Highly Accurate GPS by averaging?

jarvist

New member
We (Imperial College CC) are hoping to finally re tie-in the surveys of the caves on our summer expo to a central GPS location. Does anyone have any recent experience (post WAAS & the end of SA) with improving the accuracy of GPS fixes by averaging, and estimates of potential error?

Our current plan is to define a PSS / trig-point on the surface near the cave systems with a good view of the sky, setup our cheap garmin eTrex on a tripod above this point & data-log WAAS/EGNOS GPS locations to a connected laptop once a second.
Currently we don't seem to be able to get DOP (dilution of precision) data to be logged.

Does anyone have any suggestions for methods of discarding or weighting low-quality data, either through knowledge of the predicted DOP or by sampling the scatter in measured signals by time?

I also understand that further increased accuracy can be achieved by post-processing the data + including information from survey ground stations, such as the UK Ordinance Survey RINEX network - does anyone have any experience with this?

I also understand that due to the main ionospheric nature of the GPS signal degradation, that readings are most stable during the night - is it worth staying up late to get a better fix?

That said, considering that a single WAAS/EGNOS fix now has an accuracy of <3m, perhaps any higher accuracy is unnecessary when compared to survey errors and the rate of spread of tectonic plates!
 

graham

New member
jarvist said:
That said, considering that a single WAAS/EGNOS fix now has an accuracy of <3m, perhaps any higher accuracy is unnecessary when compared to survey errors and the rate of spread of tectonic plates!

Accuracy? or Precision?

That said, do you actually need anything better than that? I am assuming from what you say that you will be mapping all entrances in relation to your PSS by non-GPS means. That will give you what, 0.5 m precision at best? What matters it if you are a bit more than three metres adrift from the rest of the world?

This from someone who spent an afternoon last week wandering round part of Mendip with a borrowed differential GPS picking up - potentially - centimetre accurate readings on cave entrances.
 

jarvist

New member
Well, indeed - of course :)
Precision I guess is most important, as the motivation of this is to improve the accuracy of the survey tie-ins for far away systems, as a more accurate method than laborious surface surveying.
As a sideline we're also going to try and do some more accurate surface survey work with tripods used on the stations, mounting the normal suunto instruments + laser disto to avoid hand shake and using a card target for the laser beam mounted orthogonal to the leg.

The limiting step for accuracy, at least as far as getting to the registered Gauss-Kruger cave entrance coordinates as required by the Kataster Karst registry, is probably still limited by our imperfect conversion from WGS/lat-long into the local system.

I assume you mean carrier-phase GPS if you're getting cm resolution?

On the subject of ludicrous accuracy surveys, has anyone ever considered the curvature of the earth for a cave? My back of the envelop calculation suggests a 1m vertical error after 3.5km.
 

graham

New member
jarvist said:
On the subject of ludicrous accuracy surveys, has anyone ever considered the curvature of the earth for a cave? My back of the envelop calculation suggests a 1m vertical error after 3.5km.

I have, yes, with similar results to you.
 

robjones

New member
jarvist said:
On the subject of ludicrous accuracy surveys, has anyone ever considered the curvature of the earth for a cave? My back of the envelop calculation suggests a 1m vertical error after 3.5km.

I was instructed on the basis of "an inch in a hundred yards".  :)
 

kdxn

New member
An answer that may be too late but thought it worth posting for future.......

Best not to bother with averaging.

If you want more accuracy from a Garmin, try a RINEX logging system for some Garmin receivers and post processing against the IGSCB GNSS receiver network.
Try these links for more info:
http://artico.lma.fi.upm.es/numerico/miembros/antonio/async/
http://igscb.jpl.nasa.gov/network/netindex.html

Alternatively get a geodetic quality GNSS receiver and do Static Network Post Processing or Precise Point Positioning (PPP).

Some factors for you to think about.

What is height ? GNSS very good at spheroidal height differences but these are not the same as water height differences (like mean sea level etc.) you need a geoidal model to convert between the two.

How accurate do you need to be ? How accurate is your cave surveying ? How much money have you got ?

If you have the money, why not contact a Land Surveyor or Geodesist who has a geodetic GNSS receiver and is willing to take your money assist......like me.......
 

footleg

New member
Only just caught up with this thread.  :-[

If you are logging readings every second from one stationary GPS, and you also log readings every second at each of your cave entrances for a few minutes each, what error would be expected in the relative positions of each cave entrance with respect to the stationary receiver? If WAAS gives ~3m accuracy (or do I mean precision? Please remind me!) which is not too different to the probably survey error from in cave surveying, then it only has to be a bit better than this to be as useful as it needs to be. I would assume that you would calculate the relative difference in position between the two GPS receivers for each point in time, and then average these difference readings over the time period of measurement for each entrance. That should be simple enough to do in practice without expensive equipment (well assuming a couple of laptops can be borrowed, or a PDA program written).
 

