Garmin Accuracy

Did you test them all at the same time? Different satellite configurations (where they are "above" you) can change the precision.

Were you at rest? Likewise.

Yes, I was at rest and they were all powered on at the same time.

The satellites that the less accurate (older) Garmins didn't receive were definitely showing less signal strength on the newer ones with 10 foot accuracy.

I think the newest ones might have been getting signals from Galileo.

I will have to check to verify, but I don't think mine get GLONASS signals.

To establish accuracy of a device I have to compare its output to some reference that is known to be correct. What was the reference in this instance?

I am referencing the display on the Garmin in this case.

https://support.garmin.com/en-US/?faq=CGLbyjTCVcAw055Oj9rHi5

Using the Satellite Information Page on an Automotive Device
The Satellite Information page displays the strength of the satellite signals you are receiving and your GPS accuracy. Generally, the accuracy of your device will range from 3 to 5 meters, or 10 to 16 feet. It also displays the coordinates (latitude and longitude) and elevation of your current location.

To access the Satellite Information page on most automotive devices, touch and hold the satellite reception bars in the top left corner. On the nüvi® 300 and 600 series devices, the satellite reception bars will only appear if the antenna flap is up.

To return to the main menu, touch the Back or arrow button in the lower-left corner. If one is not present, touch anywhere on the screen.

Red Icon is your Heading/Direction or Bearing
Blue Numbered Icons are the GPS acquired satellites
Pink Numbered Icons are the Galileo acquired satellites
NOTE: Not available on North America Garmin Drive 53 series devices
Green Numbered Icons are the Glonass acquired satellites
NOTE: Glonass is featured in European edition devices only
Gray Numbered Icons are the satellites not acquired

To Access the Satellite Information Page on a nüvi® 295W:
Touch Tools
Touch Where Am I?
Touch Menu
Touch Satellite Info
To return to the main menu, touch the Back button.

There needs to be some form of known reference for the sake of comparison.

As an example, Montauk Point Lighthouse is a lighthouse at the end on Long Island. It's latitude and longitude are well defined on navigation charts as 41.070928 N latitude 71.857205 W longitude. These coordinates are used by ships at sea.

If your four devices came up with the same coordinates, as compared to the reference, then they are accurate. If they came up with differing coordinates, then they suffer from some inaccuracy. Keep in mind that (1) minute of arc of latitude is one nautical mile which is just short of 6080 feet.

There is also something similar and often confused called precision. As an example of the difference, say we have a basketball player that can hit 10 shots out of 10 from the foul line. He is accurate (and exhibits precision). But say another basketball player can't hit any shots from the foul line, but consistently hits the right side of the rim on all ten shots. His accuracy sucks but his precision is spot on.

So when you say the Nuvi 500 had 62 Foot Accuracy, the Nuvi 3597 had 18 Foot Accuracy and the DriveLuxe 51: 10 Foot Accuracy; as compared to what?

I got your point the first post.

I am just posting that is what my Garmin's show for accuracy.

My GPSMAP 64ST uses Glonass and GPS.
My DriveSmart 51LMT-S uses Galileo and GPS.

I never made an effort to find the real meaning of the displayed GPS accuracy. I presume that it is saying something like ... it is 10 feet with a confidence interval of .95 or somthing.

I have never owned a handheld GPS. What do you use your GPSMAP 64ST for?

My first Garmin was a handheld device, not too unlike the GPSMAP in purpose. I used it primarily for hiking. I once loaned it to a friend, a very avid hiker known to go off on multi day hikes. He called it “hiking with knowledge.”

Without comparing the coordinates to a known spot that stated accuracy is hard to verify. Certainly the elevation is off by a considerable amount, noted if you are ever driving at a known elevation like the ocean level.
Use Google maps to zoom in on your home and take those coordinates and compare them to what your GPS says. Chances are that they will not be the same and off by more than 10 feet. But then.. which one is the true value???

You guys have WAY too much time on your hands.
I'm happy if my devices can get me from point A to point B.

I used to have a Montana (hand-held) that was great for non-auto useage.

Bingo. My county GIS has coordinates for my farm, but I would need to determine if those are the dead center of the property or something else.

I make the inference that you don't find the topic interesting. A lot of my working life was spent measuring things. I find it interesting.

We've used it for hiking. My wife uses it instead of a pedometer.

I sometimes set waypoints for something I may want to find again. For example we found a rosebush in a nearby state park that wasn't wild and there was other evidence of settlement nearby. We knew someone who'd want to see it,

Once, for reasons that escape me, we used it for cross country navigating. That requires a navigator since a driver can't respond to the beeps and read the messages.

