Over the past few years, I have used the Garmin Edge GPS units with considerable success to keep from getting lost on more than a dozen 1200km brevets in numerous countries. Some of those brevets had fairly inscrutable cue sheets. I have used the Edge 705, the Edge 800, the Edge 810, and most recently, the Edge 1000.
To make the Garmin work for me, I focus on five key things. (Or perhaps these define how I work for the Garmin, but let's leave that philosophical question for another day).
Maps. Early on, I used Garmin's maps. Frustrated by the price and by Garmin's idiotic policies about transferring maps to replacement units, I switched to open source maps from the Open Street Maps project and have never looked back. Before traveling, I visit this GarminOSM and create a map for the countries to be visited. The coverage of OSM maps varies by country - sometimes under-inclusive (missing streets in Korea) and sometimes over-inclusive for my purposes (hiking trails, dirt paths). But they work. Inasmuch as OSM is an ongoing project, the more recent the download, the better the maps.
Backup Plans. I don't place complete faith in the GPS to stay on track. I also use the cue sheet and a smartphone application. When I started randonneuring, sonny, I had none of these new-fangled GPS thingys. My navigation came from the cue sheet and sometimes from paper maps (now replaced for me with maps on phones). My first long brevet was a 1000km from one end of Vancouver Island to the other and back. Cues were minimal and navigation not really a factor. My second was Paris-Brest-Paris and the route was signposted and navigation was again not a factor. (I did recover from a group navigation error by offering up my own hand-marked Michelin maps to some accompanying French riders, who were quite able to dope out the correct answer from there). My third long ride, however, was the 2001 London-Edinbugh-London 1400km brevet with, as I recall, a 14-page dense cue sheet filled with cues like "take right on unmarked lane." Fearful of getting lost in a land where I did not speak the language, I paid a lot of attention to that cue sheet - by studying with maps in advance, by trying to dope out the local customs of cue sheet presentation, and by paying careful attention to it on the road. Although cue sheets are not always perfect, I still consider them to be a very important navigation resource and a key backup to the GPS. In addition to the cue sheet, my second GPS backup is a smartphone mapping app the use of which I I've described here.
Power. One significant weakness of the Garmin Edge units for me is that their batteries don't last as long as our rides do. So I add external power. Usually that involves bringing on a "power bank" type of device that charges from a USB outlet and provides power to one or two USB ports. I use these to charge the Garmin (and sometimes my phone as well). My preferred approach (works for me; others may have good reasons to do differently) is to have some idea of how long I can run the Garmin on it's internal battery. With my 810's bluetooth features off, it seems that I can rely on at least 12 hours of runtime. Based on that, if I expect to ride for 20 hours in a day, I'll run the GPS normally from its internal battery for 8 hours, then plug in the power pack and ride that way until the device shows 100% charge. Then I unplug the power pack and let the Garmin run down until I stop for the night. If possible, I charge it from the wall at night and repeat the next day. If not, I'll charge it up from the power pack while I sleep. With drop bag support, I'll have four smaller capacity power packs and carry a fresh one on the bike each day. With no drop bag support, I'll carry one large one sufficient to power the Garmin for the whole ride (and/or bring a charger for opportunistic recharging of the power pack and/or Garmin when time at a power outlet presents itself). As a backup, I carry a lightweight USB power pack that runs from 4xAA batteries. (Sometimes I carry it empty and sometimes with four lightweight Eveready Energizer Lithium batteries installed, but with some electrical tape over the ends to prevent accidental discharge.) With AA batteries fairly readily available at stores, this provides a backup power plan. One related problem to manage is that attaching the USB cable to the Garmin compromises the waterproofness of the Garmin. So some effort is required to manage the charging in inclement weather. Instead of following the script above, I'll take advantage of a dry spell or protected rest stop to charge up the Garmin or I'll use a long stretch without cues to put the Garmin, cable, and power pack all together in a waterproof bag and get some charge into the Garmin.
