Testing GPS accuracy

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rinsemesocks
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Testing GPS accuracy

Post by rinsemesocks » 22 June 12 11:44 pm

This is probably a silly question, but how do I test the accuracy of a GPS device.

It's not like testing a speedo by using a stopwatch over a set distance. So if I am standing at what my device says is s33 xx.xxxx e151 xx.xxx, how do I know that that piece of ground is actually what I am on?

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Chwiliwr
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Re: Testing GPS accuracy

Post by Chwiliwr » 23 June 12 12:55 am

rinsemesocks wrote:This is probably a silly question, but how do I test the accuracy of a GPS device.

It's not like testing a speedo by using a stopwatch over a set distance. So if I am standing at what my device says is s33 xx.xxxx e151 xx.xxx, how do I know that that piece of ground is actually what I am on?
The following is an assumption that you are talking about the consumer GPS/phones.

The only way to test a GPS is to go to a known location and see how close the coordinates in the GPS are.

The problem is that the test is only for that moment in time, position and environmental condiditons. You cannot rely on it to tell you if a coordinate is accurate somewhere else at some other time as there are quite a lot of variables at play that become different over time and distance.

For caching a general test is whether the GPS takes you close enough to a GZ for you to find a cache. If it does it consistently then don't worry about how accurate it actually is.

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gmj3191
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Re: Testing GPS accuracy

Post by gmj3191 » 23 June 12 10:46 am

rinsemesocks wrote:This is probably a silly question, but how do I test the accuracy of a GPS device.

It's not like testing a speedo by using a stopwatch over a set distance. So if I am standing at what my device says is s33 xx.xxxx e151 xx.xxx, how do I know that that piece of ground is actually what I am on?
Most GPS receivers have an option to display their accuracy. I think they can calculate it given their design accuracy plus the number of satellites they can see plus other other things too complex for mere mortals to understand, except for Muzza.
I was out the other day and my Oregon said it's accuracy was 15m at the time, but it unerringly and repeatedly lead me straight to a cache. I was intrigued and came at it from different directions and it homed in perfectly, despite having no confidence in itself.

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pjmpjm
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GPSr Accuracy -- 'You Are Here' (Or Are You?)

Post by pjmpjm » 23 June 12 12:50 pm

rinsemesocks wrote:This is probably a silly question, but how do I test the accuracy of a GPS device? It's not like testing a speedo by using a stopwatch over a set distance. So if I am standing at what my device says is s33 xx.xxxx e151 xx.xxx, how do I know that that piece of ground is actually what I am on?
I guess the only sure way would be to compare your GPSr readings to a spot where you're absolutely sure that you know the exact coordinates. But where would you ever find such a place? I agree that you can only go by the built-in 'accuracy rating' given by most GPSrs.

The coordinates of a place where a cache is hidden were of course determined by another GPSr, which may or may not be accurate.

Not a silly question at all, but a tough one to answer.

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PesceVerde
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Re: Testing GPS accuracy

Post by PesceVerde » 23 June 12 1:23 pm

Hi. Also you could try Wikipedia and/or Google with something like HDOP GPS
:)

rinsemesocks
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Re: Testing GPS accuracy

Post by rinsemesocks » 23 June 12 6:53 pm

I'm not worried about what the device says is the accuracy is, because that could be inaccurate as well. I have had three phone gps side by side on a clear night, each giving different co-ords and all saying 5m accuracy over a 30 minute timeframe.

I also have a phone that does russian sats as well and the phone says 3m accuracy, but that doesn't match up with google maps.

Where can you find somewhere that has a known co-ordinate. I guess survey markers must have some level of accuracy otherwise building boundaries would be out of whack (and dont' say Greenwich, because I ain't flying there).

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Chwiliwr
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Re: Testing GPS accuracy

Post by Chwiliwr » 23 June 12 10:01 pm

rinsemesocks wrote:I'm not worried about what the device says is the accuracy is, because that could be inaccurate as well. I have had three phone gps side by side on a clear night, each giving different co-ords and all saying 5m accuracy over a 30 minute timeframe.

I also have a phone that does russian sats as well and the phone says 3m accuracy, but that doesn't match up with google maps.

Where can you find somewhere that has a known co-ordinate. I guess survey markers must have some level of accuracy otherwise building boundaries would be out of whack (and dont' say Greenwich, because I ain't flying there).
In WA there are the boating GPS markers specially set up with signs at a number of boating ramps. One, in Mandurah, actually has a cache based on them. Also there are some survey markers around Curtin Uni in Bently that were set up to test the survey quality GPS units used by the survey people. They have a lot of numbers in the reading as they are meant to be able to measure 1mm.

I'm sure there are similar types of markers in the other states than you can use.

I'm glad you are not worried about the inbuild GPS accuracy reading as it is only a 'feel good' programming feature that is always just an estimate based on a programers code analysing the signals it is receiving. The only time I take notice is when it tells me it is inaccurate not when it thinks it is accurate. A test done on some different branded GPS units at one of the survey marker sites at Curtin 15 months ago showed all of the units saying they had a good 5m accuracy or less but all were 8m from being accurate. The same test 2 weeks later had them all saying the same thing but this time they were all reading within 2m of the real coordinate.

