bilikituri

Regarding maps, in a city you can reach destination only guided by a straight line and some kind of Brownian motion you improvise. But if you are hiking for caches following trials, Brownian motion won't help, you absolutely need to take the right path at the right fork, sometimes oriented in a non-intuitive position related to your destination. The basemap in Garmins is totally useless for this use case. I personally use the ones in http://garmin.openstreetmap.nl .

Also you can buy a battery pack like http://www.mophie.com/shop/iphone-5 for example, as far as battery is concerned that's enough for a day out geocaching. I believe smartphones are superior in general. Take for example Groundspeak app, everything integrated, trivial selection of a map area and its caches for offline access. You can go to the hills, turn wifi and data off, and use just the GPS receiver. Gestures for the map are great, resolution is better, even the photos of the caches are stored for offline access. The only reason I use a eTrex 30 is that after an extensive comparison my conclusion is that the Garmin is hands down more accurate and stable, whereas the iPhone often reports 30ft of accuracy and acts quite erratic in my experience. If the receiver was better I would only use the iPhone.

Does the name "Off Road Transition" match what it does? I am a bit confused, either I misunderstood something, or the name should be different. The name to me suggests that the device is going to switch from "on road" (routing), to "off road" mode (straight line) given some configurable conditions. In my tests, even if "lock on road" is false, in "on road" mode distance and bearing do not point to a cache that is not in the road. If I tell the GPS to switch to off road mode, then it works as expected. The name "Off Road Transition" suggests to me a way to automate this switch. But the user manual describes this feature with regard to the "next point". What relationship has the next point with an off road transition? I can go from waypoint to waypoint in "on road" mode, how is off road related to this feature at all?

Awesome, thanks!

So it is more practical to perform a series of Go Tos rather than planning a route?

I am trying to understand how routing works in the eTrex 30. Before asking any questions let me go straight to see if someone has accomplished what I'd like to do. I have a routable map of OpenStreetMap, and the eTrex is able to draw a route following trails. I'd like to define a route whose waypoints are geocaches, that routes me through the trails instead of drawing straight lines. When a geocache is off the road, I'd like the GPS to follow me to the cache just fine reporting estimated distance and bearing. I'd like the GPS to assume I am not done with the waypoint until I tell it to. When done, it should route me to the next point according to the order defined in the route (even if there is another waypoint closer). Repeat. Is that possible? Or is routing designed for a different use case?

Your waypoint has an error itself that you need to add to the later estimation. The coordinates of a geodesic marker are virtually exact for our purposes, so only the estimation error of the device remains.

And probably my last post here. I decided to do another comparison between de iPhone and the eTrex 30, but this time with total certainty about the correctness of the GZ coordinates. In order to do that, I chose a geodesic marker in Barcelona. This marker is at the top of a castle, which is located at the top of a hill near the sea: the Fortress of Montjuic. Sunny day, totally blue sky, no clouds. This is the marker data: ftp://ftp.geodesia.ign.es/Red_Geodesica/Hoja0421/042112.pdf I parked my scooter at about 90m (300ft). I let both devices to settle, and the iPhone reported 95m (accuracy 5m), while the eTrex reported 85m (accuracy 3m). I walked a few steps, the iPhone would report 82 all of a sudden, then jump to 88m. The eTrex transitioned steadly. To reach to the geodesic marker at the top of the fortress you need to go inside a tunnel, and then go up some stairs. I was in the terrace for about 15 minutes. When I was exactly at GZ with a tolerance error way beyond what these units can offer, the eTrex reported accuracy 3m, and estimated distance of 2m. When walking around near GZ the iPhone told me anything from 1m to 12m, jumping and with any bearing you can think of. The eTrex did not jump, distance and bearing where really really consistent and accurate. My conclusions of this thread: * I don't believe the test that started the thread was significative. * The eTrex 30 is consistent in being accurate and steady. * In comparison, the iPhone 5 has proven to me at least to be much more erratic. Either because the hardware is inferior, the software less smart, or a combination of both. I can't really tell because there is little information out there about how these things actually work. * I no longer believe the behavior far away from GZ is significantly more stable than at GZ, as disproved by my test today. The geometric argument explained above yields some light, and my simulations report some probabilities to test. But I do not know if the model in the simulation is too simplistic, in particular if the distribution is uniform in the disc, if you can actually trust the estimated accuracy, etc. Those are conclusions not based on technical knowledge unfortunately, so take them with a grain of salt. They are based on the information I have been able to gather from trusted sources, and my own field tests. Thanks all for your contributions!

Followup: I contacted Joe Mehaffey from www.gpsinformation.net by email, he was very kind to confirm that Garmin devices implement some kind of hysteresis. That is, they do some kind of weighted average or something like that that stabilizes the estimated position, though the exact algorithm is not disclosed. That matches my experience, which I describe as "stability" as an end-user. My hypothesis would be that the iPhone 5 (or at least Groundspeak's application, since I don't know if that should be done by the hardware, or the operating system, or the end-user application), does not do that averaging and thus appears to jump more. I would also conjecture that this jumping does not mean the eTrex does not jump the same way behind the scenes (assuming same accuracy), it would only mean that it remains stable on the surface. It is also the case that in my tests the accuracy reported by the eTrex is often smaller (smaller estimated error/radius) than the one reported by the iPhone. Unfortunately they have not reviewed the iPhone, so could not provide information related to that and other questions that have arisen in this thread.

