I’m just learning to use GPS & mostly paddle streams that snake back and forth continuously. If the GPS always assumes straight-line distance, then I guess you have to make waypoints very close together or it could give a very wrong reading for speed/distance? Also, if it always shows straight-line direction between waypoints, this will often not be the direction you have to paddle to get there. Using a compass is also challenging on windey streams…is there a better way?
GPS will measure distances
even with turns and bends fairly accurately. The device is taking readings from the sattelites constantly (or at least every few seconds) as you are moving, so bends in a stream aren’t likely to be much of a problem. I sometimes run with a handheld GPS, and I notice that when I take a right angle turn on a street, the next reading (a few seconds later)will suggest my running speed has dropped dramatically, from, let’s say, 6.5 MPH to 4 MPH. This is because the GPS assumes you took the shortest distance between its two most recent points–that you “cut the corner.” Even with these little anomalies, a cheap handheld GPS is a pretty darn accurate gauge of my miles run. Canoes and kayaks generally don’t turn at right angles, even on windy streams, so the affect of turns would be even smaller on your journey.
It is true that the GPS unit will tell you the distance from your start point as a straight line distance. But it should also measure how far you’ve actually travelled fairly accurately.
I’ve been in significant debates over what distance was actually traveled because different GPS units behaved differently in capturing all the bends in a travel path on the water. The perfect way to handle this would be, as above, to punch in a waypoint every time you turned but even on open water I don’t care enough to do that.
Some of the diff may be in the settings - take a look at your unit’s settings in case there is one that would cause it to record finer changes of direction. Also, I suspect this is the one place where an electronic compass could make a difference. But units with that cost a lot more, so is it worth it?
If it is adjustable turn the sampling rate (number of fixes /minute) up to the maxumum.
It helped me on the Suwannee
If you have a river mileage chart you can track your distance with the GPS and know exactly where you are on the river at all times. If I didn’t have the river mileage chart I am not sure the gps would have helped me much. I have found that preparation is the key to making a GPS productive on a trip. Having a mapping program on your computer that allowes you to print maps with waypoints will help when you get in the field. I always make maps and laminate them for the trips. I include all information on the map including any relevant emergency contact information or instructions. The map is also an important part of your float plan that should be left with a family member.
maps for any trips. I paddle lots of twisting rivers . Just don’t worry about every bend and turn. Imark start and finish waypoints and any camp spots we use. Also we learned to mark any dump or rest points where we get out. That way if ya lose anything ya have a start point to look for it.
The GPS leaves trackpoints
and updates every second. You don’t need to set waypoints to monitor your distance. In very slow speed tight turns it might not appear to be accurate but will be after a longer period of time or distance.
High banks and tall trees
will sometimes block the southern horizon where most of the satelites are found. In that case the GPS will loose contact momentarily. It won’t have much effect on the distance recorded but will give you an exagerated maximum observed speed as it assumes the missing distance was covered in the update period. On the Pere Marquette river in Michigan I often see average speeds of about 4-5 mph with a maximum speed of 25+mph. Trip distance and waypoint to waypoint distances are two different readings.
I can set my GPS to sample every second
Also, if I reset the “trip computer screen” to zeros before starting then it will compute my distance traveled and not strictly on a “straight-line” basis. To prove this I can set a “goto” to a pre-established waypoint. For the sake of the example it might tell me that the waypoint is 3 miles “as the crow flies” but of course the river may not allow me to take that straight line directly to my destination. The trip computer will measure how far I have actually travelled to get to that waypoint. It will always state that I have travelled further than the original 3 miles since I can’t paddle in a straight line to my destination. This of course works not only when paddling but also when driving in a car.
Setting up a “goto” from Philly to Lewes, DE says that it is 100 miles as the crow flies but after travelling it, my trip computer states that I have actually traveled 112 miles due to all of the twists and turns on the highway.
