How to Digitize Topographic Maps using Microdem

The following was submitted by Michael Long. I want to thank Michael for sharing this information with all of us and taking the time to write this and allowing me to post it.

I recently visited Randy's site hoping to find a link on obtaining older maps on the internet. I clicked on his USGS link, and found that Reston, VA. has an extensive library of older topo maps. I ordered several copies of 1902 maps of my area (Baltimore,MD). I tried transferring coordinates manually from the older topo's to recent maps, with no luck. I then checked into Microdem, a computer mapping program written by Peter Guth of the USNA. I found out that I could scan my older maps, digitize them, transfer coordinates to my recent topo's and GPS, and be within 100' of the site I was looking for.

I use a H.P. Scanjet 3300C to scan my maps, then send the files to Photoshop (any image editor will do). I name my file, then save as a .bmp image. I then start Microdem, click on "File", "Data Manipulation".When the new window opens, click on "Create", "Digitize scanned contours". Highlight your .bmp image, then click "Open".

An "Open registration" window will open. Do not highlight anything, just close the window. A message box behind window will say "Register map now". Click "Ok". You need to know the datum of your map and your UTM zone before you go any farther.

This info can be found in the lower left hand corner of your map. Pick projection parameters window, highlight your datum, click "Ok". Type in your UTM zone and hemisphere then click "Ok".Your scanned map should come up, and at the bottom of your screen should say "First registration point".

You need to know the NW coordinate, NE coordinate, and SE coordinate (Top Left, Top Right, and Bottom Left corners of scanned copy, not original map). Place your cursor on the first coordinate and double click. Click on Lat/Long, enter coordinates, then click ok. Do this for the next two coordinates. Save registration points, and now you have a digitized map.

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