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mrp

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Everything posted by mrp

  1. I'm not sure I understand the situation, but I'll try to guess. Using EasyGPS, you can't get your laptop to talk to your GPSr (a legend as well?) but your laptop does communicate properly with your friend's legend. Is that correct? First thing to check, the communcation protocol on both GPS receivers. On the legend you can access this from Main Menu->Setup->Interface and it should be set to "Garmin" for uploading and downloading waypoints and such. For live tracking most (but not all) programs use NMEA in/out at 4800 baud. If your GPS isn't a legend, then you need to make sure that it's interface is set properly (refer to your user manual), and also that EasyGPS is set to use the proper protocol for your brand of GPS. -- Mitch
  2. quote:Originally posted by CreagerStone Family:ACCURACY Since all of the caches are listed with positions in the thousandths of a minute, I'm a bit confused. Are the newer models of GPS accurate to thousandths of a minute or is the extra digit added by the website mapping software? This a perfect example of the difference between accuracy and precision. The precision (in the coordinate format) is 1/1000th of an arcminute of latittude or longitude. This comes out to about 6 ft N/S and 4 ft E/W for most latitudes in Europe and North America. Now your typical GPSr isn't quite that accurate. It's usually accurate to within 20 or 30 ft since SA has been turned off, but if your unit only displays 1/100th of an arcminute, then the precision is 40-60ft, and it can't express the location to the proper level of precision. Some people argue that we should use the coordinate format of decimal degrees to the 1/100,000th of a degree (40.49341 degrees as opposed to 40d 29.605') because the precision is 2-3 ft as opposed to 4-6 ft. with decimal minutes. But this misses the fact the accuracy of the GPS receiver is still (at best) only 15-20 ft, and expressing greater precision doesn't change that. Similar arguments can be made for the UTM coordinate format, but have the same problems. Older receivers only display 1/100th of a minute becuase before SA was discontinued, the accuracy of the GPSr was only good to within 200-300ft, and entering (or producing) coordinates more precise than that didn't make sense. AFAIK, The only receiver still on the market that has this limitation is the Magellan 310. You see them for sale at dirt cheap prices occaionally ($60 or so), but they're not useful for geocaching. (Selective Availability refers to the practice of scrambling the timing signals slightly for non-military receivers so that the accuracy was degraded. However, by May 2000 there were enough workarounds to get better accuracy anyway that the military disconintued the practice. Now they have a system that called something like "selective deniability" and allows them to blank the system for certain geographic regions. Though my understanding if spotty and I could have this wrong.) -- Mitch
  3. I recently orderd a RAM mount from Fresh Track Maps. Their prices are decent, and it shipped the next business day. However when using their order tracking system, I discovered a bug (or maybe mis-feature) that presents some security problems. It looks like a fairly minor issue on the surface, but I know that with juat a little research and social engineering it could be used to steal credit cards and home addresses (all that's necessary to make fraudulent charges). I'm not going to give too many details (yet) because I want it fixed, not exploited. I sent email to the contact address on the web-site about 6 days ago, and he hasn't answered. This is not an obscure bug, and it's almost trivially easy to stumble accross. I know how to fix the problem, (it's just a PERL script, and I have experience writing secure PERL.) but I don't want to sound like I'm trying to create business for myself by manufacturing a fake problem and offering to solve it. Should I email him again offering to solve it at my standard consulting fee? Should I publish the details to increase the pressure? -- Mitch
  4. That's a very odd reply. I've always thought of the Vista as just a slightly beefier version of the legend, and that most of the firmware was identical between the two. I suppose that it's possible that there's a technical reason why it's not feasible to port that feature to the Vista (higher current drain when using compass/alt. throwing off voltage measurements?!?) but I expect you got a (bogus) answer from somebody who doesn't understand the question. I'm using 2.40c in my Legend, and the battery selector works fine for NiMH batts. -- Mitch
  5. quote:Originally posted by majicman:I guess I can never post again! RATS!! (At least until we get an "Off-Topic Forum". MAN! I would RULE that!!) --majicman I would think that in the OFF-TOPIC area, all threads would start pre-closed for your convienence.
  6. quote:Originally posted by majicman:I guess I can never post again! RATS!! (At least until we get an "Off-Topic Forum". MAN! I would RULE that!!) --majicman I would think that in the OFF-TOPIC area, all threads would start pre-closed for your convienence.
  7. I suspect it was me.. but I can't prove it yet! -- Mitchsuko
  8. I've heard that placing units right next to each other may cause them to interfer because the few microwatts of RF leaking out of the oscilator circuts will cause problems. However, when I've had two units within a few feet of each other, they seemed to agree on the location down to +/- 1/1000th of an arcminute of Lat/Long, which is as much agreement as you can expect. As to whether different manufacturers use the same "formula", well.. yes and no. They're all trying ot solve the same problem, which is delta_x_n^2 + delta_y_n^2 + delta_z_n^2 - (c*delta_t_n)^2 = 0 where n represents the index of the satellite (there are a few other corrections in there, but that's the gist.) However, the algorithm the manufacturer uses to solve this problem is a closely guarded trade secret. I imagine that the different approaches will not greatly affect accuracy directly, but will instead have an effect on how quickly the answers converge (I'm assuming an algorithm of sucessive approximation), and how well they deal with occsionally spurious data from bad signals, reflections, etc. -- Mitchsuko [This message was edited by Pneumatic on July 23, 2002 at 09:25 AM.]