Modern GPS Units
Most newer and higher end GPS units have 12 dedicated channels and can track up to 12 satellites at once (if 12 were in view). These are best because the lock on and track all satellites in view. Some of the earlier hand held units had only 1 or 2 channels and could only track one or two satellites at once. These used a special technique to track and rapidly switch between the various satellites and then calculate your position. I think these were referred to as fast sequencing, but don’t hold me to this as it has been a while since I was involved with GPS research. These older units were not quite a accurate in calculating position and also were a little slower because they were constantly switching between satellites. I’m not sure if fast sequencing units are made anymore or if everything is now 12 channel, but generally the more channels the better. It is rare that anyone in an inland area would be able to see and track more than 8 satellites at once anyway, so 12 channel is really overkill. I have tracked 10 satellites in coastal areas with open horizon to the South. As mentioned in other posts there are generally more satellites to the S when in the northern hemisphere.
When you set waypoints and a route the GPS unit calculates the straight line distance between the two points you are interested in (e.g. A to B, or A to C). As was mentioned, usually you cannot go directly in a straight line between points, so the GPS unit provides you with some useful calculations and information. These features are usually found in the navigation screens of your GPS and vary in name from manufacturer to manufacturer, but basically you will find SOG (Speed Over Ground), COG (Course Over Ground), VMG (Velocity Made Good) or SOA (Speed Of Advance), etc. SOG is your actual speed. It is independent of the direction you are heading, but you must be moving for it to work. It is very accurate in GPS units as long as you are moving. COG is your bearing. VMG or SOA is the speed that you are making toward your target, so say you wanted to go due North (0 degrees) and the waterway took you NE at 45 degrees your VMG would be less than your SOG. You are still making progress toward your target, but not directly toward your target. VMG or SOA cannot be any higher than SOG.
Your GPS unit will also show you XTE (Cross Track Error) or something similar. This is the perpendicular distance away from a straight line that the unit calculates between point A and B and tells you that you need to head more to the right or left to get back on course. As you wind through a stream channel you may move to either the right or left of this straight line. The further away you are from this straight line the slower your SOA or VMG.
Bottom line is your GPS will do a lot for you and can be enhanced if you use it with a compass to check your bearing. You really need to understand waypoints, routes and the navigation features to get the most from your receiver.
Fast or slow sequencing
3 satellites or 12. Even the cheapest Garmin handheld is going to be accurate enough to serve the original poster’s needs–to help them figure out how far they’ve gone. The error rates in the cheap ones–even on windey streams over many miles–will not amount to anything significant.
Track point differences
I believe there is a difference in the way Garmin and Magellen record trackpoints. Garmin records the points at equal intervals so your track will tend to cut across bends. Magellen records more points when your direction is changing and less when it is staying nearly the same. That way it will be more accurate where it needs to be (although not perfect) while saving memory where it can.
Sorry, I seem to have lost the link that explains this really well. In any case it wouldn’t hurt to put in waypoints where you need to make major changes in course.
"I believe there is a difference in the way Garmin and Magellen record trackpoints. Garmin records the points at equal intervals so your track will tend to cut across bends."
I have a Garmin 60c. If the tracking option is set to “Auto”, it will record more points when changing direction, less when travelling a straight line. There is an option to set points according to time or distance, which would tend to “flatten out” curves.
You may be right about the cheaper…
… Garmin models, but that amount of “inaccuracy” will never be enough to notice. I’ve taken my Garmin E-Trex on walks around the neighborhood, just to become familiar with how it works, and it only takes about one or two seconds for the unit to detect a change in travel direction and accurately note the new heading. There is no way that you can “fool” this cheap GPS unit into “cutting off your corners” to a degree that you could actually measure. To anyone who thinks the existing degree of “error” that might occur on turns is a problem, I say quit worrying, because no consumer-brand GPS can map your location to within the distance covered in one or two seconds of paddling anyway. Even if it could, how would that serve you better?
Also remember that unless your GPS is WAAS enabled and receiving a WAAS signal the accuracy of any point is about within about 30 feet of where you truly are located. If you track every second and are moving slowly it is possible that a next point your receiver calculates will plot behind the first, even when you are moving forward. This is one reason why GPS units cannot give you an accurate bearing when you are standing still.
