Here is a tutorial on how to create an overview .tdb and .img file needed for MapSource.

The easiest way to load the map into your gps is to make a MapSource product. City Select, City Navigator and MetroGuide are examples of products. A product will have many .img files with 8 digit filenames. These are the individual map tiles. It will also have a .tdb which is a file which tells which .img files make up the product and the area that is covered by that tile. It will also have an overview .img file so that you can see an overview map in MapSource with a border around the tiles you can select. A product will have a 3 digit code as well. Lets just pick 876 for the code that we're going to use. We are going to use gpsmapper to produce the overview .img file and .tdb for our map. We will need a file to input to gpsmapper telling it what we are going to call our map and which .img files will make up our map. This is the file I used. Just cut and paste the following into a file called "HomemadeMaps_pv.txt":

;Start of file, don't include this line
;Sample file accepted by cGPSmap to create new map data-set
;
;In order to create new TDB and preview map file run cGPSmap :
;cGPSmap pv test_pr.txt
;
;
;FileName      = file name for new IMG and TDB files, no more than 8 characters
;                must be without extension!
;
;MapVersion    = map version, must be between 100 and 999 
;                add 1 every time you modify one of the detail maps
;
;ProductCode   = unique product ID, each GARMIN product has its ID number
;                I suggest to use values above 40, but I cannot guarantee that
;                those values are not used!
;                You can check products id in windows registry under path:
;                HKEY_LOCAL_MACHINE\Software\Garmin\MapSource\Products
;
;MapSetName    = map name in GPS
;
;MapsourceName = map name in MapSource programm
;
;CDSetName     = CD set name - visible in MapSource menu
;
;Copy1,Copy2,Copy3 = copyrights info

[Map]
FileName=BaseMap
MapVersion=100
ProductCode=876

MapsourceName=Homemade Maps
MapSetName=Homemade Maps
CDSetName=Homemade Maps
Copy1=Compiled from data provided by the USGS
Copy2=
Copy3=

; The lowest zoom level in the PV should be the highest in the detailed map.
; The preview is displayed up to this switch over point.
; When you zoom in more in MapSource the GPS detail map is displayed.
; Example for a detail map:
; Level0=24
; ..
; Level4=14
; Zoom0=0
; ..
; Zoom4=4
; In this case you have to set Level0=14 and Zoom0=5 for the overview map

Levels=2
Level0=14
Level1=12

Zoom0=5
Zoom1=6
[End-Map]


;
;IMG files to be included in new map-set
;!!REMEMBER to fix the path to IMG file!!
;
[Files]
img=08071000.img
[END-Files]       
;End of file, don't include this line

Now open a command prompt window and run the following command. If cgpsmapper isn't in the same directory as your maps you will need to set the path.
cgpsmapper pv HomemadeMaps_pv.txt

You should now have 3 new files, BaseMap.img, BaseMap.tdb and BaseMap.reg.

Edit the registry to add the new map to the list of map products.
Instead of editing the registry directly lets use a .reg file that we can then just double click on and windows will add the contents of the file to the registry. First make sure that the product code matches the one we picked "876" and make sure that the paths are correct. If you make a typing mistake and the path isn't correct then you will get an error when you run MapSource and the program will exit. If that happens just edit the .reg file and correct your mistake and then double click on the file again. This will replace the wrong values with the right ones. This is an example of what the .reg file should look like. Lets call this file "BaseMap.reg". In this example BaseMap.img, BaseMap.tdb and 08071000.img are all located on your "C:" drive in the directory "CraterLake\MapSource".
 
;Start of file, don't include this line
Windows Registry Editor Version 5.00

[HKEY_LOCAL_MACHINE\SOFTWARE\Garmin\MapSource\Products\876]
"Bmap"="c:\\Crater Lake\\MapSource\\BaseMap.img"
"Tdb"="c:\\Crater Lake\\MapSource\\BaseMap.tdb"
"Loc"="c:\\Crater Lake\\MapSource\\"
                                                   
;End of file, don't include this line

Here is a .reg file to uninstall just in case we need it:

;Start of file, don't include this line
Windows Registry Editor Version 5.00

[-HKEY_LOCAL_MACHINE\SOFTWARE\Garmin\MapSource\Products\876]
;End of file, don't include this line

Remark: If you don't use XP you have to change the first line to REGEDIT4

And that's all there is to it!! You just made your first map. Now you can start MapSource and then choose "Homemade Maps" from the pulldown. You can now select your map (and other maps from other products) and download them to your gps. 

If you want to expand the map to cover more area or different areas, you can make more maps using the same procedure that we did for 08071000.img (using a different id number), then add them to the HomemadeMaps_pv.txt file in the [Files] section and then just recompile your BaseMap (cgpsmapper pv HomemadeMaps_pv.txt). Your basemap will now include the multiple map tiles and you can selct them individually in MapSource.