But in my tests I do stop and walk around GZ-style.

After sleeping on the problem, I believe a geometrical explanation would be the most reasonable one I have to say, since it would apply to all devices, only distance and accuracy would matter (assuming the software is equally smart, something I don't really know, and suspect is not the case if I had to bet). However, I have run some calculations that, if I am not mistaken, do not fully explain what I observe. Let's say we are at a point in the trail, far from GZ. Say 900 meters away. Let's say our position has a radius of margin of 10 meters. Which is the probability that our location jumps in a step of 4 meters or more? (Assuming within the error circle the distribution is uniform, that is, the GPS can locate you at any point evenly.) The points 4 or more meters closer to GZ than our location are those that overlap our circle of radius 10, with the circle centered at GZ with radius 900 - 4 = 896. Visualize an overlap with the shape of a lens. The points 4 or more meters away from GZ than our location are the complementary of the overlap with the circle centered at GZ with radius 900 + 4 = 904. Visualize an overlap with the shape of a moon. According to my calculations, if you are at GZ you should see jumps of 4 with probability 0.84. But if you are 100, 200, 300... meters away, you should still see them with probability 0.50. This is the Ruby program I've written to compute this: https://gist.github.com/fxn/7114945, and the output is below the token __END__. While the probabilities are certainly different, the ones away feel still to high. I don't see jumps of 4 meters half of the time.

Actually, 2) may not be relevant. If the correct coordinate may be anywhere within the circle, being away from GZ there's much more area where being off affects less the estimation.... Yeah, your reasoning makes a lot of sense after thinking more about it.

Well, I have to admit this explanation would match my tests if: 1) At GZ the iPhone had 10m accuracy. 2) When I looked at the distance decreasing, accuracy was better (5m). I'll try to do tests taking 1) and 2) into account.

Hmmm, not sure I follow. Jumping 4, 6, 8, meters when you do small steps in random directions should happen no matter whether you are at GZ or at 500 km of distance, if the erratic behavior happened at any given spot. Only your position is estimated, once the GPS assumes some coordinates, the distance to the target is a computation.

I do not question your claim. But you have not yet explained how such property would explain why the distance seems to be stable away from GZ, and unstable near GZ.

OK, I have done three known caches carrying the iPhone 5 and an eTrex 10. These caches are in Barcelona in wide streets. Buildings are not very tall, and there's one near the stadium of FC Barcelona. The streets had trees in the sidewalks, you know, but nothing like the backcountry of course. While going to the caches I walked often in the very center with clear sky and good margins in both sides. I walked all the way monitoring both devices. These are my findings: While navigating to the cache, the estimated distance rarely matched. They were often off of one another by 10 meters or even 20 meters. Sometimes less, but almost never equal. None of them seemed to be inconsistent when they were away from GZ. The distance more or less was decreasing in a reasonable way. I stopped sometimes and did some random steps in different directions. Both devices remained more or less stable (we are talking 300 meters away from GZ, for example). To my surprise, the iPhone reported 10m accuracy often. The eTrex had full signal strength always. At GZ, the iPhone was erratic as I have explained in this thread, and the eTrex stable. The iPhone could report 6m, then 14m, then 12m, only doing small steps. And the eTrex consistently reported 3 meters, 2 meters, 1 meter. The eTrex did NOT jump at all, and the distance was actually much better than 5 meters. So that may suggest the erratic behaviour I was asking for it is not due to the way GPS works in itself, a theoretical limitation. If that was the case, the eTrex could not be stable. There has to be something else, either the iPhone receiver is really inferior, or the eTrex has smarter software or hardware. But definitely the eTrex was stable. Next time we go to the backcountry I'll run a similar test.

Still do not understand. So, if I collect data points by hand and run a statistical analysis of some sort, you claim I'll obtain a random walk. That's fine, but, so what? What is the point? If the devices ignore previous data points, the measurements are independent. Why is it relevant to this discussion that the data points collected by hand by you yourself have this or that property?

Sorry, I don't understand the relationship between these tests and my question. My question is: I observe that while following a route the estimated distance to the destination is quite steady overall, but when you are really close to the target, the distance often starts to jump. Why does it get so inconsistent? Then you point to an article about random walks. In what sense the article or your tests answer my question?

But for that to be relevant to the thread, the device should have some tracking buffer and algorithms to correlate measurements using moving averages, markov chains, ... stuff that computes a new data point taking the previous ones into account, rather than the crude one-shot measurement from the satellites that ignores any other previous data. Do GPSs do that? If they do, could it be the case that a handheld device like the eTrex has a better algorithm than a phone? And a 62s a more sophisticated algorithm than the eTrex? Unfortunately manufacturers seem to be quite opaque about everything beyond "supports GLONASS", or "has a quad-helix antenna". Difiicult to compare other than trying actual devices.

