Create Garmin Custom Maps with Global Mapper
Creating your own maps to upload to the Garmin Colorado, Oregon, and Dakota hand-held units.
This review is based on using the Dakota 20  ° 12 Jan. 2010
For specific questions about this product, log onto the GPSInformation.net FORUM

GENERAL:  Garmin claims you can create Custom Maps in five easy steps (HERE).  'Your mileage may differ,' but using Google Earth to create Custom Maps was a time-consuming pain for this reviewer.  For the serious hiker, it seems that the USGS 1:24,000 DRG topo maps would be the most useful and legible on the GPS screens.  Google Earth isn't especially helpful in converting these maps to the required .kmz maps needed for the Garmin units.

Eventually I came across Global Mapper (about $300) which among its many features is an ability to import the georeferenced DRG .tif files and export them as a series of smaller georeferenced .kmz map files.

PROGRAM SETUP:
Exporting the data files as .kmz files to the Garmin unit is easy as 1,2,3,4.

Click on these thumbnails for larger views
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Open DRG Files                             °             Open  Control Center                                              Open GPS Tab

1.  Open as many DRG geo-referenced .tif files as you like with Ctrl-O.  Notice, however that the borders are visible and overlap on each other.
2.  Open the Overlay Control Center: Tools, "Control Center..."  or (Alt-C), and highlight all the DRG maps you want to trim off the borders.

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3.  Select: Options, Cropping, check "Automatically Crop Collar......", OK.  The borders now should all disappear and the maps joined seamlessly. (Close)
4.  Now select: GPS, "Send Raster Maps to Connected Garmin Devices" (above), check "Display on top of Roads and Trails" box,  Also select desired "Image Quality" (described below).  OK.

The program should find the unit's  \Garmin\CustomMaps folder and automatically save 96-100 Cells (maps) to the GPS.  (Remember the GPS unit has a limit of 100 maps per Custom Map).  Also, the Basemap (which is required to be active in order to see your tracks) will not display the inaccurate Basemap highways.

CAUTION:
Always wait for the progress bars to completely disappear before unplugging the unit.  There is a delay from the time the bars show "complete" to when the .kmz files have actually been created and transferred to the unit.

CONCERNING IMAGE QUALITY:
In the Garmin Raster Export tab (above) is a place to adjust the "Image Quality".  This actually only adjusts the exported .jpg compression before the maps are converted into .kmz files.  I experimented with various Image Quality selections, and discovered that one could reduce the "Quality" to 10% without any discernible decrease in map resolution on the GPS screens.  Using a 10% Quality factor results in .kmz files that are about 1/3th the size as when using the Default value of 50%.  This can reduce computer time significantly.

Dakota Screen Shots with various Image Quality settings
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0%              2%               5%                10%                              50%

Dakota screen-shot resolutions with various number of DRG maps loaded - This is independent of the Image Quality setting.
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4 DRG             6 DRG          8 DRG           16 DRG
Resolution at 10% Quality is not degraded up to six DRG Quads uploaded.
Eight and 16 Quads result in some reduction in resolution due to the unit itself and not the Quality setting.

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Basemap Enabled         Scanned Paper Map

THE BASEMAP PROBLEM:
In the Dakota, one must make the Basemap "Enabled" in order to see tracks.  Manually determining the number of cells per DRG will not hide the Basemap highways.  One must use the "GPS" method (described above) which incorporates a Draw Order to hide the basemap,

DATUM:
Most DRG maps are in the NAD-27 datum.  However, some of the later maps are in the NAD-83 datum. And even though some of these are georeferenced incorrectly to NAD-27, Global Mapper corrects this automatically.

As with ANY map, to avoid UTM Northing errors of approximately 200 meters in the GPS unit, the Position Format should be set to the datum of the map being displayed.  Lat/Long errors from an incorrect datum are in the range of the GPS errors and are therefore negligible.

Now don't get lost out there -youall!
jack yeazel