My experience is different
I have the cheapest e-trex. I was walking a 5 mile circuit each day and I carried the unit to observe what it recorded. The course is primarily through a park on a rec trail that has no tree cover at all. It is windey but not so much that it should matter. The first mile is on roads with one key point - two places where direction changes about 120 degrees or more - in other words, nearly reversing course - like this: > .
Because I measured this 25 times or more it is a great sampling. Since my plan was to run this course (I was recovering from an injury) I wanted mile markers. If you are running you’d like these to be very accurate.
My results: the one mile mark varied by up to 200 yards. Over 5 miles the variation was the same. So for a short distance with a sharp bend - you could be off enough to care. Over a few miles, it will probably be consistent.
In my opinion the one mile error was a combination of the hard turn and the fact that the road was tree lined. You could see the sky but I wouldn’t be surprised if it lost contact for half a minute or more.
If you are racing a mile, a 200 yard difference matters but not if you are simply paddling and wondering how far you went.
thanks to all…
…my speed/distance question is well explained, but I am still unsure how useful GPS is for navigation on windey streams with many intersecting tributaries (typical of salt marsh areas). Unless you continuously set lots of waypoints along your paddle (the breadcrumb method of retracing your path), it is possible to take a fork or two and become disoriented. The GPS will tell you in which direction you want to go (to get to a waypoint), but not which windey path will take you there. This can happen in just minutes, & noting the sun or using a compass does not help much either.
GPS Navigation in open water is a cake-walk, but in salt marsh its much more of a challenge.
This is a different problem
The error you see is not due to too infrequent a sampling interval. It is due to the unit being occasionally unable to pinpoint your position accurately enough to show that you followed the same route each time. I have the same model E-Trex, and if I stand in the same spot in my driveway and take a location reading on 20 consecutive days, no two readings will be the same, and if I plot the locations of those waypoints on mapping software, a small proportion of the individual readings will be as much as three hundred feet apart. You are trying to increase the accuracy of a measuring whose precision is not up to the task.
However, I stand by my previous statement regarding adequate sampling intervals. If I am traveling at a walking or paddling speed and suddenly change direction, the unit will correctly note that change within one or two seconds without fail, every time (unless trees or buildings are obstructing reception). When reception is good, the unit does a great job of keeping track of speed and direction and changes thereof, even when I try to fool it by doubling back at sharp angles, etc. I’m convinced that the problem you see is NOT that the direction changes don’t get recorded accurately due to widely-spaced samping intervals, but that the unit can’t pinpoint your location at any given time accurately enough for the recording of these routes changes to have any meaning. Sure, the unit can tell us that we made a 270-degree turn within one or two seconds of the event, but if we go back to the exact same spot tomorrow and the unit says that we standing at a spot that is 250 feet away from there, what’s the point in fretting over such minute zig-zags in your route in the first place? This is just like measuring a 500-foot distance by pacing, but substituting a yardstick for your final step so that you can accurately record that 500-foot distance to the nearest inch. It just doesn’t work that way. You can’t inject greater accuracy into your measuring system than the limited amount of precision you are already stuck with.
You are correct
You could eventually find your way back, but maybe at the expense of heading down several dead end channels. The only way to record your path is to take a waypoint at each significant intersection and then backtrack point by point using routes and the navigation screens.
I get your point
But I think the turns are very important. You say it registers that … ok, but consider this: if it only knows your location within 15 - 45 feet at any point in time then every sampling it takes has that same error. So again my V shaped turn: < As I near the apex it and then come around the waypoint will not necessarily come up as any different than where I was on the first axis. It’s within that 30’ margin of error. Now if you have a number of turns it really can’t plot that because it isn’t tracking a pinpoint on the globe, rather a circle with a diameter of between 15’ - 45’. So 10’ to the left then 25’ to the right will not necessarily register.
Anyway, I’m sure you know more about it than I do, but I find it interesting.