GPSMAP 66ST Accuracy?

Discussion related to the Garmin GPSMAP 66s/st GPSr
oldcrashtestdummy
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GPSMAP 66ST Accuracy?

Unread post by oldcrashtestdummy »

Several times now I have marked a waypoint using my 66ST, iPhone 11 using Gaia GPS, and a Trimble Juno SB using TerraSync. When I compare the waypoints, the iPhone is 5 to 20' away from the Trimble, but the 66ST can be as much as 150' off. I have also recorded a track out by walking out to a point, marked it with all 3 devices, then returned to the vehicle. The 66ST track and the iPhone and Trimble waypoints are very close to each other, however the waypoint from the 66ST is 100' or more off. It seems to me that the waypoint should exactly match the turn around point on the track since they were recorded on the same device at the same time. Am I missing something here?
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GPSrChive
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Re: GPSMAP 66ST Accuracy?

Unread post by GPSrChive »

Can't do much to help you without track and waypoint files to inspect....

They can be emailed to webmaster at GPSrChive dot com to maintain privacy.

Which firmware version is your GPSMAP 66 running?

Find a Benchmark near you and compare the posted coordinates to the coordinates displayed on the GPSMAP 66 while standing at the Benchmark.

Share your results.
Spiney
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Re: GPSMAP 66ST Accuracy?

Unread post by Spiney »

The horizontal position accuracy of the 66st has been independently tested by the USDA Forest Service as having a 95% accuracy of 2.37m in open conditions using GPS+GLONASS without WAAS correction – see https://www.fs.fed.us/database/gps/mtdc ... /index.htm

The equivalent results for the Apple iPhone 11 are also reported but with a slightly different setup. In open conditions using GPS+GLONASS with WAAS correction, the data shows slightly worse accuracy at 4.31m.

The Trimble Juno SB has also been tested, but also under different conditions again. The results for the GPS-only constellation with WAAS real time correction enabled are slightly worse than the 66st, but better than the iPhone 11 at 3.23m for the same number of averaged positions (60).

Based on this data we would not expect to see much of a difference between the three devices provided an adequate number of positions were averaged and it was performed in an environment free from satellite signal obstruction (masking) or differential signal reflections (multipath errors) i.e. away from tall buildings, cliffs, mountains, slot canyons and heavy tree canopies etc.

If the environment is subject to masking and multipath errors, then more accurate results would most likely be achieved by using more than one GNSS constellation as it increase the number of "good" satellites to get a fix from. I believe that the iPhone GPS chip is set to use multi-GNSS signals all the time. The Juno appears to only use the GPS constellation, whereas the 66st can be set up to use GPS+GLONASS as per the USDA testing or GPS+GALILEO.

How do you have your 66st set up? Which GNSS constellations were being used (GPS, GPS+GLONASS or GPS+GALILEO)? Also was SBAS correction enabled (WAAS AND EGNOS) or not? Also did you note what GPS accuracy your 66st reported at the time?

I agree with gpsrchive's suggestion and would add that you get a position fix on the benchmark with at least 20 repeat observations (preferably more) to minimise getting outliers by chance.
987
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Re: GPSMAP 66ST Accuracy?

Unread post by 987 »

I have noted on other garmin GPS units, but not yet on the GPSMAP 66st, that sometimes (perhaps 1/5 of the times) the unit shows a good accuracy (3-6 m), but the pointer and track are a bit off the actual location. It is often around 10-20 meters. After 5 to 20 minutes, it suddenly corrects the position to be much more aligned with the actual shown accuracy. It is especially easy to see if one takes two or three laps on a route/track which is about 15 minutes long per round. Running a loop, driving back and forth a few times, etc.
I read that one should allow the unit to get its signal for 30 minutes to make sure accuracy is good. This seems to be a good general rule of thumb.

