Hand held GPS device advice requested.

[Badger74]

Good afternoon collective,

I need some advice about something I don't know much or nothing about.

Can I get a handheld device that will give me latitude and longitude, to within a couple of metres, as well as height (height accuracy could be as important)?

I want to record location of test points during construction site works, but with out the need of a full time site engineer with all the gear to record points.

Any ideas appreciated.

Chris

 

[The Lord Poncho]

Yes - many hand held GPS will include a barometric altimeter that will give you height. Those that don't will usually provide a less accurate height by triangulating satellite locations. Now that the built in error that was applied via satellites to civilian GPS units has been turned off, accuracy to within a few meters is usual unless you are in very heavy cloud cover, surrounded by tall buildings/terrain that confuses the signal, or under thick tree cover. Garmin foretrex 401 would be a fair starting point.

 

[Badger74]

Thanks TLP, I looked at that. I also looked at a few more and none of them mention accuracy or at least "within X metres".

 

[bearbait]

Assuming it's for European or N. American use one that is WAAS/EGNOS enabled will improve accuracy. Haven't read up on GLONASS yet but I believe that will help in getting a faster initial fix.

 

[Pterodaktyl]

Thanks TLP, I looked at that. I also looked at a few more and none of them mention accuracy or at least "within X metres".

Unfortunately none of them will specify the accuracy because even with WAAS/EGNOS/GLONASS enabled it still varies constantly depending on weather and surroundings. In real world use of a Garmin GPSMAP 64s, which has a larger antenna than most handheld units, the best accuracy I saw, on an open hillside with a clear view of the sky and with EGNOS/GLONASS turned on, was about 4m. Height accuracy is going to be even harder to achieve - GPS height calculations are less accurate than the lat/long ones, and barometric height is going to drift over time as the local pressure changes.

 

[Keithlard]

GPS accuracy is quite variable, depending on the antenna size, the signal quality, number of satellites visible, etc. It's perfectly good enough for navigation (’a few metres' is about right), but not for the kind of surveying work you're talking about. You'd need a differential GPS system: one that's corrected by signals from a fixed base station. That will give you sub-metre accuracy.

 

[bigcas]

look at the handheld etrex range, great device and helped me navigate around Africa very accurately. personally i'm very biased towards this brand due to how great it was out there. I don't think i'll ever trust another device.

 

[Badger74]

GPS accuracy is quite variable, depending on the antenna size, the signal quality, number of satellites visible, etc. It's perfectly good enough for navigation (’a few metres' is about right), but not for the kind of surveying work you're talking about. You'd need a differential GPS system: one that's corrected by signals from a fixed base station. That will give you sub-metre accuracy.

I know, that'll do down to the millimetre, but that is a MASSIVE expense on something that only requires a couple of metres and is not a full time occupation.

I've been playing with android apps that will give latitude and longitude. These seem to have it down to within 5m at the moment and in stormy conditions here on the west coast of Ireland.

 

[santaman2000]

Yes - many hand held GPS will include a barometric altimeter that will give you height. Those that don't will usually provide a less accurate height by triangulating satellite locations. Now that the built in error that was applied via satellites to civilian GPS units has been turned off, accuracy to within a few meters is usual unless you are in very heavy cloud cover, surrounded by tall buildings/terrain that confuses the signal, or under thick tree cover. Garmin foretrex 401 would be a fair starting point.

Yeah anything masking the signal could affect GPS readings; but to be truthful weather would also affect a barometric altimeter as pressure varies with the weather.

 

[bearbait]

Yeah anything masking the signal could affect GPS readings; but to be truthful weather would also affect a barometric altimeter as pressure varies with the weather.

Exactly right. But you could improve the altitude readings slightly by a) calibrating the barometric altimeter at a known point at the start of the survey, and b) checking the altimeter reading again at a known point at the end of the survey. Assuming the final reading is 10m adrift you could then apply this proportionally to the observations made during the survey. Of course, this is subject to weather errors as SM2K says, e.g. a fast moving depression during a long day's survey.

