+Birders Posted January 6, 2005 Share Posted January 6, 2005 We have several times been out with other geocachers running a different make GPS to ours. Sometimes theirs seemed to be more spot on than ours, and vice versa with other caches. A few days ago we were out with some other friends who were running the same make GPS as ours, but a much more up to date model. First cache we did... the "needle" on ours shot off 90 degrees to the left with 10m to go but our friend said: "No, it's still 60 feet ahead" so we followed on and, sure enough, there was something which fitted the clue. A search revealed nothing so I strolled back along the track, following our GPS, and almost immediately found the cache. Their GPS still said 60 feet to go. Unfortunately I didn't have the presence of mind to check the exact co-ord readings on both instruments but will do so next time it happens. We only did two caches and the second one didn't need a GPS - it was laying on the floor in the open! We've heard talk of some GPS receivers having some sort of "delay" built into the reading but ours seems to keep up with exactly where we are.. Is this behind the discrepancy between various receivers? Problem is, if our old 10-year old clockwork Garmin GPS12XL fails what do we buy as a replacement? Quote Link to comment
markandlynn Posted January 6, 2005 Share Posted January 6, 2005 After a really bad day with accuracy doing the zodiac series i have found that fresh batteries can make quite a difference when cache hunting, as can prevalent weather conditions. I did see somewhere that as a school project some students took thier GPS out side to the same spot and took coordinates this was done at the same time and they got some interesting results. Snowdon summit also shows this variability as all these GPS'r s are in the exact same spot. Can't help with your replacement issue though Quote Link to comment
+harrogate hunters Posted January 6, 2005 Share Posted January 6, 2005 I have an Magellan 310.... yes that old ! Its so old there is oly two decimal places e.g. N53 51.12 W001 01.23 and only enough storage to store 10 waypoints. Its quite bulky. There is no onscreen data apart from direction to travel, distance to go (in KM e.g. 0.01 km) and speed travelled. Despite all this, I commonly get to caches on trips out with other geocachers before them....... How.... I dont know, but old is sometimes better ! I am obviously worried about upgrading..... so many contradicting advice..... So.... I am staying put until mine blows up... Quote Link to comment
Leoness Posted January 6, 2005 Share Posted January 6, 2005 I have an Magellan 310.... yes that old !<edit> Despite all this, I commonly get to caches on trips out with other geocachers before them....... <edit> Only because my Extrex needed new batteries and was losing accuracy! (That's my story and I'm sticking to it!!!) Quote Link to comment
+Fangsy Posted January 6, 2005 Share Posted January 6, 2005 I have an Magellan 310.... yes that old !<edit> Despite all this, I commonly get to caches on trips out with other geocachers before them....... <edit> Only because my Extrex needed new batteries and was losing accuracy! (That's my story and I'm sticking to it!!!) Sounds like a good excuse to make a belt-clipped super-long life battery pack and cable to power your etrex good for several days non stop power. Someone should make and sell such a thing! Quote Link to comment
+CuplaKiwis Posted January 6, 2005 Share Posted January 6, 2005 Sounds like a good excuse to make a belt-clipped super-long life battery pack and cable to power your etrex good for several days non stop power. Someone should make and sell such a thing! Not sure about for a GPSr, but last time I went climbing in NZ (up around Mt. Cook) my uncle decided he wanted super long battery life for his Video Camera. He wired up a Lead Acid/gel type battery (ie what you'd find in many motorbikes) as a external power source!! All 3.3Kg of it At 7000 feet Mad Bugger. He didn't run out of power tho'... Quote Link to comment
+tonycollins Posted January 6, 2005 Share Posted January 6, 2005 My magellan 315 goes into averaging mode when stationary. Unfortunately it also does this if you are moving slowly (less than 2-3 mph) This effectively puts a lag into the reading as you current position is averaged with your previous positions. I can get around this by running for a few paces which switches the averaging off. as long as you dont move while it is averaging there is no problem. Sensitivities of different recievers will also affect accuracy. If one GPSr has 10 sats locked, and another only 4, the first will give a more accurate reading. Then the other possibility. If you are using the wrong map datum (anything other than wgs84) then there will be a fixed offset introduced which can be as large as 300m (I think) Quote Link to comment