Kerry
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Re: Testing GPS accuracy

Post by Kerry » 24 June 12 10:14 am

There is no standard GPS that can indicate accuracy as accuracy is relative to a known position of truth and typically no receiver knows where it actually is.

What many SPS receivers indicate is Estimated Position Error (EPE) but this is a computed value and can be rather biased, is not computed the same by all manufacturers and should be used as nothing more than a relative indication as it is certainly not absolute.

Only way to test accuracy is to compare against a mark which the position is known in absolute terms. System accuracy spec is also based over a 24 hour, 1 second data interval so simply comparing to a known point is only relative at that time as the system is dynamic.

All states maintain a survey database but not all PSM's are coordinated (accurately) and the PSM accuracy can also be variable from high order geodetic down to fixed by handheld to even scaled to none, so one needs to read the PSM details. If you know the PSM number then the details can generally easily be sourced.

For anybody in Qld I could provide position details if you have a number.

Similar to WA there were also public type marks installed in Qld (parks, boat ramps etc) as part of Q150.

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Dik:
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Re: Testing GPS accuracy

Post by Dik: » 25 June 12 2:26 pm

Your GPS should be able to accurately calculate your position based on a mathematical model of the earth.
The errors are not in your GPS, but in atmospheric disturbances that affect the path and speed of light and radio waves.
This changes over time. So 2 readings taken within a short period of time will be correct relative to each other. That's why your GPS can give very good speed readings.
Therefore the only way to know your true position at any given time is to compare it with a known position and time, using DGPS.
There are a number of DGPS systems around, but for consumer applications are far to expensive.
Some starter reading here: http://www.amsa.gov.au/Shipping_Safety/ ... _Sheet.asp
All thse systems use reference to "known locations"

If you want to check your GPS at any given time for interest use a Trig Point, which are all listed as GCA caches.

Rainbow Spirit
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Re: Testing GPS accuracy

Post by Rainbow Spirit » 26 June 12 12:49 am

Where's geomatica when you need him? He is/was a surveyor, and would know the answer to this question..

Kerry
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Re: Testing GPS accuracy

Post by Kerry » 26 June 12 8:18 pm

Rainbow Spirit wrote:Where's geomatica when you need him? He is/was a surveyor, and would know the answer to this question..

Is there more to the answers above?

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Re: Testing GPS accuracy

Post by Kerry » 27 June 12 3:34 pm

Not that the following is really applicable in this situation (yet) but something to think about when considering accuracy.

Reason why it is not applicable is the typical SPS type receivers used here can not determine accuracy comparisons (limited both by accuracy and precision) less than around 2.6 metres (max).

Coords provided by any state authority (PSM wise) are GDA94, handheld GPS is WGS84, the difference between these depending on location in Aus is now around 1.14 metres.

There will become a time in the medium term when accuracy capability will come down further, difference between GDA94 & WGS84 will continue to increase and receiver accurarcy/precision will improve and at some point simple handhelds will require a transformation from WGS84 to GDA94.

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Re: Testing GPS accuracy

Post by The Coffee's » 01 July 12 9:02 pm

Kerry wrote:
Rainbow Spirit wrote:Where's geomatica when you need him? He is/was a surveyor, and would know the answer to this question..
Is there more to the answers above?
There are many variants to this question but as a general rule of thumb in an open area such as a oval you can expect a accuracy of around 3 metres, in other words draw a dot on the ground and draw a circle of 3 metres all around your dot and that is most likely as good as you can get, however!:
it can also depend on: the position of the satellites above you, are they tightly grouped or are they spread out evenly,

what is the PDOP? Positional Dilution of Precision. It indicates the accuracy of a 3D GPS position based on the number of satellites and the geometry of satellite.

What is the HDOP? measures of the best possible horizontal or vertical precision for a given configuration of GPS satellites

What is the EPE? estimated positional error? GPS reciever's estimate of it's own inaccuracy at the moment, in other words where it thinks it is in relation to its own thought of where it thinks it is not.

Next we get to other errors relating to ionosphere errors cloud, rain, fog and refraction errors.

The best advice is average your GPS position when taking coordinates. It does not matter what you are using when placing a cache as long as you average your readings. OK place your cache and place your GPS as close to possible to the cache and try to have a clear view of the sky above, now leave it still for the first minute then take a reading, without moving your GPS repeat this over 30 minutes at 3 minute intervals and choose the one that is in the middle of all those readings. This should give you hopefully the best reading for your new cache, and I do this over 2 days.

In using this procedure I have been able to get the same coordinates on a Magellan, Garmin, Nuvi, iPhone and iPad as they all do the same thing they read the satellites above us. Dont Chuck, Read and Run, if you like your cache TLC it with a bit of averaging.

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Dik:
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Re: Testing GPS accuracy

Post by Dik: » 26 July 12 2:09 pm

There is another answer, or further information, or whatever, related to this topic, that I have already given it's own topic, but probably worth a link here>
http://forum.geocaching.com.au/viewtopi ... 12&t=17531

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