  9. I've heard that placing units right next to each other may cause them to interfer because the few microwatts of RF leaking out of the oscilator circuts will cause problems. However, when I've had two units within a few feet of each other, they seemed to agree on the location down to +/- 1/1000th of an arcminute of Lat/Long, which is as much agreement as you can expect. As to whether different manufacturers use the same "formula", well.. yes and no. They're all trying ot solve the same problem, which is delta_x_n^2 + delta_y_n^2 + delta_z_n^2 - (c*delta_t_n)^2 = 0 where n represents the index of the satellite (there are a few other corrections in there, but that's the gist.) However, the algorithm the manufacturer uses to solve this problem is a closely guarded trade secret. I imagine that the different approaches will not greatly affect accuracy directly, but will instead have an effect on how quickly the answers converge (I'm assuming an algorithm of sucessive approximation), and how well they deal with occsionally spurious data from bad signals, reflections, etc. -- Mitchsuko [This message was edited by Pneumatic on July 23, 2002 at 09:25 AM.]
  10. The thought of a vespa scooter with satellite navigation is enough cognitive dissonance to give me a headache. -- Mitchsuko
  11. The thought of a vespa scooter with satellite navigation is enough cognitive dissonance to give me a headache. -- Mitchsuko
  12. Mitsuko who? Never heard of her! -- Mitchsuko
  13. The Latest version of the firmware for the Legend has just been posted on Garmin's Site. -- Mitchsuko
  14. quote:Originally posted by peter:I agree with the first part, but you're still exagerating the effect. On a 20% ski slope the difference between slope speed and horizontal speed is less than 2%. You're quite right. I had a brain fart when I made up those numbers and did sqrt(1.2) instead of sqrt(1+(0.2^2)). quote:Originally posted by vstanescu:The car measures the speed using wheel rotation. The wheels are rotating the same number of times on a given distance, and the fact that this distance is covered climbing a hill, descending a slope or going straight has no relevance for the measurement method. It is just the distance along which you are going. The GPS might (or might not - I don't know about this) calculate the speed between two coordinates using only lat&long. If it uses the height also (calculating the distance between the 3D coordinates) it will give the speed that the car is showing. Thanks for being patronizing and wrong at the same time. The speed your spedometer gives can be wrong for a whole lot of reasons, most notablely that they're usually poorly calibrated at best (+/- 10%, or roughly 5mph at highway speeds), and the fact that even if precisely calibrated, they're highly suceptible to issues like tire size/inflation and slippage. Now, lets say you're on the maximum slope allowed on major roads, roughtly 10%. That means that you descend (or rise) 10 ft for every 100 ft that you go horizontally. (This is pretty steep. most highways have a maximum incline of 6 or 7%) For a a 10% grade, for every 100 ft horizontally you would actually travel sqrt(100^2 + 10^2) ft, or roughtly 100.5 ft, or less than 1/2 of a percent difference, which is paltry compared to the 10% error that most spedometers already have. Now most GPSRs claim a +/- 0.5 mph accuracy, or roughtly 1% error at highway speeds. The difference between measuring horizontal and "true" distance is already below the accuracy threshold, and anybody who would drive 50 mph down a 10% slope is a ballsier man than I. Now, lets say the GPSR tried to use vertical displacement. Most GPSRS are far less accurate in the vertical plane than in the horizontal, which means that vertical position may vary by up to 50-100 ft in subsequent readings. Since a typical leg legth of a track is 40-60 ft, this means that occasionally the GPSR will see a slope of 100-200%, and will show speeds that are 40-75% too high, drastically REDUCING the accuracy of the measurements. Of course, if you skiing down a 200% hill (pretty extreme even for expert skiers), your speed measurements will only be 58% of what they should be, but then you should use the tracklog and calculate the average yourself. This is a small price to pay for great accuracy in "normal" situations. If you're sky-diving, then you probably want 100's ft/min or ft/sec anyway, and most GPSRs have one of those as an option. -- Mitch
  15. quote:Originally posted by peter:I agree with the first part, but you're still exagerating the effect. On a 20% ski slope the difference between slope speed and horizontal speed is less than 2%. You're quite right. I had a brain fart when I made up those numbers and did sqrt(1.2) instead of sqrt(1+(0.2^2)). quote:Originally posted by vstanescu:The car measures the speed using wheel rotation. The wheels are rotating the same number of times on a given distance, and the fact that this distance is covered climbing a hill, descending a slope or going straight has no relevance for the measurement method. It is just the distance along which you are going. The GPS might (or might not - I don't know about this) calculate the speed between two coordinates using only lat&long. If it uses the height also (calculating the distance between the 3D coordinates) it will give the speed that the car is showing. Thanks for being patronizing and wrong at the same time. The speed your spedometer gives can be wrong for a whole lot of reasons, most notablely that they're usually poorly calibrated at best (+/- 10%, or roughly 5mph at highway speeds), and the fact that even if precisely calibrated, they're