Are you sure that's the explanation? I have a degree in math, can read the page if I grab a cup of coffee , but in what sense that models approaching GZ? When the GPS devices I have tested report 1357 meters of distance to the target, they do not start jumping like crazy all the time, 1350, 1364, ... Sometimes there might be a little fluctuation, but generally speaking they report more steadily as far as I have tried. But that remarkably changes in my experience near GZ. There has to be something related to both points being close that affects this somehow. One point is your estimated coordinate, and the other point is the target coordinate which is an exact point in the grid. The target coordinate may be off relative to the cache, but that doesn't matter for this discussion, as far as the GPS system goes you've entered a concrete coordinate as destination, so the only error variation for doing the math happens in your end (I conjecture): once you have your estimated position, the target coordinate is known and exact relative to that pair and the computation of the distance carries no more error propagation (I conjecture). Regarding the eTrex 10. Thanks all for the advice! I indeed purchased an eTrex 30 some days back. It couldn't calibrate the compass, systematically failed in the 3rd step (it is a known error, there are threads about it). Then I went and replaced the unit with the 62s and did a cache with it. I am not a hiker, only recreational hiking to go find some caches in the backcountry, so I wouldn't use the majority of features in those devices. Also, the interface is from the 90s. Do caching with an eTrex, and then do it with your iPhone, OpenCycleMaps and all the cache metadata available offline (including photos). Drag the map, zoom in and out,... all fast and snappy looking awesome in your retina display. Case with battery pack to have enough for an entire day and done. Now, if it was true (something I do not know) that these devices are more stable near GZ than the iPhone 5, I'd consider an eTrex 10 just to have bearing and distance. I can throw the rest of features, stable bearing and distance is all I need to complement the phone. Bearing does not depend on your orientation, it is an imaginary line between your estimated position and the target point, so I don't even need (and never actually use) a compass. In particular the lack of maps and type of compass of the eTrex 10 don't matter to me (Edit: though the device needs to know my orientation to be able to draw the arrow. In that sense the electronic compass would make the bearing arrow more responsive, but I don't really care where is North). I am going probably to test one, do some caches, systematically compare their behaviour, and report my findings in this thread.

The 62S should not jump even under canopy and with a clear sky will be scary rock solid....we use 2 of them and have been around many more. Firmware can be an issue and on the 62S I have found ver 3.90 to be the best ( the latest is not always the best )......I've used almost every unit out there and prefer the 62S for geocaching. Once in a great while I will visit a location where GPS operation is very unstable...metal buildings, overhead power wires, and sometimes no apparent issue ( maybe GPS jammer in the area )......again, this is rare. 99% of the time the 62 is VERY accurate and smooth at GZ......I recommend getting another, put on ver 3.90, put in fresh batteries, calibrate the compass, and have fun. Re iPhone caching, I have a 4 and a 5 and while I use the GC app and its great features I'm not a fan of getting to GZ with an iPhone. Aha, that's what I expected. The accuracy error is fundamental to the way the location is computed, but I expected a good device to be more stable near GZ than an inferior device. I know accuracy is impossible, I just expect the estimated distance to behave more consistently. You clearly have a lot of experience, so my test had to be flawed or I had bad luck. I would like to understand why that jumping happens. By now nobody has explained it, I purchased a book about GPS to research this. On the practical side, I don't want to spend 300 euros to get to GZ, though I certainly have thought what you suggested. Navigate to the cache with Groundspeak's app, and then narrow to GZ with maybe an eTrex 10, since I only need bearing and distance. Assuming the eTrex 10 is going to do a better job than the iPhone 5 there (which is only a guess, since I don't know the technology behind them).

Well, no, because even with the rough accuracy as I walk up, my PN-60 reliably takes me to within 20 feet, more like 6 meters. And I do have to let it average for a couple minutes before it really knows where it is. I'll try that. When it gets crazy, I'll leave the iPhone level somewhere and will wait a couple of minutes to see if it converges to something. Then a posteriori evaluate if the distance was off within accuracy error, or way off.

Yeah, we know that accuracy is not possible due to how GPS works. That's fine. We also know coordinates may be off. But one thing is to get zero or near-zero distance, put the device in a pocket, and take a radius of about 5-10 meters for guessing, and a totally different thing is to have the slightest idea of whether you are close to GZ or GZ is actually 20 meters apart because the distance reading gets so inconsistent and tells you 6m now and 14m seconds later. In an area off-trail with rocks a radius of 20m is a Universe!

Interesting. My question is theoretical so to speak, would like to understand why does the distance jump like that. But in practical terms, do you mean that when the GPS reports you are close, say 20 meters, or 15 meters, you already consider that to be GZ and start looking around? I thought people tried to get 0 distance, and then assume it was in a circle within 5-10 meters.

So are you saying that the way GPS works implies that if you get data D now in GZ, and 10 seconds passes, the new data received by the GPS may be totally different and that explains the jumps? If that was true, the same kind of jumps should be observed along the way, but when in the route I don't observe this as crazy as I do at GZ.