Les W

Active member
Surely if the error is fluctuating you would need to take the corresponding readings at exactly the same time. If you took them at 1 second intervalls but half a second out of sync then the error might be different.

Just a thought.
 

footleg

New member
That is the question. How quickly do they drift? When I fixed all the entrances in West Kingsdale a couple of years ago, I spent a lovely sunny Summer afternoon starting at Valley Entrance, and taking a 1 minute averaged reading at each entrance. Ending up at Valley Entrance a few hours later I repeated the reading there, and got the same reading (OSGB grid to 1m precision). But how do I know if that was a coincidence? Did the readings drift around all over the place all afternoon and I just happened to get the same reading back at the same spot by chance? Or is the error more to do with the conditions on the day and satellite positions at that time, and so 'differential positions over a 4 hour time window' could actually be very accurate? But readings taken on different dates might differ by up to the 3m accuracy quoted for WAAS fixes?
 

graham

New member
Firstly, how do you know for certain that the fluctuations are only time dependant? How many different factors are involved and how will they vary across time as well as space?
 

khakipuce

New member
This gives a pretty good overview of GPS errors
http://www.kowoma.de/en/gps/errors.htm

And I agree with what footleg said, in that probably what really matters to cave surveyors is relative positions of features to each other, rather than exact position on the surface of the earth (which is fraught anyway as the earth is not sphere, the sea isn't level, etc). So it may be better to get a GPS to each entrance and take readings at the same time (better to take many readings say every 5 minutes over an hour) and then rotate the GPSs (i.e use each GPS at each cave entrance) then average the result.

This ought to help compensate for instrument errors as well as atmospheric errors - however the multipath error (in a steep sided valley) may be harder. It may be that getting up out of the valley and fixing the position of a survey station and then using traditional surveying techniques from there to the entrance may be better. But I also guess if you look at the variance of the positions and it is much larger, you know you have a problem.
 

Bucket

New member
New year's eve last year, I bust my toe and couldn't go caving  , so I surveyed between all the cave entrances in Burrington Coombe (using a suunto).  I augmented this with GPS measurements, using a Garmin eTrex, at several points on the hillside  (all with a good clear view of most of the sky).  At each point, I took GPS data for haf an hour on half a dozen different days.  Averaging the data suggested that (a) GPS co-ordinates appear very repeatable to within 2m horizontally but (b) they cycle vertically over periods of 5 minutes and > 1 hour - often varying by as much as 10m about the mean.  Base on that experience I would log data  for an hour or so and repeat this once or twice more if possible.  That should give you more than enough data to average laterally (to within 2m) and give you enough data to work out the vertical variation so that you can guess the mean height.  Overall, I was quite impressed by how repeatable the data was, even over the course of 3 months!  I have the data in a spreadsheet if you want to get an idea of what to expect.

Graham , did you ever try adding my surface data to your Burrington model ?
 

graham

New member
Bucket

I went straight from where we were to adding surface data from NASA satellite data using TerrainTool. However, the next job with that model is to go back to your data and use it to calculate an offset between what we say and what NASA say with regard to entrance heights. Interestingly enough in all the areas where I've done this so far, our data shows the entrances lower than the surface as shown by NASA, especially on/under steeper surfaces. In Cheddar, for example, the swallet entrances are all in close agreement, but the Gorge entrances are all "below ground".

See http://www.ubss.org.uk/resources/surveys/survex/Burrington.lox for the Burrington model and

http://www.ubss.org.uk/resources/surveys/survex/Charterhouse.lox for the Charterhouse/Cheddar one.

Both of these are Therion models and require the loch viewer. Download Therion from http://therion.speleo.sk/
 

footleg

New member
graham said:
Firstly, how do you know for certain that the fluctuations are only time dependant? How many different factors are involved and how will they vary across time as well as space?

You are making the assumption that I am making assumptions!  ;)

Let assume that I know nothing about errors in GPS, apart from that they are not accurate to the nearest 1m. What I am asking is whether it is possible using two GPS receivers and data logging to determine positions of cave entrances to within 1m relative to each other, using one of the GPS receivers located at a fixed point and measuring the relative position of each entrance to that point? I am not really concerned with what might be causing the variations in the readings. My only concern is the error I could expect in the relative position of each entrance versus any other entrance if I difference readings taken within 0.5 seconds of each other. (i.e. I log readings every second at both locations. So the largest time difference between pairs of readings will be 0.5 sec. ). If I want the relative position of the cave entrance GPS fix compared to the fixed point, then I have to assume the same error at both locations. i.e. If the relative distance between the receivers varies by 1m over successive readings then I have to assume 1m error in the fixed point value I use, and also 1m in the position of the entrance. This is because I am going to repeat the experiment at multiple entrances using the same fixed point in order to place all the entrances relative to each other. So really to get 1m accuracy between different entrances I need to be getting 0.5m accuracy in the relative positioning of each entrance with respect to the fixed point.

