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DRGs - Digital Raster Graphics
Background:
The DRGs available on the download page were created by Beartooth Mapping out of Colorado and delivered to TNRIS in a GeoTIF format with a UTM NAD 27 projection. TNRIS created a new world file to re-project the image to UTM NAD 83.
Projection information has been application dependant in the past. For example: ESRI software at the time read the world file, Microstation read the header, and AutoCAD didn't read either. In order to correct this situation for all types of software, TNRIS has incorporated a different version of the DRGs clipped and rectified using the following process.
A DRG is imported to GRID via the 'imagegrid' command in ArcInfo. The appropriate 'clip' coverage is selected from the master listing of 'clip' coverages based on USGS topo grid system. The GRID of the DRG is then re-projected from its UTM zone to geographic, decimal degrees (DD), NAD 27, to form a second GRID. The resultant GRID from the projection routine is then 'clipped' using 'gridclip' command against the clip coverage (as described in Step 2; also DD NAD 27) to create an intermediate working GRID. The 'No Data' areas in the intermediate working GRID are then reset to the value '13' (13 is the first available number in the color map for standard USGS DRGs). To minimize the areas where the value '13' occur, the '.bnd' of the working GRID was deleted and then replaced by the '.bnd' from the clip cover to produce a final GRID. The final GRID is then both exported to TIF format using the 'gridimage' command in geographic DD NAD83, and re-projected to UTM NAD83 before being exported as a TIF image in UTM NAD83; also using the 'gridimage' command.
To modify the DRGs for seamless alignment, follow the steps below.
ArcMap:
1. In the Table of Contents, right-click on the DRG and open up ‘Properties’.
2. Click on ‘Unique Values’.
3. Scroll to <VALUE> 13 and right-click on the Symbol.
4. Select ‘Properties for selected colors…”
5. Click the ‘No Color’ button at the top of the color chart.
AutoCAD:
Type in ‘imageclip’ and follow the steps to draw a polygon around the image you want shown. This procedure will cut off the excess allowing the images to join properly and not overlap, which basically changes the frame of the image to not have the extra blank space.
Note: The procedure above has been tested by an AutoCAD user.