Thread: Accuracy figure of horizontal position fix on GPSMAP 66 satellite screen?

Well I have been using GPS's now for 20yrs, I still have an use my Garmin Etrex, I have only ever seen 4mtr accuracy, since it only uses the US "military sats" and found out it was SA "selective availability" and the Glossnos Sats are Russian, so do you think they are more accurate ???. If your a professional Surveyor your equipment is offset for SA and works to mm acc. I have taken info from Topo/Geological map systems for years either from paper or digital maps and walked to an abandon mine and when been within 4mtrs of the hole it says "arriving destination" so I don't see why you are so worried about accuracy, am not going to send a missile through your window huh.
This the way.

@Bushwalker8 and babj615,

Your technical info is super and very informative even if you two do not see eye to eye on issues.

That being said the GPSMAP 66 series GPS units are for recreational use they lack being about to attache an external antenna to them. In fact there are only a couple of GPSMAP series GPS units that let you add an external antenna to improve the accuracy of the GPS units.

Most users are satisfied with the GPSMAP 66 series with updates come from time to time to improve them.

This does not taking away from what you both have pointed out but will have little or no effect on the average recreational user of these GPS Units....

Even the forum GPSrChive that is dedicated to Handheld units has not responded to the original post on their forum.

Since WGA1994 Australia has moved 1.8mts West so now the new maps needed to be coordinated to GDA2020 and with SA of 4mtrs and WGA84 1.8mts out you are now upto 5.8mtrs away from true coordinate on the ground.


have a read of this

Code:

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I think you may be confused by the basics; your underlying assumptions are very wrong.

Firstly the earth is not round and the lines of latitude and longitude are not fixed, they are centred in many different places in the centre of the earth as a best fit for each of the various local datums and can be up to hundreds of metres out. So, no matter where you are their exact location depends on which datum you are on.

And you still haven�t said which local datum you are using so it�s hard to be specific.

If you are using global datum WGS84 then it�s not accurate at all, its averaged over the world and not directly meaningful in any one area. Everything is continually moving under it. OK for basic navigation but no good for accuracy. It�s a virtual GNSS reference frame about which the satellites orbit, and on which all the accurate local datums are linked.

Have a good read of this: [Only registered and activated users can see links. ]
An extract here: �The International Hydrographic Office and many international navigation systems quote the datum they use as simply WGS 84. As we have pointed out, this is, strictly speaking, insufficient for a datum. The version of WGS84 is commonly not quoted nor is any reference epoch.�

And this: [Only registered and activated users can see links. ]
Extract: �Because Earth is an imperfect ellipsoid, local datums can give a more accurate representation of some specific area of coverage than WGS 84 can. OSGB36, for example, is a better approximation to the geoid covering the British Isles than the global WGS 84 ellipsoid.�

Haven�t you ever wondered why your country that developed GPS still spends enormous resources refining and adjusting your current � official - US local datum NAD83? And why all the other countries do the same with their datums? It�s more accurate.

And it also seems you have missed in the link I posted previously that even in your area NAD83 has a number of different flavours �one for each of the North America plate, Pacific plate and Mariana plate. Why? Because the coordinates are static, fixed to the plates and move with them.
And your new datum that you will be moving to in a few years - and that is better aligned to GNSS - will still also be split into separate datums, one fixed to each plate for the same reason.

Same with most datums, both our GDA94 and GDA2020 are plate fixed, the coordinates are static on the ground and move with us. Read here very carefully: [Only registered and activated users can see links. ]
Extract: �GDA94 is a �plate-fixed� or �static� coordinate datum based on the International Terrestrial Reference Frame 1992 (ITRF92), held at the reference epoch of 1 January 1994.�
And funnily enough the coordinates of the GDA survey marks I use here are still measuring the same within 1cm around 25 years after I first used them�..

And it certainly does matter which coordinate system you chose and they are definitely not all linear and equidistant. One local example here, look at the NZMG which is mathematically twisted around NZ and needs special calculations. It�s now superseded but maps still exist and are used: [Only registered and activated users can see links. ]
Extract: "NZMG is not based on a geometric projection (transverse Mercator is based on a cylinder). Instead it uses a complex-number polynomial expansion. This has the advantage of exhibiting minimal scale distortion over New Zealand; however it is a projection unique to New Zealand and so can be difficult to use or program into computer software or positioning devices (eg, GPS receivers)."

And there are also many other aspects you apparently haven�t considered, for example how earth curvature is treated as it�s also a significant factor in calculating the distance between points.