highly suceptible to issues like tire size/inflation and slippage. Now, lets say you're on the maximum slope allowed on major roads, roughtly 10%. That means that you descend (or rise) 10 ft for every 100 ft that you go horizontally. (This is pretty steep. most highways have a maximum incline of 6 or 7%) For a a 10% grade, for every 100 ft horizontally you would actually travel sqrt(100^2 + 10^2) ft, or roughtly 100.5 ft, or less than 1/2 of a percent difference, which is paltry compared to the 10% error that most spedometers already have. Now most GPSRs claim a +/- 0.5 mph accuracy, or roughtly 1% error at highway speeds. The difference between measuring horizontal and "true" distance is already below the accuracy threshold, and anybody who would drive 50 mph down a 10% slope is a ballsier man than I. Now, lets say the GPSR tried to use vertical displacement. Most GPSRS are far less accurate in the vertical plane than in the horizontal, which means that vertical position may vary by up to 50-100 ft in subsequent readings. Since a typical leg legth of a track is 40-60 ft, this means that occasionally the GPSR will see a slope of 100-200%, and will show speeds that are 40-75% too high, drastically REDUCING the accuracy of the measurements. Of course, if you skiing down a 200% hill (pretty extreme even for expert skiers), your speed measurements will only be 58% of what they should be, but then you should use the tracklog and calculate the average yourself. This is a small price to pay for great accuracy in "normal" situations. If you're sky-diving, then you probably want 100's ft/min or ft/sec anyway, and most GPSRs have one of those as an option. -- Mitch
  16. quote:Originally posted by Geek Boy:I'm only speculating but since the receiver is aware of elevation changes, wouldn't it be able to calculate the change in distance both vertically and horizontally over time to determine velocity? The calculation is trivial compared to all the other math it's doing (solving multiple nonlinear simultaneous equations of intersecting 4 dimensional parabolic hyperboloids) However, in almost all practical cases the difference in speed is trivial, and since vertical dilution of position (vertical error) is relatively high, and delta_h can fluctuate quite wildly, putting it in would lead to a much larger error in speed calculations that just leaving it out. Even on a relatively steep ski slope (20%), the difference would only be 10% or so. If you're hurtling down the ski slope, you may be interested in the vertical speed readout. -- Mitch
  17. quote:Originally posted by BrianSnat:I don't believe the website will approve buried caches anyway. Buried caches are a no-no, but only if you have to dig to retreive them. If the container is in a small hole flush with the ground, then that is okay provided you got permission from the lang manager. -- Mitch
  18. quote:Originally posted by BrianSnat:I don't believe the website will approve buried caches anyway. Buried caches are a no-no, but only if you have to dig to retreive them. If the container is in a small hole flush with the ground, then that is okay provided you got permission from the lang manager. -- Mitch
  19. quote:Originally posted by Cache Magnet:I just heard form Garmin that new legislation has been handed down from the CA state court system requiring Garmin to include a warning on any of the gpsr's that have antennas due to the chance of falling on an extended antenna and impaling and oriface That's why I only use etrex GPSr's... no sharp corners on which to hurt yourself. -- Mitch
  20. quote:Originally posted by Cache Magnet:I just heard form Garmin that new legislation has been handed down from the CA state court system requiring Garmin to include a warning on any of the gpsr's that have antennas due to the chance of falling on an extended antenna and impaling and oriface That's why I only use etrex GPSr's... no sharp corners on which to hurt yourself. -- Mitch
  21. quote:Originally posted by georgeandmary:Do I need to change any setting to get Easygps to work with the Magellan? Two things: First get the latest version of easygps, since I think support for some magellan units was a recent addition. Second, I think you need to change the preferences to use the magellan protocol, since maggies speak a different language than garmins. -- Mitch
  22. I'm not sure I entirely follow your narative, but if you're trying to take the same way back that you took out, you can use the trackback feature, though I've had mixed luck with that. But it sounds like it did take the the most direct route home, though probably not accessible since you can't really take you bike on the freeway. (or was the point that you were already close to home, and the legend just showed you a quick way?) Maybe I'm just dense today. -- Mitch
  23. I'm not sure I entirely follow your narative, but if you're trying to take the same way back that you took out, you can use the trackback feature, though I've had mixed luck with that. But it sounds like it did take the the most direct route home, though probably not accessible since you can't really take you bike on the freeway. (or was the point that you were already close to home, and the legend just showed you a quick way?) Maybe I'm just dense today. -- Mitch
  24. Here's a solution that doesn't even involve hacking up the case, you can make a re-radiating antenna, that also acts as a power source for the etrex. -- Mitch
  25. OGJ, jura eha guebhtu EBG13, tang orpbzrf gnat, naq ivfr-irefn. Qb lbh xabj bs nal bgure jbeq cnvef gung ner EBG13 cnyvaqebzrf?
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