GPS Basics

[bpeschka]

I'm trying to understand all the components or layers involved in GPS'ing for bikepacking.  Specifically, I'm interested in Tour Divide, at this time but figure these concepts apply to most events of any scale.

My current plan is to buy a Garmin Etrex 30 which includes a simple basemap.

It seems that I then need

   a topographic map for the U.S.
   
   a route map for TD showing the path to follow, in my case.
   
Can someone, with definite knowledge help me confirm and quantify this?

Do I have to buy the $99 Garmin topo map or can I get this from Topofusion.

A question out of order ... where DOES Topofusion fit in here?  On their site, it looks like I might be able to get the topo maps free through them.

If so, it seems I could get by with the Basic, rather than Pro version.

Where do I get the route map data file for TD?

I've seen mention on the forum about limits on the number of data points that could be loaded for a route map.  Maybe specifically mentioning the Etrex 20 and 30.  Like 10,000 points.  In this regard, I've seen talk of dealing with the 10,000 data point limit by either compressing the data (removing some points) or breaking up the file into smaller files.  Also, an REI employee recently told me, no problem, you just use a bigger SD card.  So, I guess my question here is, if not compressing, can I use a big SD card or do I break things up into multiple files on separate cards. 

Thanks in advance,

[aaron w]

Lot's of questions...I'll try to answer a few. 

You can get topo maps for your unit from various places..direct from garmin, free from various internet sources, etc.  But you don't actually need to have any map on your unit at all.  I prefer for my unit just to show my track on the screen and nothing else.  That way when you quickly glance down at your screen you dont have to "find" your track on the screen amongst all the topo lines, roads, etc.  In fact you might want to hide the basemap that came with your unit.  It's not super accurate and it has confused me more than it has helped.  IMO for racing all you need to know is are you on course, or not.  No maps necessary.   

Topofusion cannot be used to put maps on an etrex.  However I've found TF to be an invaluable tool for adding tracks and waypoints to my gps, for manipulating files, and for lots of other stuff.  Get TF, you won't be sorry. 

Yes your etrex has a file size limit of 10,000 points.  But you can have quite a few files...I can't remember how many, maybe 10?  So you have plenty of space for long tracks like the tour divide route without even using a SD card.  If I remember correctly the TD track has something under 20,000 points.  So you would use topofusion to split this file into 2 tracks of 10,000 points or less.  Then when you load the track it would occupy 2 "slots" in your folder of tracks.  Click "show on map" for each of these files and you can then see the entire route on your unit.  No need to "compress" or simplify the file, you have plenty of space. 

I hope that helps a bit. 

[AnatolyG]

Do I have to buy the $99 Garmin topo map or can I get this from Topofusion.

check out this thread for a good discussion on topo maps. http://www.bikepacking.net/forum/index.php/topic,5518.0.html

The short of it is that there are free topo options that are very high-quality.