The accuracy requirements (and funding) of your survey are the key to the solution adopted.

 

[Keithlard]

I know, that'll do down to the millimetre, but that is a MASSIVE expense on something that only requires a couple of metres and is not a full time occupation.

Then what you're really asking is "Will accuracy of around 5m be good enough for my project?" Only you can answer that.

 

[santaman2000]

Then what you're really asking is "Will accuracy of around 5m be good enough for my project?" Only you can answer that.

It sounded like he was stating it rather than asking it in that last post. The OP asked if it was possible to get that accuracy from a hand held GPS (not if that accuracy was good enough)

 

[SCOMAN]

I know, that'll do down to the millimetre, but that is a MASSIVE expense on something that only requires a couple of metres and is not a full time occupation.

I've been playing with android apps that will give latitude and longitude. These seem to have it down to within 5m at the moment and in stormy conditions here on the west coast of Ireland.

Bear in mind that any phone will use it's signal reception to tie down its position. Once you go a way off shore most phones GPS must be treated with caution. Every GPS unit will vary it's accuracy even the military encrypted ones. The most accurate would be a differential GPS capable unit. Most of the medium level GPS have a barometer function which is aligned by you putting in yr altitude at time of setting and it then works out the altitude sampling the air pressure as you gain/lose altitude. The Garmin Foretrex 400 and Fenix watch were pretty accurate in my experience.

 

[Geebe]

The Garmin Etrex 30x has a Barometer. I have the Etrex 20x (same but no barometer) and I am very happy with it, but I only use it for navigation. Waterproof, rugged etc.

 

[santaman2000]

....... Most of the medium level GPS have a barometer function which is aligned by you putting in yr altitude at time of setting and it then works out the altitude sampling the air pressure as you gain/lose altitude. The Garmin Foretrex 400 and Fenix watch were pretty accurate in my experience.

Most of the ones sold here work off the satellite triangulation for altitude as well. Or at least that's the way they're advertised and the way the included manuals state. To be honest I have no idea how accurate the handheld ones are but then ones in the cars seem accurate (altitude wise) to within a few feet (bear in mind that Florida's relatively flat and a few feet make a huge difference)

 

[Robson Valley]

Airports have altitude benchmarks, you may have to ask where. YXS "see that triangle of grass beside the control tower? The benchmark is in the middle." I saw reception from 6 satellites.
Not too good around McBride, in the narrow side valleys, useless. But then again, follow the water instead.

 

[SCOMAN]

Most of the ones sold here work off the satellite triangulation for altitude as well. Or at least that's the way they're advertised and the way the included manuals state. To be honest I have no idea how accurate the handheld ones are but then ones in the cars seem accurate (altitude wise) to within a few feet (bear in mind that Florida's relatively flat and a few feet make a huge difference)

A GPS 3D fix is possible on most modern receivers, I haven't seen enough accuracy to completely trust it and have relied on the barometer function on the GPS sets I have for altitude. Not too say I'm up to speed on every set or the most modern kits.

 

[santaman2000]

..... Not too say I'm up to speed on every set or the most modern kits.

Nor am I. We'll figure it out though.

 

[bearbait]

Most of the ones sold here work off the satellite triangulation for altitude as well. Or at least that's the way they're advertised and the way the included manuals state. To be honest I have no idea how accurate the handheld ones are but then ones in the cars seem accurate (altitude wise) to within a few feet (bear in mind that Florida's relatively flat and a few feet make a huge difference)

As you say some work off the satellite triangulation for altitude. Some, at least, of the barometric altitude GPS units use the satellite-determined altitude to calibrate the barometric altimeter at boot-up time. However they do allow you to enter the known altitude of that point to correct the initial "boot" altitude.

 

[Badger74]

Thanks all for your comments. I think the consensus is "it depends"

I'll look into the ones mentioned and then go try them at the shop before purchase.