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Creating GeoTIFF Format Georeferenced MVUMs

Home : Where To Go : Forest Service Motor Vehicle Use Maps : Creating Georeferenced MVUMs

A georeferenced MVUM is the starting point for creating MVUM background maps to use on cell phones, GPS units, and laptops for real-time navigation. Georeferenced MVUMs are also useful to see where you have been after you've recorded a GPS track while out riding or driving.

Needed Software

Two pieces of software are used to convert an MVUM into a georeferenced TIFF file. The first piece of software is used to rasterize the Adobe Acrobat PDF MVUM file into a Portable Network Graphics (PNG) bitmap file. I used Adobe Photoshop CS3 Extended to perform this task, but other, less expensive software can also be used.

The second piece of software is used to georeference the PNG bitmap and convert it into a GeoTIFF file. I used the full version of Global Mapper 11. Fugawi and OziExplorer should have this capability too. In addition, software such as Google Earth, Garmin MapSource, or National Geographic Topo can be useful for checking the accuracy of the newly created georeferenced MVUM.

Converting the PDF into a PNG File

In Adobe Photoshop CS3, select File -> Open from the menu bar. Use the open file dialog box to find the PDF of the MVUM on the hard drive. Select the MVUM and click Open.

Photoshop will present an Import PDF dialog box. In this dialog box, select the following then click OK. Now would be a good time to go get a cup of coffee or other snack.

  1. Select crop to bounding box.
  2. Check Anti-aliased.
  3. Check Constrain Proportions
  4. Enter 300 pixels/inch for the resolution.
  5. Select Grayscale for the mode.
  6. Select 8 bit for the Bit Depth

When the import is completed, some areas of the MVUM will have transparency. To eliminate the transparency and render the map on a white background, select Layer -> Flatten Image from the menu bar.

Since I like to display multiple MVUMs side-by-side in Global Mapper, I prefer to eliminate most of the details outside the map and the map collar such as the legend, instructions, etc. To do this, I use the Crop Tool (C) to crop my MVUMs to just the map and map collar. This is strictly a personal preference so if you'd prefer to keep this information in your GeoTIFF, you may skip this step.

Now for a tricky step. If you examine the collar of the map, you'll notice latitudes and longitudes printed around the collar of the map. Find a latitude along the left or right edge of the map and a longitude along the top or bottom edge of the map. Find the intersection of these values on the map. Look for a faint tick mark at their intersection. Most, but not all, MVUMs will have a faint tick mark everywhere the latitude and longitude values intersect. If the MVUM has these marks, skip to the next step, otherwise you'll need to add your own tick marks.

To add your own tick marks, do the following in photoshop:

  1. Select black as the foreground color.
  2. Select the line tool (U).
  3. Draw a line through a latitude tick on the left edge of the map all the way across the map and through the same latitude tick on the right edge of the map.
  4. Repeat for one more latitude and two longitude values.
  5. You should now have two horizontal lines and two vertical lines that intersect in four places.
  6. Select these four layers in the layers palette. (Window -> Layers controls whether the layers palette is displayed or not).
  7. Select Layer -> Merge Layers to rasterize the lines into a single bitmap layer. You should now have two layers, one containing the MVUM in the background and one containing the lines in the foreground.
  8. Now use the Eraser Tool (E) to erase each line except for the areas where the lines cross each other.
  9. Select Layer -> Flatten Image
  10. You now have an MVUM with four tick marks at known latitude and longitudes!

Once you've identified that your map has tick marks or you've created your own tick marks, save the MVUM as a PNG file:

  1. Select File -> Save As from the menu bar.
  2. Select PNG (*.PNG) for the file format.
  3. Click Save.
  4. Select None for Interlace and click OK.

Close Photoshop.

Georeferencing the PNG File in Global Mapper

Launch Global Mapper and select File -> Rectify (Georerence) Imagery. Global Mapper will present the Batch Image Rectification Options dialog box. Make sure the following options are clicked:

  1. Added Rectified Images to Main Map View
  2. Show Rectified Images in Reference View
  3. Export Rectified Images to New Files (This one is usually unchecked but must be checked before continuing.)
  4. For export format, select GeoTIFF and Grayscale - Min is Black then click OK.

In the Open dialog box, find and open the .PNG file you previously saved in Photoshop and click Open. Once the map is opened, click Select Projection...

The MVUM has projection information in the map collar. Most Colorado MVUMs are UTM, Zone 13, NAD83, but it's worth checking the map collar to make sure. Select these options in the projection dialog box then click OK.

Now click and drag on the middle map to find a tick mark and its associated latitude and longitude. Use a paper and pen to right down the latitude and longitude of the tick mark you found. Now zoom in on the mark by clicking and dragging some more. If you zoom in too far, right clicking zooms out. Now single click in the middle of the mark. A red dot will appear in the middle of mark. In the lower left hand corner of the screen, in the Ground Control Point Entry box, the x and y values will be filled in. Now enter the latitude and longitude values for this point that you copied to paper earlier. Global Mapper requires just the numbers separated by spaces. For example, 106 deg, 30 min, 0 sec West longitude is entered simply as -106 30 0. The (-) denotes west longitude. Click Add Point to List. Click Yes to convert the coordinates then click OK in the point name dialog box.

Repeat this step for three more tick marks. I try to use tick marks that are as far away as possible from each other such as one from each corner of the map.

Once you've accurately located four points, click OK. When the Exporting GeoTIFF dialog box disappears you have a georeferenced MVUM on disk in GeoTIFF format! These files will be in the same directory as the .PNG file you opened but their names will end in _rectified.tif and _rectified.tfw.

Cleaning Up

You will always need both the .tif and .tfw files. I recommend creating a single .zip file containing both the _rectified.tif and _rectified.tfw files so they don't get separated. To do this, select both files in Windows then right click and select Send To then select Compressed (zipped) folder. Once the .tif and .tfw files are in the .zip file, they're no longer needed and may be deleted.

Checking Your Work

Since you may be using these files on your phone, on your GPS, or on your laptop while navigating in the woods, you need to check your work to make sure the coordinates embedded in the GeoTIFF file are accurate. The easiest way to do this is:

  1. Open Global Mapper
  2. Open the MVUM .zip file you just created
  3. Select File -> Export Web Formats -> Export KML/KMZ...
  4. The default settings should work so click OK then save the .kmz file.
  5. Once the .kmz file is created, open Google Earth and view your MVUM overlayed over the Earth's features.
  6. Does your map align accurately with the roads and highways that Google Earth superimposes on your map? If not, you'll need to erase the .zip file and start over.

Next Steps

Now that you have a georeferenced MVUM in GeoTIFF format, you can do a few neat things with it such as overlaying GPS tracks from where you've been in the woods over the MVUM, displaying the MVUM on a mobile phone, displaying the MVUM on some Garmin GPS units, and using the MVUM on your laptop while navigating using software such as Global Mapper, Fugawi, or OziExplorer.

Notes

Here are some additional notes and clarifications on the process:

  • While I did everything above using degrees latitude and longitude, Global Mapper is also capable of working directly with UTM coordinates when georeferencing an image.
  • If the MVUM is at an angle in the PDF, the latitude and longitude for a tick mark may be on the same edge of the map. Enter the coordinates like you would for any other tick mark. No need to rotate the image in Photoshop either. Global Mapper will automatically orientate the map with North pointing up once it has been processed.
  • If Photoshop crashes or runs out of memory while opening the MVUM PDF file, make sure no other applications are running then try again. If that doesn't help, try opening the PDF at a lower DPI. Good results can be had down to about 150dpi.