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Handheld Accuracy to 1m


SewerSleuth

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Hi all, newbie here.

 

I know nothing about GPS devices and have never used one before. That's better, now I've got that off my chest. <_<

 

Please help. I'm looking for recommendations for a handheld GPS device that can give me decimal co-ordinates to the nearest metre anywhere in the UK. I don't need any mapping, or fitness services. Altitude information would be useful but not essential. The device needs to be very easy to use, rechargeable with a good battery life and waterproof.

 

I need to gather GPS data on the locations of sewer manhole covers, and with my non-expert head on, I imagine standing on top of a manhole holding one of these devices and it giving me the Eastings and Northings that I require, in a decimal format to 1m accuracy.

 

This is for an industrial application, not leisure, and we would most likely need maybe 10 of these for our engineering crews.

 

Is such a device available on the open market?

 

Thanks in advance.

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Hi, and welcome.

 

Such devices are available - but they ain't cheap. I work in managing public rights of way for a local authority in the UK. Our needs include being able to plot/locate public rights of way to the nearest metre accuracy. The device we use is an Ashtech MobileMapper 100, onto which we are able to load OS Mastermap data and our public rights of way datsets (which are all held on an ESRI ArcGIS corporate GIS system).

 

The device is invaluable to our work and saves us a great deal of time and effort (and therefore money), but it does come at a cost - the handheld device alone costs about £3000, with software costs on top of that.

 

I have no affiliation to them , but we bought our device from a company called Positioning Resources of Aberdeen, and get excellent backup and support from them. Google them, if you're interested, they have a website, and are also happy to talk to people on the phone.

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Google GPS post processing. There is software that compares data for two GPS receivers with one at a known position. Here in the states the government has data for fixed stations you can use as one of the two devices.

 

Give Delorme a call. They have some folks that work with business and professionals.

Peoria Bill :>)

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Just curious, do they have WAAS ground stations in the UK? My little knowledge of post processing came when I found out you could do it with cheap GPS's that output raw pseudo range data, like Sirf based units.

 

I don't know about WAAS ground stations (and I'd think it unlikely because we generally can't pick up the US WAAS signals this far east), but we have a few EGNOS ground stations here in the UK which feed into the EGNOS corrections which we can receive.

 

The EGNOS ground stations are shown here...

EGNOS Ground Segment

Edited by RamblinBear
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I don't know about WAAS ground stations (and I'd think it unlikely because we generally can't pick up the US WAAS signals this far east), but we have a few EGNOS ground stations here in the UK which feed into the EGNOS corrections which we can receive.

 

The EGNOS ground stations are shown here...

EGNOS Ground Segment

Interesting. It looks like they provide data that can be converted to the RINEX file format that post processing uses.

 

RIMS2RINEX

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I thought these professional sub meter units used differential GPS with "local" base stations in order to get the real time higher accuracy.

Yes, they can be used that way - with an additional bunch of kit - but that typically gets you into the 10-centimetre (or better) range of accuracy. That's more accuracy than I need in my work, and more cost than my employer would want to bear. The thing is, the basic data I'm working from isn't accurate to that level - so the extra precision form the receiver would be "wasted" in that application.

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I thought these professional sub meter units used differential GPS with "local" base stations in order to get the real time higher accuracy.

What is a local base station?

http://www.esri.com/news/arcuser/0103/differential1of2.html

To attain accuracy levels on the order of one to 10 meters, differential correction is essential. The three main methods currently used for ensuring data accuracy are real-time differential correction, reprocessing real-time data, and postprocessing.

 

http://www.trimble.com/gps_tutorial/dgps-where.aspx

Edited by Red90
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From my limited testing:

I set a SporTrak Pro, Meridian Color, and a ProMark 3, all to display centimeters in the User Grid, and let then sit on the top of a wooden storage unit for for 20 minutes. Then I watched for another 10 minutes as the SporTrak stayed within 3 to 32 cm's of the ProMark, and the Meridian was within 5 to 37 cm's of the ProMark. Setup on a E-W plane 19 cm's apart. The ProMark was showing most of the movement, as usually after about 15 minutes the SporTrak and Meridian's cm's will settle in the averaging mode.

I haven't found a way to setup a ProMark 3 the same way that Garmin "does" the User Grid, but when set to hddd.ddddd, and not averaging, the yellow GPS 60 read the same as the ProMark 3, and the Sirf 60CSx was showing one number different. On a 60 I can cheat on the Scale and show grid feet in Trans Merc by putting in 3.2808333, but with the Lat tied to the Equator, it screws things up when comparing to the Maggies and the ProMark.

When I had the 62s I ried to setup the User Grid to display in feet, but failed, as it would only show in meters, in Trans Merc, and the Lat is still tied to the Equator.

Dang it, I sure wish that Garmin's units could be setup to display in cm's, so I could compare them to the Magellans!

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The search for a GPS with 1m accuracy is futile in my opinion. The problem is the signals pass the ionosphere, which is a mass of charged ions that constantly shift. The ions slow down the radio waves in varying amounts. Say a packet is sent out from a satellite and arrives at the two GPSs.The fixed GPS with a known position knows the error caused by the ionosphere and the error can be corrected for the the GPS with unknown position. They can compare because each packet has unique stamp and they fix like stamps.

