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 georeferenced DRG .tif or Google Earth .jpg files and export them as a series of 100 or less georeferenced .kmz map files.

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

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.

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 1/3rd the size as when using the Default value of 50%.  This can reduce computer time significantly.

CREATE CONTOUR LINES FROM DEM DATA:
DEM data are nothing more than elevation points in the US and around the world.  The points in US are in grids with 10m or 30m squares.  It's up to outside programs (like Global Mapper or Garmin/Magellan maps) to compute the contour lines and add terrain and other features.  In the case of Global Mapper, the DEM data is stored on their own servers for faster downloads.

1.  File, US Data, Highlight: Download Online Imagery/Topo/Terrain, US Elevation Data (10m resolution).
2.  Pre-determine the location central point of your area of interest.
3.  Check: "Within".
4.  Specify the radius (1 to 2 miles) and Lat/Long of the central point in the hddd.ddddd format, OK.
   (A relief map of your area should download and appear).

5.   In the Shader dropdown box, you may want to choose Atlas rather than Color Ramp.
6.   You can save this area of work with Ctrl-S.
7.   File, Generate Contours, elect Contour Interval, (usually 10', 20', or 50') OK.
     (Contour lines should appear on the relief map.)
8.   Activate the Overlay Control Panel with Alt-C.
9.   Uncheck the US Elevation Data, and highlight the GENERATED CONTOURS.
10. Select: Options, Line Styles, check: Assign Random Colors to Features.
    (In Select Line Style, you should use a width of 1, 2, or 3 pixels depending on the roughness of the terrain), OK, OK.
11. The contour lines should now have random colors.
12. File, Export Vector Format.
13. In Select Export Format, choose KML/KMZ, check Display Labels for Area and Line Features, OK.
14. Give the exported .kmz file a logical name.
15. Clicking the resulting .kmz file should make it appear on Google Earth with some elevations labeled.

The .kmz file will superimpose the contour lines on Google Earth -such as (HERE).  To identify an unknown contour in Google Earth, click on it and a box will show the elevation.

Working with Garmin Custom Maps:
After generating and coloring contours in 11. above:
12.  GPS, Send Raster Maps to Garmin Connected Device, OK.
13.  Give the exported .kmz file a name.
14.  This file can be uploaded to Garmin units that support Custom Maps.

NOTE: There is a practical limit to the size the Garmin Custom Maps can handle without the contours smearing and losing resolution.  This occurs with maps downloaded with greater than 2 to 3 mile radiuses from the central point, creating files greater than about 2 MB.

But these files will have 33 or less tiles allowing three, four, or five .kmz maps to be displayed on the unit at the same time and stay within the 100-tile Custom Map limit.  One can determine the number of tiles by changing the .kmz file to a .zip file which will show the number of tiles.

Alternate Method for Large Maps:
After generating and coloring contours in 11. (above) with radiuses of 4 miles or greater.  (The upper limit is unknown at this time).
12.  GPS, Send Raster Maps to Garmin Connected Device.

                                         Thumbnail                                                                                                                                        4.5 Mile Radius

...

13.  In Garmin Raster Export Options, click on "Click Here to Calculate Spacing in Other Units."
14.  Select 'Units', 'meters'.  Enter "2.4" (DRG standard) for 'X/Longitude Spacing', OK, OK.  (The correct Y/Latitude value will then be calculated).
15.  Give the exported .kmz file a name. The file will have 99 tiles.
16.  Only one file of this size can be uploaded to Garmin units that support Custom Maps, but it will have high-resolution contours.
 

EXTRACT CONTOUR LINES from  GeoPDF Topo Maps (v. 13 required):
The new GeoPDF Topo Maps can be downloaded free from (HERE). These maps are similar to the USGS topo quadrangle maps and have the same names.  However, they contain 'layers' of geographical features that can be viewed separately.  At present Global Mapper is the only program we know of that can extract individual features and display them on Google Earth.

Google Earth aerial photos can not show contour lines, although their images can be viewed in 3-D.   Global Mapper v. 13 can extract the contour lines as a .kmz file, which can then be displayed on Google Earth.  The procedure is as follows:

1. Load the GeoPDF file.  The 'Select Layers to Render' dialog box will appear.

.....

A .kmz file will be created in the folder with the GeoPDF file which when clicked on, will superimpose the contour lines on Google Earth -such as (HERE)..

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