I think you are also confused with basic �Accuracy� vs �Precision�. Google it or read here: [Only registered and activated users can see links. ]

Accuracy � this topic - is how close you are to a known true point. Precision is how closely your data points group together, regardless of where you are.

So for a Garmin the accuracy relates to how close you are to the known point�which has to be the ground coordinates it gives you and the spot where you are on the ground. There is no other known or true point, it�s not giving you anything else to measure against including satellites. It doesn�t matter if the coordinates are in either datum, or are right or wrong, it�s simply telling you the distance it thinks you may be from that point on the ground where you are. So you would normally expect the accuracy to be the same under the same time and satellite conditions and your test is not really testing anything.

Originally Posted by Bushwalker8

Lat Lon is a projection, you don�t say what datum you are using. The current local one for your area is NAD83 and like other national datums across the world it�s regularly adjusted over time (epoch) so even it has multiple versions. You should only use NAD27 there if you are using old maps or have obtained coordinates in NAD27.

Here�s a description from your responsible authority the NGS of an NAD83 2011 adjustment which specifically mentions tectonic activity and earthquakes in Western US: [Only registered and activated users can see links. ]

And this is where you are going next: [Only registered and activated users can see links. ]

Garmin devices normally have a selection for NAD83, however the transformation is simplistic, the epoch date isn�t specified and it isn�t updated.

Thank you for those links, I have read through them and multiple pages linked from them. I find they all have one thing in common. They are all discussions about different systems and methods used to map the surface of the Earth.

None of them have any bearing on how a GPSr calculates its distances from multiple satellites orbiting the Earth to trilaterate a three dimensional position relative only to those satellites.

Only after the GPSr has made this determination does it then display coordinates based on the projection and datum specified by the user.

Originally Posted by Bushwalker8

So forget all the nonsense about accuracy, you won�t get it.

Won't get what?

Each GPSr has unique capabilities and will provide 'some level of accuracy' in the position it reports.

I believe the OP wanted to know 'how' the Garmin GPSr 'Accuracy' was calculated, which I suspect Garmin engineers are not going to be allowed to divulge.

However, I do believe that accuracy value is an indication of the GPSr confidence in its reported position relative only to the satellites being used to make the calculation, and has no bearing what-so-ever on the projection or map datum selections chosen by the operator.

In fact, I just ran a simple exercise on my Garmin GPSr where I repeatedly changed the Map Datum settings and compared the coordinates and estimated accuracy displayed for each Datum selected, and the accuracy value never changed, confirming it has no relevance to the map projection or datum configured on the device.

Originally Posted by Bushwalker8

Just use it and enjoy whatever it is you are doing they are great devices to support almost any activity.

I think we can all agree on that!

Originally Posted by Bushwalker8

The answer is simple, they are two different sources or error and you need to add both together to understand the total accuracy or distance to a known point.

There are indeed multiple sources of error to take into account when calculating the distance to a known point and the accuracy of that calculation, including:

1. Current GPS reported position errors.
2. Map errors present at the time the current map location is determined.
3. GPS location errors present when the 'known point' position was calculated.
4. Map errors present at the time the 'known point' map location was calculated.
5. Map errors in all calculation due to changes (movement) in the Earths crust over time.
6. ...and the list goes on...

However, these are all referencing the activity of land surveying, and have nothing to do with the GPS receivers reported position accuracy.

Originally Posted by Bushwalker8

The device gives you one, a horizontal accuracy number based on its calculations to where it thinks it is in the 3D reference frame. The OP�s question related to this calculation to better understand the accuracy.

Yes, 'The device gives you one,' is correct.

The devices 'Accuracy of GPS' data field displays the margin of error for the calculated location 'in a 3D reference frame', a reference frame that is based entirely on it's relationship to the satellites being used to make that calculation.

After reviewing this data field on many Garmin GPSr devices, a common theme has become apparent. The GPS Accuracy data field always displays one of the following text options based on the satellite constellations in use:

1. 'GPS' (when only GPS is enabled)
2. 'GPS + GLONASS' (when GPS and GLONASS are enabled)
3. 'GPS + Galileo' (when GPS and Galileo are enabled)
4. 'GNSS' (when Multi-GNSS is enabled)

Can you see the common theme?

They are always indicating the reported position accuracy based on the satellite constellations enabled.

This data field is exactly what it says it is: "Accuracy of GPS".

This data field has no relationship to the accuracy of the maps or map datums being used.

