coshida

I have a refurb Dakota 20 from Garmin USA. All is fine and the handset looks new. (Don't forget to check whether you have household or bike insurance to claim from.)

Below is a screen grab from a commercial GNSS. The large circle is a 5 meter radius, the small a 1 metre radius. Every blue dot is a sampled position. The green circle is average position calculated from the samples. Note the 10 metre plus total drift North-East and South-West. The averaged position error in the pic is only 0.23 metres, but it's taken 880 samples to get that. (15 GPS sats in view, & 1 Egnos.) If you randomly sample one position from that scatter, you could be ten metres out north-east or south-west. The sample rate is one per second. The receiver will calculate your travel direction (COG) and speed-over-ground (SOG) between any two consecutive points. More scatter between points means the receiver's indicated position 'fluctuates'. It can do this every second: ie every time it calculates a 'fix'. This is why receivers often indicate you're doing 2-3 MPH even when you're standing still. Every 1 second 'fix' produces a different set of numbers. 1 MPH is 0.44704 metres per second. If the receiver 'fix' is 10 metres from the last, one second later, (see pic) you've 'travelled' at 4.4 metres per sec, (SOG) for that time. That's 2.24 MPH. Receivers mostly have SOG filters in them to mitigate this. From this you can infer that a spot-sampled GPS position is often no better than 10 meters or more, and that averaging is needed for better position. 15 minutes should do it. Regarding satellite movement; the receiver knows this so it's not an issue. There are small timing errors in the receiver which add to varying indicated positions but they are very small at 0.000786 parts per million or so in a modern receiver. (Tested in a U-Blox 622 GNSS) Clock drift is also very small at 0.5 us/s (micro-seconds per second) on an average receiver, but all of it adds up to 'spider tracks' on map displays and swinging needles on handsets, made worse by multi-path signals and moisture in the air. As an example, the screen shot below was taken in clear sky. An hour later cloud cover had arrived, and the averaged position worsened to 5.67 metres, twice as 'bad'. Water vapour attenuates radio signals and refracts them so you might see more spider tracks in cloudy weather or fog. You can see from the above that averaging a position, (available on most Garmins) is the best way of getting a close to absolute position fix if you are placing a geocache. If you are searching, remember the 10 metre (or worse) differences between 1 second position 'fixes'. Also remember that the quoted accuracy for a GPS handset is an 'averaged' accuracy, not an instantaneous and repeatable one. If you work to an accuracy of ten metres, that's pretty good on a sunny day on flat plain. The 'any instantaneous fix can be >10 metres out' is a reason why two receivers placed in the same spot will rarely exactly agree. For flight purposes the accuracy of non-corrected GPS (USA system) is quoted at 17 metres. Averaging and /or differential corrections improve that: EGNOS-corrected flight GPS quotes 1 metre instantaneous. That's how it is going to stay until we get the ionosphere repaired and add atomic clocks to the insides of our handhelds :-) Finally, if you have nothing better to do here's an early report on GPS Averaging. No one need ever ask the question again after this post. :-) http://i58.tinypic.com/ettug4.jpg

Good old Garmin-speak: 'likely be resolved'. Tempe (and Chirp) work well on the Etrex. I sometimes get the feeling that Garmin staff work in sealed boxes so that they can't communicate with each other.

To complicate the discussion, many of the Garmin units have 'Sight 'N Go' or similar which lets the user point the hand-held at a heading and walk to it in the manner of a bearing or 'marching' compass The basic Magellans (110)and upwards have a user-switchable 'compass' overlay (heading indicator) which can be overlaid on the map screen. There's a second 'compass' on one of the Magellan dashboards. For geocaching any pointer which points to the target might be considered useful. In the question of whether an electronic compass is useful, it's down to the users. Hikers like them, I'm told. The choice is between a two-axis, which needs to be kept level, and three-axis which doesn't. Three-axis compasses are useful in aircraft, I might add, but both consume battery power, if that's an issue.