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Sorry for bumping this thread, but I too am curious about this subject. I'm not familiar with all of the companies that make DGPS products besides Trimble.

 

I looked at Trimble's options and I think their cheapest 6000 series handheld unit cost about US$5000. Are there cheaper units available?

 

Basically I'm looking for a unit that will record GPS at a 1 second interval into a GPX (or any other format) with an update frequency of 1hz or more per second. I know that if I want better accuracy than what the recreational units will provide will obviously cost more money. What is the cheapest DGPS unit that will fulfill my needs? Can I get something that's under US$2500?

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If your idea is just get to a place get the coordinates and ready, it will probably work only if you have a good view of the sky at least for about 270 degrees.

On the moment highrise (a city or any kind of canyon) is around you gps will receive multi-path signals and calculate a wrong position.

 

The gps technology isn't an exact technology, the original signal might be a bit off from the start, as explained above there will be interference in the ionosphere, there might be (heavy) clouds, trees and buildings, or powerlines.

Signals can be distorted by transmitting towers and so on.

 

Once the signal is in the gps the software calculates what it has, even when it has a distorted signal, this gives a position as it is.

 

The main error many people make is, believing the gps is spot on, in the car satnavs this is done in the software AFTER the coordinate calculations, if the gps thinks it has acceptable coordinates, the software will glue your position on the nearest road.

 

As mentioned above on some Garmin models (other models I don't know) they have an Averaging waypoint calculation, but in fact this takes time like 20 minutes or even more and you might have to do it again on a different day.

 

Important to know is also if you have a real good accurate coordinate, then what? If you approach it with a cellphone gps you can see it once you are near as you will with a cheap dedicated gps as you will wil a real professional expensive gps.

 

So for finding it back a visual of about 30 feet will do also.

Edited by splashy
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If your idea is just get to a place get the coordinates and ready, it will probably work only if you have a good view of the sky at least for about 270 degrees.

On the moment highrise (a city or any kind of canyon) is around you gps will receive multi-path signals and calculate a wrong position.

 

The gps technology isn't an exact technology, the original signal might be a bit off from the start, as explained above there will be interference in the ionosphere, there might be (heavy) clouds, trees and buildings, or powerlines.

Signals can be distorted by transmitting towers and so on.

 

Once the signal is in the gps the software calculates what it has, even when it has a distorted signal, this gives a position as it is.

 

The main error many people make is, believing the gps is spot on, in the car satnavs this is done in the software AFTER the coordinate calculations, if the gps thinks it has acceptable coordinates, the software will clue your position on the nearest road.

 

As mentioned above on some Garmin models (other models I don't know) they have an Averaging waypoint calculation, but in fact this takes time like 20 minutes or even more and you might have to do it again on a different day.

 

Important to know is also if you have a real good accurate coordinate, then what? If you approach it with a cellphone gps you can see it once you are near as you will with a cheap dedicated gps as you will wil a real professional expensive gps.

 

So for finding it back a visual of about 30 feet will do also.

 

Can you please address my questions rather than giving me your opinion on whether there is a need for real accurate coordinates?

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You could try getting a unit with an external antenna connection. The GPSMAP 62 and the Montana both have a spot where you can connect an external antenna. It's not going to be as good as a professional-grade unit, but if you're under heavy tree cover or in challenging reception conditions it helps out.

 

The Garmin GLO updates at 10 Hz. It's a bluetooth device that can connect to your cell phone. I've never used one.

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ublox neo 7 modules will do sub 1m - provided you use gps, egnos (sbas) and their ppp carrier phase averaging - you need full clear sky view and a few minutes of being stationary,

 

In the uk with an old etrex yellow with the WAAS on you get about 6ft radius error - you need to hold the unit level and still. The original use for WAAS was for north American shipping.

 

The data format is the same in the Egnos Europe sats - So put WAAS on and you decode Egnos and go from 20ft radius error to 6ft radius error.

 

sat 33 is an Egnos., in the low south geostationary. Helps if you aim your screen towards the satellite to get a faster initial decode

 

Steve

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They only quote 2m best in any of their literature for the Neo-7, and that assumes a clear sky and no weird PDOP issues at the time of measurement.

 

WAAS was developed by the FAA for air navigation, not shipping.

 

About the only part I can agree with is that it's beneficial to use WAAS/EGNOS where available.

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WAAS was for north American aviation, although it is mainly, currently used in shipping and farming.

 

neo 7 SBAS + PPP (GPS): < 1 m CEP

as is neo 6 also. You do need to stand still and allow the carrier phase averaging to converge. You would benefit from a reasonable phase centre antenna like some of the old Sarantel designs.

 

http://www.u-blox.com/en/gps-modules/neo-6p-/neo-7p.html

 

Steve

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Are we not missing the point... He requires a GPS that will allow him to find

a manhole cover... Unless there are two or more manhole covers at his GZ then any of the modern

GPSs will be suitable, especially the latest Glonass enabled units.

 

Its all right being ultra precise if the person who is next to find the Manhole cover is using the same equipment as yourself, but spending 10 x £5000 is a bit different to buying 10 £120 units.

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