Free USGS Topo Maps

My recent post about OziExplorer accepting .gpx and .kml file types got me thinking about one of the great benefits of Ozi — it’s ability to utilize free maps. While Ozi isn’t free (though there is a shareware/demo and a trial version that allow you to try out almost all its features), there are lots of sources of free maps to use in it. I’m speaking here of digital raster graphics (DRGs). These are scanned versions of USGS topo maps. They typically come in a .tif format, which can be opened in a basic photo or graphics program. But you need a program like OziExplorer or Fugawi to georeference them. Once you do that, your computer knows the coordinates of every pixel on the screen, and you can transfer GPS data (waypoints, tracks and routes) back and forth between your GPS receiver and PC, layering them on the DRG map.

Here are my favorite sources of free USGS maps:

• This listing of USGS DRGs has lots of information about various sources of maps, state by state.  Compiled by a frequent contributor to the OziExplorer discussion board, this site has great notes about datums and file formats, designed to assist in importing them into Ozi in an already georeferenced format.
• From their website, "The purpose of the Libre Map Project is to aggregate and make digital maps and related GIS data available for free."
• The Mostly Free DRG Reference Page has this to say about it’s name…"Between the various state and other references on this page you should be able to download a DRG for just about anywhere in the country without having to spend a dime (although I have included a few low cost DRG sites)."
• Of course, you can always go straight to USGS.

There is a chapter in my book, GPS Mapping: Make Your Own Maps on this subject called Maps 201: Downloading Free Maps. It delves into other types of free maps (and aerial photos) and various technical issues that you might run into. There are also full chapters on both OziExplorer and Fugawi.