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GPS Data Capture for GIS

 
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jeffvan
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Joined: Jan 07, 2004
Posts: 3
Location: Bedfordshire, England

PostPosted: Thu Jan 08, 2004 11:14 am    Post subject: GPS Data Capture for GIS Reply with quote

I am currently researching GPS units for our iPAQ 3970s that we have. The main use of these units will be for data capture using CadCorp's mSIS GIS product http://www.msis.info/. Data capture will be done at OS MasterMap level and will help to populate our land and property gazetteer (BS7666).

I will need fairly accurate information and would like a light weight alternative (and less costly!) then going for a Trimble, Leica or Garmin GPS units.

The bluetooth solution looks like the best alternative right now (particularly the SysOnChip Bluetooth GPS), but which are the best units for reliable/consistent accuracy (2D)?

Any help would be greatly appreciated.

Jeffrey Thumbs Up
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DavidW
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Joined: 17/05/2003 02:26:21
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Location: Bedfordshire, UK

PostPosted: Thu Jan 08, 2004 12:41 pm    Post subject: Reply with quote

To be honest, I'd ask the software vendors if they have any particular hardware recommendations.

That said, I'd go for something that supports SBAS, and intend to use EGNOS when it becomes available. That's the best accuracy you're going to get outside of specialist surveying units - but be aware that the information you're capturing really isn't survey grade. There's a reason that the high-end Leica GPS gear is so expensive - it's doing all sorts of tricks (particularly on the dual band models) that a consumer GPS unit is not.


It concerns me somewhat that you're talking about using OS MasterMap data, which is high detail expensive mapping, then using a consumer GPS where, even with EGNOS available (which it isn't at the moment - in the UK, the only satellite currently broadcasting a test EGNOS signal is too low to the horizon to be any use), you only have accuracy of about 2m horizontal and 5m vertical (both with 95% confidence). I'm unclear whether that specification from ESA refers to a single reading, or an averaged reading - and if the latter, I'm unclear whether the CadCorp software can carry out averaging (averaging multiple readings improves accuracy).

I really would talk to CadCorp about what their software can do and what kind of accuracy they expect when using a consumer GPS (which all have pretty similar accuracy figures). Only you can then decide whether a consumer GPS is accurate enough.



David
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jeffvan
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Joined: Jan 07, 2004
Posts: 3
Location: Bedfordshire, England

PostPosted: Thu Jan 08, 2004 1:10 pm    Post subject: Reply with quote

I am aware of the difference in the quality of the data captured between the survey grade GPS and consumer GPS. To be honest, I am just researching alternative methods for data capture. I think the only route to go down is the Trimble or Leica GPS, but it is a matter of persuading others this! Rolling Eyes

I can purchase rather easily a recreational GPS unit, but will need to seek approval from others for the Survey grade GPS.

Do you know the approximate accuracy of recreational GPS units? My understanding is it is around 5 - 10 m and sometimes more depending on how open the sky is.

You mention SBAS and EGNOS, but which units that are bluetooth support this? Are there any recreational GPS units that you can get with DGPS or some sort of differential correction?

Thanks
Jeffrey
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DavidW
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PostPosted: Fri Jan 09, 2004 5:59 pm    Post subject: Reply with quote

SBAS is wide area DGPS - conceptually it's a little different to the type of DGPS you receive from beacons, as it separates out the various sources of error in the transmitted corrections, so that the system works across a wide area.

At the moment, the European SBAS system, EGNOS, is not operational. The test signal currently isn't receivable in the UK; the only satellite transmitting at the moment is far too low to the horizon. This should change later this year; firstly the signal will become available from other geostationary satellites that should be visible in the UK, and EGNOS itself should hopefully reach full operational status later this year.


Most of the Bluetooth units are SBAS capable, once you find the right software to turn on SBAS - I believe the TomTom unit is the sole exception. The feature may be described as WAAS or WAAS/EGNOS - it all means the same thing. Gradually the term SBAS seems to be becoming the standard for WAAS, EGNOS and all similar systems (current or planned).


So far as accuracy - it depends what confidence interval you want to set. I gave ESA's figures for operational EGNOS in my previous post - 2m horizontal and 5m vertical with 95% confidence, though I'm unclear if that requires averaging multiple readings. In other words, 95% of the time, the position you're given by the GPS is going to be within 2m horizontal and 5m vertical. That's rather better than without SBAS; it's hard to give figures on that, as the official specification for GPS gives wildly pessimistic figures, seemingly dating back to the days when Selective Availability was on, which introduced deliberate errors into civilian GPS.


For survey use, I'd say you really do need a decent Leica or similar setup. However, for 'quick and dirty' data collection, an EGNOS capable consumer unit won't be that bad.



David
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cultivator
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Joined: Oct 02, 2003
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PostPosted: Sat Jan 10, 2004 6:39 am    Post subject: GPS1000 Plus? Reply with quote

In addition to the high-end, submeter systems from Trimble, Ag Leader, etc, and the relatively low-end 2-5 meter receivers like the Emtac, Socket, Navman, etc, you might consider the mid-range Ag Leader GPS 1000 Plus (new version just released). Not sure of the cost outside US, but here it's around $750 USD, with accuracy of 2m or better and DGPS WAAS and EGNOS. So it's fine for yield mapping, site verification, application rate logging, soil sampling, scouting, etc, but not a good choice for guidance or other highly precise ag applications. One new feature is the ability to output a radar speed signal for input into your rate controller or other equipment that accepts radar speed input. It's also weatherproof and durable - big pluses for ag use. Not quite the same type of animal as the GPS Sleeves or CF units, but it can definitely be used with an iPAQ and is flexible for future expansion. It's often used as an inexpensive second GPS receiver.

It can be used with an iPAQ running Farm Works Software, so it might work for your software as well. Check with Ag Leader's international dealers for more information. Or there is probably something similar in other companies offering ag-specific GPS receivers in your area.

Spec sheet: http://www.agleader.com/docs/bulletin-gps1000plus.pdf
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