Cerise Posted January 8, 2005 Share Posted January 8, 2005 (edited) We have several times been out with other geocachers running a different make GPS to ours. Sometimes theirs seemed to be more spot on than ours, and vice versa with other caches. A few days ago we were out with some other friends who were running the same make GPS as ours, but a much more up to date model. First cache we did... the "needle" on ours shot off 90 degrees to the left with 10m to go but our friend said: "No, it's still 60 feet ahead" so we followed on and, sure enough, there was something which fitted the clue. A search revealed nothing so I strolled back along the track, following our GPS, and almost immediately found the cache. Their GPS still said 60 feet to go. Unfortunately I didn't have the presence of mind to check the exact co-ord readings on both instruments but will do so next time it happens. We only did two caches and the second one didn't need a GPS - it was laying on the floor in the open! We've heard talk of some GPS receivers having some sort of "delay" built into the reading but ours seems to keep up with exactly where we are.. Is this behind the discrepancy between various receivers? Problem is, if our old 10-year old clockwork Garmin GPS12XL fails what do we buy as a replacement? There are many influencing factors to GPS accuracy. I am sure you are well aware that its dependant on satellites, weather, environment (dadgum those trees) and if in doubt and you know your close triangulate. There are differences between GPS devices and all seem to have good and bad days. I have a Rhino 130 right now and that seems to be very accurate and as a bonus it has a radio facility which means I can send my little ghoul out ahead with her walky talky and keep in touch with her. If you know others with this unit you can share coordinates - its fun! The type of unit for you depends very much on what you want from it, I wasn't worried about colour just wanted a functional unit the walky talky function just meant my young daughter felt more involved and enjoyed the search more. All the best Edited January 8, 2005 by Cerise Quote Link to comment
+minstrelcat Posted January 8, 2005 Share Posted January 8, 2005 We have Magellans - the 315 and 330. We have been out a couple of times with Garmin users in wooded areas and the Magellans have performed better both times. Lisa Quote Link to comment
+The Forester Posted January 8, 2005 Share Posted January 8, 2005 We've heard talk of some GPS receivers having some sort of "delay" built into the reading but ours seems to keep up with exactly where we are. The symptoms you describe are exactly what would happen if the GPSr software has a filter to smooth out random errors in the fixes. Some models have what is called a Kalman Filter, which is a piece of mathemagic which dynamically alters the strength of the smoorhing depending on severeal parameters which measure the performance of the fixing. Usuallly this is done for use on vessels or vehicles. When walking you can turn through 90° much quicker than most vehicles or boats can, so a heavily filtered GPS output will momentarily show you still moving on your original heading. Iy sounds to me that the reason why one of the GPSrs was showing these symptoms while the other wasn't is an indication that one is filtered and the other is just outputting raw data. Cheers, The Forester Quote Link to comment
+Birders Posted January 10, 2005 Author Share Posted January 10, 2005 (edited) Thanks to The Forester for the information. We're curious as to why there should be any "averaging" on handheld devices employed for walking. What does it achieve? Yesterday we were out with some friends and we had three GPS receivers between us - our old Garmin GPS12XL and two others - a Magellan and a Garmin (can't recall exact models but both were "modern" - the Garmin was just a few months old). Our old Garmin consistently came up with the goods - taking us straight to, or very close to, caches when our friends were busy searching 50-60 feet or more away. The owner of the "new" Garmin was somewhat dis-chuffed at the performance of his GPS.. and the Magellan occasionally gave very weird results, saying a particular cache was some miles away when it was quite close! Is "averaging" a switchable option on some modern GPS receivers or is one stuck with "new technology". We guess that the low-battery problem is known about, although we are surprised that it happens. Edited January 10, 2005 by Birders Quote Link to comment
+The Forester Posted January 10, 2005 Share Posted January 10, 2005 (edited) why there should be any "averaging" on handheld devices employed for walking. What does it achieve? There are two different things here. Averaging is not at all the same thing as filtering. The reason for filtering is to smooth out the bumps in the track while dynamically fixing (kinematic trajectography, in the jargon). Averaging is an extremely powerful tool for refining a static position fix, such as when you want to measure the co-ords of a fixed point such as a trig pillar or a geocache. There's a very good analysis of the effect of averaging here and another page here which describes and quantifies the effect of averaging WAAS fixes. As for the reason for having a filter on a "walking" GPSr, I don't know. Perhaps it's a sales-driven gimmick which is to make the trackplot of a boat or vehicle trip look better on the screen. Raw data, plotted at a very large scale can sometimes look like the trackplot of a drunken crow with inky feet walking over a sheet of paper. Smoothing the track does little to improve the accuracy of the derived co-ords, but it does a good cosmetic job. An alternative explanation might be that the 'new' GPSrs were suffering from the fabled errors which are supposedly introduced by WAAS, but I've never seen any fresh or recent data which supports the anecdotal stories that a WAAS fix is somehow less accurate than an unaugmented one. Cheers, The Forester Edited January 10, 2005 by The Forester Quote Link to comment
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