Courses. Most of my preride efforts as they relate to GPS navigation revolve around creating a "course" file in Garmin's TCX format. My goal is to have a file that has both a "breadcrumb" track that follows the course along with "course points" along that track that represent the entries on the cue sheet. I use RideWithGPS, but depending on what may be available from the ride organizers or fellow riders, the exact process may vary. Perhaps a rider who uses the same approach to their Garmin has created a course in RwGPS with cues for turns inserted automatically by the RwGPS application. Perhaps the organizer has provided a GPX file that traces the route that I can import into RwGPS. Perhaps I only have a cue sheet, in which case, I will make the effort to create a RwGPS route. From any of these starting points, I'll painstakingly follow the cue sheet along the course and edit the RwGPS route file. My goal is to have a course point in the TCX file for each line on the printed cue sheet. (RwGPS puts a course point in the TCX file for each of the route's cue sheet entries). If I start with a bare track, this means using RwGPS "add to cue sheet" function to add a cue for each turn or other feature (like controls). If the track has cue sheet entries already, I'll follow along, one-by-one, adding entries from the paper cues sheet that are missing in RwGPS route, deleting extraneous entries, or editing existing entries. Once done, I export a TCX file from RwGPS and copy it to the /Garmin/NewFiles folder on the GPS. Good practice is to divide a 1200 into smaller chunks, but if I have one that has fewer than 200 cue sheet entries, I'll often live dangerously by running a single 1200k course. (Side note: the Edge 800 can't record a ride longer than around 450-500km without crashing hard. So I reset the unit every 400km or every day, restarted recording, and then restarted the course).
Defeat the Garmin. I'm forever at war with my Garmin and its engineering team. They want to navigate for me. Provide them some parameters - destinations, waypoints, a track, whatever - and they'll figure out how to get me there and give me turn-by-turn directions. I want something else; I just want it to help my find my way around the course that I spent all that time creating. Garmin doesn't get it; so we are locked in battle. My happy place with the Garmin happens when I load the course, tell it to ride the course, turn off that annoying "virtual partner", and then have the GPS display a few pieces of key information to me as I ride. The two most important things that I want to see are the map and a field called "Course Pt Dist" that will tell me the distance, along the course, from where I am to where the next course point (i.e., cue sheet entry) is. The map should show my location and the line that is my course. With sufficient attention, this could be the only navigation aid. One would ride to stay on the course line and use the "off course" warnings to cover any misses. I've done that, but it's much better for me if I can see the distance to the next cue sheet entry. As that decrements towards zero, I'll look at the map screen or the paper cue sheet (or both) to familiarize myself with my next required course action. Usually that's good enough, but if I get it wrong, the "off course" warning usually helps me to recover. The Garmin folks have reluctantly conceded that one might want to navigate to a course, as opposed to destinations and way points, but they will not surrender easily their desire to calculate a route to do so. That route will get stuff wrong, so I don't want it. So I try to defeat that route calculation effort. Most of the time I can do so by starting the course, then going into the currently active "Activity Profile" and change the navigation method to "Direct Routing." Annoyingly, it will not save this a setting in the "Actiity Profile," so I have to do it every time. Idiots! I have occasionally resorted to turning off the routable maps in the GPS, starting the course, and then re-enabling the maps. A real nuisance.
On the road, I usually set up the Garmin to display five separate screens while I ride. One is a map screen with two data fields added. One of those fields is the Course Pt Dist that will tell me when I'll reach the next cue. The other may vary (sometimes I use a battery indicator if that's a key consideration, sometimes I'll use Distance to Go, sometimes current elevation. Second is the elevation profile screen that will show me the profile of the course behind and ahead of my current position. I'll usually put two fields there too - one with my all-important (to me) Course Pt Dist field and one the current elevation. Third is a screen, created automatically, that shows all the upcoming course points. The entries are severely truncated, but I can, for example, scroll down to the next control to determine how far away it might be, or I might want to look at next couple of cues to know if one comes fast after another (and therefore might be easy to miss). Fourth is screen filled with data that my geeky riding heart desires - elevation, grade, speed, distance travelled, time of day, etc. And of course my Course Pt Dist. The fifth screen is deeper geek - cumulative elevation, average speed, sunrise/sunset times and the like.
It's work, but it works. At least for me. At least so far.
