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surveying question

 
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jonokimber
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Joined: Oct 03, 2003
Posts: 36

PostPosted: Thu May 13, 2004 12:18 am    Post subject: surveying question Reply with quote

Here is my suggestion, please leave some comments!

Surveyor arrives on site (500m by 500m). He is carrying a PDA with CF GPS receiver (standard unit, i.e not mega expensive).

He goes to a known position (the site origin) which is a stake in the ground. Then he wanders around the site (only 2D is important), taking waypoints as he goes. These waypoints are automatically numbered and stored.

The 'surveying' might take a few weeks but every morning he can go to the same reference point and take his first 'datum' point, from which all the other days points will be referenced.

After the whole survey is done the waypoints are downloaded, the different days collections are translated mathmatically so that any variation in the start point is corrected. Then, there is a massive list of waypoints all relative to oen fixed site datum.

One question i do have is whether this sort of 'localised datum' will be any good at being a sort of differential system thereby gaining accuracy. What makes me think it might work is because it is one waypoint relative to another that we are interested in, not where the whole set of waypoints are relative to the rest of the world.

For example. If i take waypoint 1 and then take waypoint 2, will the difference/distance between the waypoints be accurate?

Another question i have, is what software can be used on a pocket PC PDA (HP 2210 FOR TESTING) to take these waypoints? Ideally there could be some sort of data entry at the individual waypoints too.

Thanks in advance. What i might try as an experiment is to use my 2210 and tom tom to walk around the painted lines of a football (soccer) pitch, taking waypoints as i go. Then i can process these back at base and see if the waypoints descibe the same football pitch i just walked around!

Cheers in advance for any advice.

Jono
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lbendlin
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Joined: 02/11/2002 22:41:59
Posts: 11878
Location: Massachusetts, USA

PostPosted: Thu May 13, 2004 12:29 am    Post subject: Reply with quote

The statistics of the GPS position accuracy are not coupled to the time. So your approach is as (un)scientific as any other approach, and you will end up with the same HDOP numbers, no matter where and when you start.

SBAS/EGNOS or a local certified differential GPS transmitter will be the only way to improve accuracy.
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DavidW
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Joined: 17/05/2003 02:26:21
Posts: 3747
Location: Bedfordshire, UK

PostPosted: Thu May 13, 2004 11:07 am    Post subject: Reply with quote

True survey grade receivers do all manner of processing that isn't done in a consumer GPS. I only know a little about this, but I'll share what I know and hope it's accurate.


For high accuracy work, you tend to use software that works on a recording of raw pseudoranges over time to provide a much more accurate position than a single GPS position. Such raw pseudorange information can be got from SiRF receivers in SiRF mode, though I'm not aware of any software for the Pocket PC that performs this process.

You use the best data available about the GPS errors at the time of your survey as part of this process. This often involves a second receiver on a point whose co-ordinates are known to a high degree of accuracy, also use of systems like the UK's National GPS Network - see http://www.gps.gov.uk. The data available from that site is often used to process the raw data collected on site after the site work is complete.


All this gives a much more accurate position than putting a consumer GPS at a point and getting a reading!


A lot of survey grade gear uses correlation techniques on the encrypted military signals between the L1 and L2 frequencies to help cancel out especially ionospheric errors, which are the greatest source of error in a GPS position and are totally random. Becuase correlation is being used, it doesn't matter that the cryptographic keys are unknown (though if they were known, it would make the job much easier!).


If you need true survey grade results, it's really best to use proper survey grade gear.



David
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jonokimber
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Joined: Oct 03, 2003
Posts: 36

PostPosted: Thu May 13, 2004 12:18 pm    Post subject: Reply with quote

Thanks for all this info gents, it certainly enlightened me.

Just out of interest, what sort of drift are we looking at if i just leave a consumer unit in one place and let it take positions for 8 hours?

Jon

I'll try and do a field trial myself to test this, does anyone know of any software that will take a reading every 5 mins for example and log it?
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DavidW
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Joined: 17/05/2003 02:26:21
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Location: Bedfordshire, UK

PostPosted: Fri May 14, 2004 5:30 pm    Post subject: Reply with quote

There's no a position drift issue per se - the biggest source of error in a GPS position using a consumer receiver is ionospheric errors, which are random in nature.

SBAS systems like WAAS and EGNOS (the latter is, to my knowledge, not yet fully launched - but will be soon) attempt to correct for these errors. EGNOS should provide something like a 3m horizontal positioning error in most of the coverage area (95% confidence) - which means that 95 times in 100, the true position is less than 3m from the GPS position. That's still not survey grade, though.


You may find more discussion of these topics, including averaging software and the like (which is more commonly found for the PC than the Pocket PC) in the sci.geo.satellite-nav newsgroup.



David
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