The cheaper models from Garmin and Magellan are all easy to use and have reasonably simple interfaces. The cheapest are the Garmin Etrex 10 in yellow. Monochrome screen, no (usable) mapping, quite a tough unit for kids. The Magellan GC is also easy to use and about the same price. At the moment the slighter better featured Magellan 110 (with worldwide street maps is also about the same price as the Etrex 10, as the 110 is about to be axed by Magellan. Any of those will fit the bill. If you're putting waypoints into multiple units, the PC software (BaseCamp for Garmin and VantagePoint for Magellan) will enable you to set waypoints on the PC screen and download them to the handheld(s). Of the two choices, the Magellan PC software is easier to use and the cheapest handsets have colour screens. The Etrex Units have GLONASS +GPS and give faster initial fixes and arguably have better sensitivity in tree cover The Etrex (and Dakota) units support 'Profiles' so you can remove any pages or icons from the handset screens to tailor the units to your needs or simplify them. The touch-screen models (Dakota/Oregon) and the upmarket Magellans are slightly easier to use, but for 'kids and treasure hunts' the basic Etrexs or Magellans might well be enough. Out of the two, the Profiles on the Etrex 10, and the better sensitivity might be useful, at the expense of learning the slightly horrible PC application 'Basecamp'. Whatever you get, allow for a lot of rechargeable AA batteries in the budget.

I have similar issues with a Dakota 20. It connects once to Tempe or a Chirp sensor, then refuses to reconnect. Firmware is the latest 5.80. It's annoying as the Etrex 30 connects every time. The Tempe is part of a weather monitoring system, so good data-comms would be useful.

If you buy on DVD the content is locked to one handset. If you buy on SD card the maps can be moved around if you upgrade the handset later. You can also sell the original SD card if you no longer need it as it will work in other handsets. In both cases the maps can be viewed in the latest release of Garmin's BaseCamp PC software.

Yahoo!

I've owned & used both (for sailing. flying & hiking). The 62S is slightly faster, has the quad helix antenna and so on. The Etrex 20/20 are smaller, slightly slower. But they have GLONASS which is an added layer of precision (if you can see the sats) and they are smaller if you go hiking. I did sell the 62S and bought the Etrex as the smaller size was better for me. Fits in a pocket/aircraft cockpit, has terrific accuracy, SD card, fast to acquire sats. If you don't need the barometric altimeter and e-compass, the Etrex 20 is also very capable.

Go to the Magellan Web site and download the free Vantage Point software for PC. Install it and then plug in the Magellan. You'll be able to see what has been downloaded. Also in Map mode: 1) Press Menu then Map Options. 2) Make sure Geocaches/All is enabled and Icons & Names is enabled. If you still have nothing loaded, come back to Geocaching and make sure you have the Communicator plug-in loaded. The yell again.

It's a bit late now, but I have both. The Magellan 110 is a pretty nice unit with a bright colour screen. The Dakota has a touch screen which is dimmer but slightly larger. The Dakota picks up more sats quicker in a side-by-side comparision, but I think the Magellan is more pleasant to use overall as the screen is so much better. Magellan has 1.6Gb of user memory which will take OpenStreetMap topo maps, Garmin has a micro SD card slot for mapping. Garmin for speed, Magellan for the display. The Mag. is on offer at the moment at about half price if buyers hunt. The Dakoto is also being phased out, and can be had as a factory refurb for about the same price as the Mag. 110.

Bobbiesgirl: 1) On the receiver, go to Tools/Settings/Navigation/. 2) Set Coordination Accuracy to 1 metre. 3) Then go to Tools/Settings/Satellite. This shows the satellite screen. Press Menu. 4) An Options Box with Enable SBAS will appear. Set it to Enable and press back. With a good fix the receiver will show 'accuracy' of ten metres or less. (A 'good' fix in Magellan speak.) An 'excellent' fix will be found out in the open with an 'accuracy' of 7 metres or so. The 'accuracy' amounts to a circle of probability with the receiver at the centre, not a pinpoint fix. The receiver is accurate enough to both find and place caches, as long as the indicated fix is 'Good' or better. The 'accuracy' amounts to a circle of probability with the receiver at the centre, not a pinpoint fix.