Originally Posted by Bushwalker8

However, the device also transforms that 3D position into the 2D horizontal coordinates of where it thinks it is in your chosen local datum. But it�s often actually somewhere else because the transformation is simplistic, and it does not calculate additional plate movement over time. The known point is actually further away.

The 'GPSr' is not 'somewhere else because the transformation is simplistic'.

The GPSr is exactly where is says it is.

The map data displayed may be incorrect for the current GPSr location due to any number of potential map data errors.

Again, the 'Accuracy of GPS' date field has no relation to the accuracy of the map being used.

The [Only registered and activated users can see links. ] states:

"If you are navigating and comparing the GPS coordinates to a map, chart, or other reference, set the map datum in the GPS unit to the same datum as the map to ensure the most accurate navigation."

The [Only registered and activated users can see links. ] states:

"You should not change the position format or the map datum coordinate system unless you are using a map or chart that specifies a different format."

If we look at a Garmin GPSr that did not provide on-board mapping capabilities, we can see that the [Only registered and activated users can see links. ] states:

"You may select each datum applicable to your region until you find the datum that provides the best positioning at a known point."

Originally Posted by Bushwalker8

So that additional error needs to be added to the device provided accuracy number to give you the total distance to the known point if you want to understand the real accuracy of your position.

Once more, the GPSr reported position accuracy has no relationship to map data errors.

One must be able to understand that these are two different things. The GPSr calculated position in reference to the satellites being used will have some amount of inherent error, as will the maps used to display where that position falls. The GPSr uses the selected 'Map Datum' to position the map data under the devices calculated position as precisely as possible.

Originally Posted by Bushwalker8

So if the question was how does it calculate the accuracy, then one short answer is that it does it insufficiently because it leaves some of the error out.

Only if one assumes this value is supposed to be including map errors in the first place, which I do not believe it does, as I explained above.

The accuracy field is based on where the device it thinks it is in the frame, but it�s reported as a 2D horizontal number. The number relates directly to the ground coordinates it provides you, and importantly those numbers are in whatever datum you have chosen so the datum is intrinsically linked and a significant part of your overall error.

GNSS devices report horizontal accuracy and typically with a confidence level. A common method is the already mention CEP or Circular Error Probability 50%.

Note very carefully the word Circle, it is obviously 2D. It doesn�t say Spherical or anything else in 3D for the reference frame or individual satellites.

In other devices and software it�s much clearer that the accuracy number is horizontal, the software even draws it for you in 2D.

You appear to be confused with other things like satellite specific User Range Error which is quite different to the User Accuracy we are discussing here. Here is a simple overview with a picture that may make it clearer for you: [Only registered and activated users can see links. ]
Note that the User Accuracy is a 2D circle on the ground, around the coordinates the device is giving you, which will be in the datum you have selected.

This has nothing to do with land surveying, it�s the basic ABC�s of GNSS, maps and navigation.

You have quoted recommendations from Garmin advising to use local datums so that should tell you it�s fundamental and important because of potentially large discrepancies in error. And if you do so the Garmin will add a simple general offset for the datum to take out the worst of the error but it's definitely not precise because the datums are far more complex. I gave an earlier example NZ datum that shows they can require far more computation.

And if the datum is important then it follows that if the datum moves then that is also important.

So if you want to understand the accuracy number, the topic of this thread, then you should understand that the Garmin doesn�t track that movement of the datum over time. So it�s reported position � the location of person in the middle of the circle in the picture referred to above - increases in error over time. The plate moves, and with it goes you, the datum and the map. They are all connected, the Garmin isn�t because it's reference frame is left behind in your wake. The coordinates it�s giving you are no longer correct.

No-one has said the drift is in or should be added in the accuracy number. The point being made is simply that it's not, the Garmin doesn�t track it and so you should be aware of it.

Garmin clearly understands their datum transformation is initially simplistic, and that their device isn't tracking it's subsequent drift over time. They provide the raw data in the RINEX file to allow you to do the processing yourself to correct the position. The correction includes a far more rigorous and accurate datum transformation computation, and adjustments for drift using precise reference station data. That is the only purpose for the RINEX.

You don�t have to take up the correction option, it�s optional. But if you don�t then accept you won�t be correct and you won�t know by how much or in which direction.

The points here relate to the underlying basics and do not include the �map errors� you refer to which is an additional consideration.

And I have no idea what point you are trying to make by listing the GNSS combination selections, obviously whichever combination you have selected the device will still provide an accuracy number. And if it thinks the chosen constellations give it a better horizontal position on the ground then the accuracy number will reflect that.

You still haven't said what datum you are using.