Removing the Map Collar from a DRG File in Didger 3 and Exporting as a GeoTIFF without Losing Resolution

Introduction

One of the many features of Didger 3 is the ability to automatically remove the map collar from a USGS Digital Raster Graphic (DRG) file. DRG files are scanned topographic maps that typically have collars. The map collar is the area around the map that contains information such as projection, quadrangle location, latitude/longitude and UTM tick marks, map scale, etc.

The limits of the collar are stored in an associated FGD file. This FGD file is necessary to use the Image | Clip Bitmap | Map Collar command. If your DRG did not come with an FGD file, it is possible to create one manually. Open a text editor, like Notepad, and type in the West, East, North, and South bounding coordinates of the map in the following format. Save the file to your hard drive and rename the extension to *.fgd.

If you do not have the FGD file for the DRG, you can create one
manually by entering in the bounding coordinates of the map
in a text editor and saving the file to an FGD extension.

Removing the collar from a DRG with an FGD file is simple. However, Didger will always export georeferenced image files from a vector project at a resolution of 300 dots per inch (DPI). Based on the physical size of the vector project, this may result in an exported image that contains significantly fewer pixels than the original image, resulting in a loss of image quality. To export a higher number of pixels and increase the resolution, you must go to View | Project Limits and increase the size (length) of the project. Once the project is larger, you can export it to a GeoTIFF and you will retain more pixels and therefore better resolution.

To retain the original resolution of the image, the key is to determine what exactly to enter for the length values. To do that, you can follow these steps:

1 - Import the GeoTIFF, noting the original bitmap dimensions.

1. Go to File | Import Bitmap | Into Vector Project.
2. Select the DRG file (in TIF format).
3. If the File Info is not displayed at the bottom of the dialog box, click the File Info >> button.
4. Write down the Image Height shown in the File Info display.
5. Click Open.
6. In the Bitmap Import Options dialog, choose Embedded GeoTIFF Parameters as the Spatial Reference and click OK.
    
   
   

   Select the georeferenced image file, making a note of the
   Image Height in the File Info section, and click Open.

2 - Determine the default export size in pixels.

1. Go to File | Export.
2. In the Export dialog, choose Tagged Image (TIFF) as the Save as type, enter a file name (any file name will do), and click Save. This will take you to the Tagged Image (TIFF) Export dialog.
3. Write down the Bitmap size in pixels for Height that has been calculated by Didger and click Cancel. In most cases, the pixel count will be smaller than the original image size as seen in step 1d.
    
   
   

   Note the default Height in pixels and then hit Cancel.

3 - Adjust Project Limits and Scale

1. Go to View | Project Limits to display the Project Limits and Scale dialog.
2. In the Project Limits and Scale dialog, check both check boxes at the bottom next to Set Proportional X/Y Scaling and Clip Map Objects During Export.
3. Calculate the ratio between the original size of the image and the default size of the image Didger was going to export. In this example, the original height was 6720 pixels and the default export height showed 2500 pixels. This gives a ratio of 6720 / 2500 = 2.688
4. Multiple the current Length for the Y Axis Scaling by the ratio calculated above and enter this new Length. In this example, the original Length is 10.00, so 10.00 * 2.688 = 26.88. When you enter the new Y Axis Scaling, the Length of the X Axis Scaling automatically updates.
5. Click OK
    
   
   

   Calculate the ratio between the original Image Height in pixels and
   the default export Height in pixels. Multiple the Y Axis Scaling
   Length by this ratio and enter in the new number.

4 - Clip the map collar.

1. Click on the bitmap to select it.
2. Use Image | Clip Bitmap | Map Collar to remove the collar.
    
   
   

   Clip the map collar.

5 - Export the clipped map.

1. Go to File | Export
2. In the Export dialog, choose TIFF as the Save as type, enter a file name, and click Save
3. In the Tagged Image (TIFF) Export dialog box, the Bitmap size in pixels is reported. This number will be slightly smaller than the original because of the clipping. Click OK to save the file.
    
   
   

   Export the clipped image.