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Thread: Handheld GPS

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    Default Handheld GPS

    Hi guys,
    Noob here with a questions about handheld GPS.

    I want something that I can walk around the city with that will plot various points of interest.
    Preferably to 6 decimal places.
    This is important because some POI's will be quite close together, possibly a metre or so.

    I would like it to be able to store around 80 POI's which hopefully I can then export from the device to a PC.

    Any suggestions?

    Awesome forum guys too.
    Thanks.
    Luke.



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    You won't get anything with 1m resolution.

    Why not jut use maps on your phone?

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    The Garmin Etrex 10 is an entry level GPS that will save up to 1000 waypoints but only has a very basic map and the map can't be upgraded.
    The Etrex 20 can have up to 2000 waypoints and can have other maps loaded either directly into it's internal memory or on micro SD card.

    Data can be imported/exported to Garmin Basecamp which is a free program or thy can communicate directly with a PC and Google Earth.

    As previously said accuracy to 1 metre isn't possible. The best you could get out in the open is 3-4 metres. In a city with tall buildings and other structures this may be worse and this accuracy would be the same for any non differential GPS

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    I have a Garmin Oregon 450 which has been used a lot for bushwalks and around old gold mining areas. Touchscreen with maps, real easy to use and can be either downloaded to Garmin Basecamp or OziExplorer mapping software.

    Good battery life, water protected and very rugged. The model up Oregon 550 has a camera built in which would be handy for some.

    Depends how much you want to spend I guess.
    Last edited by Landytrack; 14-09-17 at 04:21 PM.

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    yeah, I've got an old Garmin eTrex Vista HCX that would do the job the OP wants, except that max accuracy is 3 metres as they all are.
    The screen is a bit small, but it does the job.

    You could probably pick up one of these second hand on Ebay for a good price. Maps are easy to come by.
    Cheers, Tiny
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    Quote Originally Posted by Tiny View Post
    yeah, I've got an old Garmin eTrex Vista HCX that would do the job the OP wants, except that max accuracy is 3 metres as they all are.
    The screen is a bit small, but it does the job.

    You could probably pick up one of these second hand on Ebay for a good price. Maps are easy to come by.
    Tiny,

    That 3 metre accuracy figure is a bit of a furphy in Australia.

    That level of accuracy can only be reliably and consistently achieved with a typical hand-held unit when used in conjunction with a SBAS (Satellite based Augmentation system).

    Two such systems are currently in use – WAAS and EGNOS – neither are usable in Australia, as the augmentation information that they relay applies to the Northern Hemisphere, and is worse than useless down under.

    There is currently a project underway, under the direction of Geoscience Australia, to develop a Southern Hemisphere SBAS – see below:



    and it appears it is now reasonably well advanced:



    From the above link:

    Drury noted that without augmentation GNSS systems such as the US GPS constellation and the European Gallileo system offers navigational accuracy of around 10 metres in the horizontal plane.

    “We’ve taken that accuracy from 10 metres down to a metre or two, and ultimately the horizontal accuracy will end up in the range of centimetres, somewhere around five to 10 centimetre accuracy,” Drury said.
    It is possible to achieve better than 10 metres with an off-the-shelf unit, but that necessitates taking a number of fixes over a fairly lengthy period of time and then applying an averaging function, which is available in quite a few retail units (the Oregon 600 has it).

    Accuracy can also be improved if the unit is capable of combined GPS/GLONASS operation – again, quite a few of the more recent retail units have this capability.

    If you’re interested, the following link details a test performed on the Garmin Oregon 600 (a unit I’ve recently acquired) .

    The unit is tested in two locations – one out in the open, the other subject to building and tree shading.

    Separate tests are conducted taking 30-second fixes over a thirty minute period using GPS only, and then GPS+GLONASS.

    The results are presented in graphical and mathematical form.

    You will notice from the results that, in the shaded site, the results, even with the dual-system operation, are fairly ordinary.



    Luke is definitely not going to get the results he’s seeking using a hand-held GPS.

    Perhaps it’s a case of….. “Use the Force, Luke”

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    Quote Originally Posted by Thala Dan View Post
    Tiny,

    That 3 metre accuracy figure is a bit of a furphy in Australia.

    That level of accuracy can only be reliably and consistently achieved with a typical hand-held unit when used in conjunction with a SBAS (Satellite based Augmentation system).

    Two such systems are currently in use – WAAS and EGNOS – neither are usable in Australia, as the augmentation information that they relay applies to the Northern Hemisphere, and is worse than useless down under.
    you are probably correct, I've always suspected the accuracy meter reading on the eTrex to be a façade, however I thought that with the ability to receive & triangulate 12 sats simultaneously that it would be close.
    I dense bush the accuracy meter reads ~16m, but out in the open on a clear day it comes down to 3m.
    Cheers, Tiny
    "You can lead a person to knowledge, but you can't make them think? If you're not part of the solution, you're part of the problem.
    The information is out there; you just have to let it in."