The land survey authority has points all over the country (sweden) and i have one not too far away which i checked against, its position is given to an accuracy down to 0.15 centimetres.
I marked one point and a second which was averaged for 5 minutes. GPSMAP 66st (GPS+GALILEO and 3 satellites reported EGNOS, D) and a etrex vista hcx (GPS, but no EGNOS reported) was was used.
GPSMAP 66st, showed 2.4 m accuracy. The etrex 3-4 m accuracy.
The differences between the non-averaged point and the averaged is negligible on both devices.
The reported location compared with the real location is 4-5 meters on both devices. Closer to 4 m on the GPSMAP, and closer to 5 m on the Etrex.

Overall, the differences were very small.
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GPSrChive
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Re: GPSMAP 66ST Accuracy?

Unread post by GPSrChive »

987 wrote: Sun Dec 20, 2020 1:38 pm The reported location compared with the real location is 4-5 meters on both devices. Closer to 4 m on the GPSMAP, and closer to 5 m on the Etrex.
Both units were 4-5 meters off the actual known coordinates for the location?

I would have hoped for a little better than that.

Perhaps I need to do a comparison test with GPSMAP 66st vs GPSMAP 66sr at the same US Benchmark.....
987
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Re: GPSMAP 66ST Accuracy?

Unread post by 987 »

Yes. Both units were placed on the point where the known location is, both reported to be 4-5 meters off and both were off in the same direction, actually the readings on the units were closer to each other, than the real location.
This point lies on a small hill top, with a few trees around (very light cover to no cover, weather was cloudy and foggy).
I agree i would have expected a bit better, but even when i have measured against some other point (a few months ago) i got very similar result. At that time it was 3-4 metres. While the GPS/GALILEO/GLONASS satellites move, the precision does change a bit. If one leaves the GPS unit on while at home, one will see how sometimes the pointer is on one side of the house and later it may be on the other.
If i would have stayed and averaged again after a few hours, the results might have become different.
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GPSrChive
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Re: GPSMAP 66ST Accuracy?

Unread post by GPSrChive »

Yes, even without trees, hills or buildings, the current weather, especially wet weather, as well as the current GPS satellite constellation status can have an affect on accuracy.

When planning GPS accuracy testing like this, it can also help to use a GPS Satellite Prediction service to help determine the best time for a specific date and location. (See GPSrChive > How To... > Waypoints > Averaging a waypoint)

The GPSMAP 66sr is expected to do better under these circumstances, but I have not yet had any inclement weather to test mine with.
987
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Re: GPSMAP 66ST Accuracy?

Unread post by 987 »

A question to the starter of the thread.
Are you sure that you did not accidentally pressed the d-pad and moved the pointer a bit, and then pressed the MARK button?

When MARK is pressed on a GPSMAP 66, it will make a waypoint where the pointer is in case the pointer is active on the screen.
If no pointer is showing then the current location is used.

This is different from for example (older) etrex, where a long press on the joystick always marks the current position, even if the pointer is somewhere else.
Marking a waypoint in some other place is done differently.


I must say i think gpsmap 66 should be changed to always mark the current position. So it can be done without looking at the device.
Marking a free position can anyway be made by pointing somewhere, pressing ENTER and then MENU and "save as waypoint".
987
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Re: GPSMAP 66ST Accuracy?

Unread post by 987 »

By the way, could we add a feature request?

GPSMAP 66 st, firmware 6.00 (gps 2.80)

Updated behaviour of MARK button.
- Short press and release. Continue to work as now.
- Long press (2 seconds or more), within 5 seconds. Open the save waypoint page after 2 seconds, but always use the current position.
If the button is released within 5 seconds the user can edit information and save or cancel.
- Continue to hold MARK for 5 seconds. Immediately save the current position and go back to display shown before the button was pressed.

Current position should always be taken at the start of the press.
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GPSrChive
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Re: GPSMAP 66ST Accuracy?

Unread post by GPSrChive »

GPSrChive > GPSMAP 66 > Common Issues > Requested Feature 34 has been posted.
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