum 1927. In this spatial reference, the corners of the DRGs are even values in decimal degrees.
For example, for the 'Norfolk South' quad, State of Virginia, ID number o36076-G3, the quad corners on the NAD 1927 datum are:
Or as expressed in degrees, minutes and seconds:
X-min -76 15 00
Y-min 36 45 00
X-max -76 22 30
Y-max 36 52 30
When this quad is projected to the North American Datum 1983 and the NAD_1927_To_NAD_1983_NADCON transformation is applied, the corner coordinates are no longer even values. The coordinates become:
Or expressed in degrees, minutes and seconds:
X-min -76 14 58.7796
Y-min 36 45 00.5328
X-max -76 22 28.7976
Y-max 36 52 30.5328
At this latitude and longitude, these differences result in a displacement of approximately 16 meters in the northing, and 30 meters in the easting for the image, in relation to projected vector data.
These offsets vary, depending on the location of the DRG within the 48 contiguous states.
Offsets also occur for DRGs for the states of Alaska and Hawaii, when transforming from NAD 1927 for Alaska, or the Old Hawaiian Datum for Hawaii, to NAD 1983.
Solution or Workaround
- Use ArcCatalog to define the coordinate system for the image data to the correct projection and datum for the raster, based on the metadata obtained from the data source.
a. In ArcCatalog, right-click the image and select Properties. On the Spatial Reference tab, examine the projection. If no projection is listed, click the Edit button, and on the Spatial Reference Properties dialog, click Select, and navigate to the appropriate projection file, based on the image metadata. Click Apply and OK as needed.
b. Select the image in the ArcCatalog Table of Contents, then press the F5 key on the keyboard to register the projection parameters to the image header, or to create an AUX file to store the parameters with the image.
To project the raster to a new coordinate system, go to ArcToolBox > Projections and Transformations > Raster > Project Raster, and project the raster image back to the original geographic coordinate system (datum) of the data. The correct geographic transformation method must be specified to insure correct results.
- In ArcGIS 8.x, use the Project Wizards in ArcToolbox to project vector data from NAD 1983 to NAD 1927 if necessary.
In ArcGIS 9.x, use the Project Raster Tool in ArcToolbox to project vector data from NAD 1983 to NAD 1927 if necessary.
Be sure to select the correct datum transformation method when projecting the vector data. See Related Information. The datum transformation to use in the lower 48 states between NAD 1927 and NAD 1983 is NADCON.
- Start a new, empty ArcMap session, and add the DRG or TOPO! images to ArcMap first, then add the reprojected vector data. If necessary, verify that the NADCON transformation method between NAD 1927 and NAD 1983 is specified in the ArcMap Data Frame. The data and images will align correctly.
The TOPO! images and some DRGs appear to be displayed in the wrong place, due to the expectation that the topo quads will have even corner coordinates. However, the corner coordinates are recalculated if the images have been reprojected to NAD 1983 and the NADCON transformation was applied.
- The alternatives to properly align vector data with the rasters in ArcMap are listed below.
* Use the Georeferencing Toolbar in ArcMap to adjust the spatial reference of the raster images to NAD 1983.
* Convert these images to grids, project the grids back to NAD 1927, then convert the images back to grids. This process requires an ArcInfo Workstation license.