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    Quote Originally Posted by Thala Dan View Post
    .........
    Two such systems are currently in use – WAAS and EGNOS ....
    Actually there's four currently, 3 in full use and 1 in partial use. Additionally to WAAS & EGNOS, Japan's MTSAT Satellite Augmentation System (MSAS) has been operational for 10 years now (Japan is also working on The Quasi-Zenith Satellite System, QZSS, still in proposal phase). The GPS Aided Geo Augmented Navigation (GAGAN) system being operated by India is not technically fully operational because it lacks dependable vertical aviation guidance so as yet can't be used for precision approaches but is routinely used by SBAS equipped aircraft for enroute navigation in Indian airspace. As well as the AU/NZ joint venture, there are others in development: Satellite Navigation Augmentation System (SNAS), proposed by China is one such. Wide Area GPS Enhancement (WAGE) is already operated by the United States Department of Defense but not available to most users being restricted for use by military and authorized receivers. Also we shouldn't forget the commercial systems which are fully operational, i.e. the StarFire navigation system operated by John Deere and C-Nav Positioning Solutions (Oceaneering) and Starfix DGPS System and OmniSTAR system operated by Fugro.

    Really for both off and on road navigation in Civilian situations, 15 m accuracy is acceptable. In aviation use for precision approach and landing in non-VFL conditions a 15 m error would likely be disastrous. Certainly agricultural users would consider 15 m accuracy as unacceptable but for different reasons.

    Clearly the Geoscience JV partners reckon they can flog their product to the rest of the world. I think they'll need to get their skates on as the others in development and testing phase probably intend doing the same.

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    Thanks, Surething,…..some excellent info there.

    I was nominally aware of the existence of MSAS and GAGAN, but just went with the two systems that seem to get most air-time in consumer GPS literature.

    Regards the Aussie SBAS….it’s sad, isn’t it……not long back we touted ourselves as the smart country, but nowadays we always seem to be the reluctant straggler, last to arrive at the party.

    Others around us are developing space industries and pursuing hi-tech development, whilst we agonise over how to dig more coal out of the ground.

    Meantime, if you happen to have any good links on the subject of consumer GPS accuracy and precision, they would certainly be appreciated.

    I’ve got quite a few, but they generally fall into one of two categories….either GPS-for-Dummies or GPS-for-Phd students.

    I’d like to find something that is written on the assumption that the reader is technically literate, but not sitting for a Masters Degree.

    @Tiny….you’re right to be sceptical of that “GPS” figure on the Garmin…..it’s a figure derived internally by a Garmin-proprietary algorithm, and there doesn’t seem to be any readily-available information as to its exact meaning.

    An early Garmin Spec sheet for the etrex series detailed the accuracy as:

    GPS accuracy:
    Position: < 15 meters, 95% typical*
    Velocity: 0.05 meter/sec steady state
    DGPS (WAAS) accuracy:
    Position: < 3 meters, 95% typical
    Velocity: 0.05 meter/sec steady state




    The 95% is presumably a reference to the R95 factor explained in the link I provided earlier.

    Essentially, that is saying that 95% of the data points collected fall within a circle of 15m radius. The other 5% could be anywhere outside that circle.

    This figure is calculated by the device, and there is no guarantee that the circle is actually centred on the true position, although theoretically it’s supposed to be.

    Later Garmin spec sheets don’t appear to provide this level of information.

    The 3m figure bandied about these days is most likely based on the CEP (Circular Error Probable), where 50% of the data points recorded are within a circle of 3m radius, whilst the other 50% can be anywhere outside of that circle.

    Again, the centre of the circle is supposed to based on the true position.

    Although the only reference I can find to 3m in the Garmin literature involves the use of WAAS.

    The usefulness of the 12 satellites providing data to the unit is a function of a number of factors, in particular the geometry of the satellites in question.

    You can see a bit more about this here:



    and here:



    So, more satellites doesn’t necessarily equate to better accuracy.

    Upshot of all this is that if your handheld GPS says that you are within 3m of where you want to be, chances are it’s more like 10-15 metres……but as Surething says, that’s more than enough for normal use.

    As a matter of interest, I’ve got an etrex10 and an Oregon 600.

    There’s a trig point on Reform Hill on the edge of town here, and, if it ever stops raining, I’m going to take both units up there and see how they pan out.

    Will post the results.

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    Back to the OP's question of finding a handheld GPS with accuracy to cope with POIs within about 1 metre of each other in a city environment. There's a few reasons why that's not currently possible here in Oz. Even high-end handhelds have some limitation as to accuracy, however in an open field with no nearby trees and a perfectly clear sky with no adverse atmospheric conditions it may be possible to achieve 1 m accuracy with the better handhelds. In a city-scape that's a pipe dream because of high buildings and other interference. The canyon-like conditions alone degrade accuracy, i.e. satellite signal blockage can occur due to buildings and bridges or even trees etc., additionally signals are reflected off buildings or walls resulting in "multipath" signals:


    The US Government has committed their GPS signal to broadcasting, in space, with a global average User Range Error (URE) of ≤7.8 m with 95% probability. They claim the actual performance nearly always exceeds the specification, quoting figures from May 11, 2016 where the global average URE was ≤0.715 m 95% of the time. To be clear, they don't mean that URE is the end-user's accuracy, rather it's the accuracy of the source signal. The actual accuracy experienced by the user depends on a combination of satellite geometry, URE, and local factors such as signal blockage/interference, atmospheric conditions and of course receiver design features and quality. Now, how could any receiver get, say, 1 m on the ground even in perfect conditions when the signal from the sats is usually not even that accurate? The obvious answer comes from enhancement by dedicated SBAS sats of course which for receivers that can process the WAAS/EGNOS/MSAS/GAGAN signals [they're inter-operable] may mean that their 'on-the-ground' accuracy is quite higher than that of the source signal in space. For non-SBAS areas like here, the accuracy is enhanced by fancy [and secret] algorithms like in Garmin's firmwares which then allow them to claim 3 m accuracy for automotive devices when we know that's seemingly unlikely.

    An aside: One little know fact is that turning on WAAS-type augmentation in an area such as Australia can currently result in getting degraded accuracy on the ground as well as increased battery usage. So you get lost quicker in the donga and have to find the paper map and compass.

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    Thanks for that Surething, I've been pondering this accuracy for civilian GPS & you've confirmed what I thought.

    Garmins algorithms must be pretty good & so are the ones in my UAV (drone) that has a built in GPS guidance system, it can land 10/10 times within a 1 metre radius of where it takes off, if I ask it to use it's automatic return to home system or a radio failure triggers it.
    The only augmentation to GPS that it has access to is it's altitude & the altimeter reading is only within 2 metres accuracy from my observations as it doesn't compensate for barometric pressure changes during flight.
    Luckily the landing programming can compensate for the discrepancy & not slam it into the ground or cut the motors at 2 metres off the ground. It wont stop it's final decent rate until it detects that it is descending no further for a few seconds & must be on the ground.
    Cheers, Tiny
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    Vertical accuracy is another thing altogether of course, exactly as you said due to barometric changes. However, those changes aren't constant. Pressure will always be higher at sea level than it is above sea level which is why pilots have to account for lessening of pressure as they gain altitude and it's also how the 'steam' altimeter in aircraft work, because in the International Standard Atmosphere(ISA) there is an assumed 1 hPa difference for each 30 feet vertical change in height, at least in the lower levels. Simplistically, disregarding other factors like temperature and humidity, given say, a change of +10hPa from the airfield's pressure means the aircraft's altitude would be 300 ft AGL, provided you've correctly calibrated the altimeter for the local pressure [it's QNH] at the departure airfield which is for example 100 ft ASL then the altimeter would read 400 ft ASL. The higher you go the more complicated things get because as well as pressure altitude you need to factor in density pressure. Also, high and low pressure weather systems might move across the flight path or the aircraft might move into them. So you must know the QNH for the arrival airfield as well even when not flying on instruments. A GPS with good vertical accuracy is a huge help for instrument approaches.

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    Quote Originally Posted by Tiny View Post
    Thanks for that Surething, I've been pondering this accuracy for civilian GPS & you've confirmed what I thought.

    Garmins algorithms must be pretty good & so are the ones in my UAV (drone) that has a built in GPS guidance system, it can land 10/10 times within a 1 metre radius of where it takes off, if I ask it to use it's automatic return to home system or a radio failure triggers it.
    The only augmentation to GPS that it has access to is it's altitude & the altimeter reading is only within 2 metres accuracy from my observations as it doesn't compensate for barometric pressure changes during flight.
    Luckily the landing programming can compensate for the discrepancy & not slam it into the ground or cut the motors at 2 metres off the ground. It wont stop it's final decent rate until it detects that it is descending no further for a few seconds & must be on the ground.
    That ability of your drone to return to within a 1m circle is more likely a function of Precision (or repeatability) than it is of Accuracy.

    The two parameters are linked, but different.

    Take the following diagrams a) and b):



    The receiver in a) has taken ten fixes…they are a bit scattered (but acceptably so), so the unit is not particularly Precise.

    But if the ten fixes are averaged out, the resultant fix will be quite Accurate, as the individual fixes are all fairly evenly distributed around the true position.

    So receiver a) would be said to be reasonably Accurate, but not particularly Precise.

    On the other hand, receiver b) has also taken ten fixes, which are tightly grouped together, so it is considered to be Precise.

    But when those ten fixes are averaged out they will yield a final fix that is well off the true position.

    Hence receiver b) would be considered precise, but not overly accurate.

    So if receivers a) and b) were rifles, you'd have a better chance of hitting the desired target with rifle a), but rifle b) would shoot a much tighter group, but off target.

    This figure gives some idea of the differing effects of the combination of Precision and Accuracy:





    Incidentally, has anyone noticed that the OP, Flukey, hasn't returned to the thread?

    Hope we haven't turned him off with all this tech-head talk

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    Actually Flukey probably walked away muttering 'coz he got his answer in Post #2 clear 'n' straight, without the later tech-head talk.

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