First printed in NATRC Region 3 Stirup in 2010. This is targeted toward NATRC Trail Masters needing to make maps with accurate distances.
Over the years, we have learned a few tricks operating our Garmin GPSs. We hope they will help you make better maps, whether you are putting on a CTR, creating maps for your personal use, or you are a rider and want to more quickly adjust to map mileages that do not agree with your GPS.
We have used several models of Garmin eTrex mapping GPSs (Legend, Vista Cx, and Vista HCx), but the tricks can be applied, we think, to all GPSs. First, let’s highlight the problem we often see with GPS’d maps.
On CTR rides we have attended over the last several years, or with trail maps we have created ourselves after roughing in a trail on a topographic map, the mapped trail distances almost always are shorter, often significantly shorter, than what our GPS reports on the ride. This occurs because the trail we ride is rarely as straight as the one we draw. Even if the trail is created using an actual GPS breadcrumb track, this still occurs. For GPSs, the reason seems to be because they have limited storage to save data, and the factory default settings seem to favor saving a little data about a lot of trails, rather than a lot of data about a few trails.
Pictured here is a small section of trail we recently rode. We don’t have the original GPS track, but it appears it was made with a sample interval of about one breadcrumb every 60 seconds (simulated here with the blue line). We have our GPSs set to drop breadcrumbs about every 10 seconds (red line). The blue line shows a much straighter trail whereas the red line is closer to the actual trail path. We can see how the mileage would be too short if we relied on the blue line for our map. In fact for this ride, the reported distance was 16.2 miles, but our GPSs reported about 18 miles.
Using an actual GPS breadcrumb track and resampling the data, we can demonstrate the issue graphically (see graphs below). To explain the graphs, let’s pretend we are riders in a CTR and have been given a map in our check-in packets. It says our ride is 15.2 miles (point A). As riders, we are never aware as to the frequency of the breadcrumb setting used to create the maps, but for example sake, let’s assume the map was made with the breadcrumb frequency set at 240 seconds (point A on the x-axis),which is an unacceptably slow sample rate. This means GPS breadcrumbs were dropped on average, about every 955 feet (point C). On the second graph, Point B says that riders can average a slow 3.05 mph to finish the ride on time (5:05 ride time, after removing P&R delays). What frequently happens is we end up riding is a very different ride. Here’s why.
Had the same map been made with the breadcrumb set every 10 seconds with an average sample spacing of 50′ (point F), that same ride would map and track closer to the actual mileage. Point D tells us the same track has a mileage of almost 18 miles and if we were to accomplish it in the same amount of time, we need to average 3.6 mph (point E).
As riders, we generally discover our problem when we don’t reach our first time point when expected. We might for example be expecting to arrive at 4.0 miles, but don’t arrive until 4.7 miles. Assuming we didn’t pick up the pace and we maintained 3.1 mph, would be about 14 minutes behind our target time.
We end up with the same ride taking on a new look. If we assume the 0.7 mile error was just a map error (rare with a GPS’d map) or a GPS glitch, we have a couple hours to make up the time. If this is a sampling error though, the entire ride is likely to be short by a similar percentage. This changes the 15.2 mile ride with an average speed of 3.05 mph, to a 17.9 mile ride with an average speed of 3.5 mph. This means we have to boogie much faster than 3.5 mph to catch up and then we have to average 3.5 the rest of the ride.
We have seen a number of rides end up being 2-4 miles off which sets a much faster pace than anyone anticipated. This is with GPS’d maps with no obvious errors (e.g. a section not counted). An advertised slow-timed ride turns out to feel like a race and it is not because the riders did not plan their times in the intervals correctly.
We have found two issues that can cause the GPS breadcrumb tracks to be short, and the fixes are easy:
- Set your breadcrumb frequency to be as frequent as practical. We set one GPS to leave breadcrumbs every 10 seconds. The other is set to evaluate time and distance between breadcrumbs and it decides when to leave a bread crumb (Auto & Most Often). On Garmin’s, this is set on the “Track Log Setup” screen. For us, on a 40 mile weekend ride, these methods drop 4,000 – 6,000 breadcrumbs. As the GPSs start overwriting data (configurable) after 10,000 points, we can’t really set the frequency much shorter, or we risk losing data. This also means it is important to turn your GPS on and off shortly before and after the ride.
- On Garmin GPSs there is a trick to saving the breadcrumbs. While you can save your current breadcrumbs to a named file on your GPS, for purposes of plotting a trail, we do not recommend this! If you do, the GPS will take the active log (up to 10,000 points), and reduce it to 500 points! While the algorithm is fairly smart, a 5% – 10%, or larger, reduction in trail length should be expected. It is best to save the active trail log directly to your GPS mapping software on your computer. This should preserve every data point.
We have a few additional hints and suggestions for those making maps using a GPS, and for riders using a GPS to track their progress:
- Maps with frequent, timed waypoints give riders time to adjust their speeds before they get too far behind.
- Use fresh batteries. The GPSs seem to do more odd things on weak batteries.
- Carry extra batteries.
- Carry more than one GPS if you will be making the map.
- GPSs can misbehave. It’s not always the map.
- Large maximum speeds (e.g. 80 mph) suggest the GPS lost its signal, had a weak signal, or experience electrical interference. This probably means extra distance has been logged.
- Large differences between GPSs on the trail indicate one had issues. We’ve occasionally had 2-3 mile discrepancies occur over short distances.
- Clear your existing track log before you start the ride, but make sure you have saved it to your computer first.
- Shortly after you start the ride, make sure your GPS is dropping breadcrumbs. For whatever reason, sometimes they don’t, and when you get home, you have no log of your ride.
- If you are a rider and there are obvious straight sections on the map not aligned with a road or pipeline, you should consider it likely the reported distances are on the short side.
An accurate ride map enables all riders to accurately plan and have a good time on the ride. For those ride managers who have started using GPSs to make their maps, kudos. They are a significant improvement over hand drawn maps, but there can still be distance issues. Even with a GPS though, distances are still only an estimate. Short of using a surveyor’s wheel, uncertainty will always remain.
We hope this information helps you create or use GPS’d maps more efficiently. If you need assistance with your settings on any GPS model, contact us and we will try to get you answers. Anyone wishing to see our data file, just send us an e-mail. We can be reached at firstname.lastname@example.org