The other question is what time period I need to take measurements over the be certain of the accuracy seen. i.e. If the relative positioning of the entrances seen on one day varies only by a small amount over a 1 hour period, but varies by much more over days or months then I might get better accuracy on one day than another. But there will be a maximum error I can expect in readings taken on any random date. Is this going to be <1m?
 

khakipuce

New member
Check the link I posted. There would seem to be two main variables between GPSs at different locations that are read at the same time, firstly are they using the same satelites and secondly is there any multipath error.

Why would they not be using the same satellites? - maybe because you are in a valley in one location and so cannot see all those used by the other GPS.

Multipath error is due to reflected signals, I would think this is unlikely with caves unless you are in something like Trow Gill where a signal reaches you by bouncing of the cliff face rather than coming directly.

You have to remember that you are dealing radio signals from moving satellites. Just like your wireless, somedays the signal will be stronger than others, and when it is weaker the AGC in the GPS will crank up to try to pick up more signal and so may start getting reflections, etc. On another day you will be getting different satellites in different positions.

I think the upshot is there is no easy answer, the pro surveyors use more advanced GPS systems (e.g. Carrier Phase)  that can get down to tens of mm. But it is not a handheld Garmin. You could be better off hiring a 1 second total station instrument for the day and doing it the old fashioned way.
 

footleg

New member
Great link, thanks! In two posts we've gone from 'can I get <1m accuracy from two consumer GPS units with data logging' to an overview of relativity theory and how the speed of motion of the satellites and the person on the surface of the earth causes the apparent speed of the clock in the satellite to be faster to the earth based observer than it really is on the satellite. Gotta love UKCaving!

Having read that whole article I think the answer can best be obtained by collecting some data and analysing it for variations. So now my question is:
Given I have a GPS receiver which can connect to a serial port on my Windows laptop, what software can I use to capture the position data and time for data logging purposes?
 

khakipuce

New member
Sorry to harp on - but I really do not think averaging will get you there. Averaging only works if the error is random and normally distributed. Suppose that at one location the data is always skewed to the north by somehere between 1.9 and 2.7 metres. An average would at best be out by 2.3m.

But I agree, take some readings and check the variance - at least the variance will give you an idea if a particular site is likely to yield accurate results at all. But then presumably your GPS does that any way (mine gives me an accuracy which is constantly updated).

In the tradition of internet forms I hadn't of course read your whole post so did not realise you had a GPS with serial out - the software you need will depend on what sort of GPS it is. Assuming we can get the data off I'm happy to help with some processing software if necessary.

If you have two it is possible to get decent accuracy by taking readings at a base station with a known location (trig pillar?) and at the cave entrance. It is then possible to reverse the calculations for the base station and get the error and then apply this to the readings from the cave entrance GPS. This is done on a per satellite basis, so for each satellite used by the cave entrance GPS a known error can be applied (this is know as differential GPS).
 

jarvist

New member
Given I have a GPS receiver which can connect to a serial port on my Windows laptop, what software can I use to capture the position data and time for data logging purposes?
I've used gps-babel under Linux, but I believe the windows version works similarly.

http://www.gpsbabel.org/

Here's the stuff on real-time tracking:

http://www.gpsbabel.org/htmldoc-1.4.0/tracking.html

If you have an eTrex, the garmin data format contains the most numerical precision in the lat + long, but only the NMEA (ASCII) sentences have any DOP info in them.
You get an update per second, tagged with the GPS-time (for post-processing / combining with an independent station).

I'll try and dig up the magic gpsbabel invocation to get the most data out of an eTrex + spit it out in a sensible format --- I spent a while fiddling with it back in London, but never quite got around to using it on expo.

I found this site very interesting, and may answer some of your questions with respect to increase in accuracy as a function of arithmetic averaging (I recall that 3hrs was a bit of a 'sweet spot'):
http://users.erols.com/dlwilson/gps.htm

If you have two it is possible to get decent accuracy by taking readings at a base station with a known location (trig pillar?) and at the cave entrance.
Hmm, I think a technique like this would be described under quasi-differential GPS.

KDXN's suggestion of getting at the raw data (the satellite pseudo-ranges) + then post-processing seems to make a lot of sense (though I'm not sure if the method he linked to, using an undocumented debug mode of the old garmin GPS 12 works on modern units), but it would also be interesting to see what you can achieve with the minimum of effort / equipment...
 

footleg

New member
I have a Magellan Meridian GPS which I believe sends NMEA ASCII data over the serial port cable. I also have a Bluetooth GPS module which I assume sends NMEA data over a bluetooth wireless serial connection as the software I use with it always needs to be set to NMEA format to connect with it. I'll see if I can get GPSBabel to log this NMEA data.
 
Top