Like I implied. These devices are made to get us from point A to point B. They are not used for aiming a missle, or anything else that requires accuracy of better that 10-60 feet. If you think it should then you are relying on that device to do more than what it was designed to do.

BTW. I have degrees in both Mathametics and Computer Science, and I find the topic worth noting (in passing), but not very interesting because of what the device is and what it is used for.

Each to their own topic to relieve boredom.....

The degree of accuracy depends on how many decimal places there are as well. My Garmin gives me 6. According to an interesting list this should be accurate to 0.11 meters. In theory.

The sign tells us whether we are north or south, east or west on the globe.
A nonzero hundreds digit tells us we're using longitude, not latitude!
The tens digit gives a position to about 1,000 kilometers. It gives us useful information about what continent or ocean we are on.
The units digit (one decimal degree) gives a position up to 111 kilometers (60 nautical miles, about 69 miles). It can tell us roughly what large state or country we are in.
The first decimal place is worth up to 11.1 km: it can distinguish the position of one large city from a neighboring large city.
The second decimal place is worth up to 1.1 km: it can separate one village from the next.
The third decimal place is worth up to 110 m: it can identify a large agricultural field or institutional campus.
The fourth decimal place is worth up to 11 m: it can identify a parcel of land. It is comparable to the typical accuracy of an uncorrected GPS unit with no interference.
The fifth decimal place is worth up to 1.1 m: it distinguish trees from each other. Accuracy to this level with commercial GPS units can only be achieved with differential correction.
The sixth decimal place is worth up to 0.11 m: you can use this for laying out structures in detail, for designing landscapes, building roads. It should be more than good enough for tracking movements of glaciers and rivers. This can be achieved by taking painstaking measures with GPS, such as differentially corrected GPS.
The seventh decimal place is worth up to 11 mm: this is good for much surveying and is near the limit of what GPS-based techniques can achieve.
The eighth decimal place is worth up to 1.1 mm: this is good for charting motions of tectonic plates and movements of volcanoes. Permanent, corrected, constantly-running GPS base stations might be able to achieve this level of accuracy.
The ninth decimal place is worth up to 110 microns: we are getting into the range of microscopy. For almost any conceivable application with earth positions, this is overkill and will be more precise than the accuracy of any surveying device.
Ten or more decimal places indicates a computer or calculator was used and that no attention was paid to the fact that the extra decimals are useless. Be careful, because unless you are the one reading these numbers off the device, this can indicate low quality processing!

More than likely, your pin point location is the aggregate of the four corners of your foundation. The GIS guy comes out with his Super Duper GIS GPS and takes a reading at the four corners of your house. Back to the office to Post Process the data and then that point is used to assign 911 addressing. Your house number is the distance from the starting point of the road to your house's actual location. 5102 Primrose Ln is 5.102 miles from the beginning of the street.

To be petty, you are talking about precision, not accuracy. Plagiarizing Wikipedia ( https://en.wikipedia.org/wiki/Accuracy_and_precision ) "... Accuracy is how close a given set of measurements (observations or readings) are to their true value, while precision is how close the measurements are to each other. ..."

Accuracy requires good raw data for your *precise* computations. The configuration of the satellites is important for better data. For example, 3 satellites all near the horizon and 120° apart will provide better data than 3 satellites all on the western horizon.

If you are serious about the true accuracy of your GPS and have some time on your hands, this is the best way to do it. The process is a bit technical but it can be fun if you're a true GPS geek.

Find a USCGS Classic Horizontal Control benchmark that has been surveyed to a high degree of precision using professional GPS surveying equipment. The accuracy is usually within .5 CM.

You can usually find one one of these benchmarks fairly close to your location by going here:

https://geodesy.noaa.gov/NGSDataExplorer/

Type your Zip Code in the search box and click go. Zoom in to view the benchmarks. Choose only those marked as "Classic Horizontal Control". Click on the mark and print out the datasheet.

Find the benchmark location in the field and center your GPS on top of the mark. Leave the GPS stationary for a few minutes and read it's location coordinates. Compare with those listed on the datasheet. The calculated offset is the true accuracy of your GPS.

Keep in mind the results obtained here are made under ideal conditions and are the best your GPS is capable of producing. They will vary greatly with location.

Finding these benchmarks in the field can be a challenge but they are always in open areas and usually well marked by local surveyors.

As always, excellent advice and a great post by bdhsfz6. You should repost this information as a FAQ!

Several years ago when I was geocaching, I did this a couple of times to test the accuracy of my Garmin Oregon 550 before placing and posting a new geocache. I added the step of using "Waypoint Averaging", a feature of the Oregon. This feature takes multiple readings over a period of time and averages them until it reaches a high confidence level. I usually let the process run for 10 to 30 minutes and found I could lock the waypoint into the benchmark coordinates within a couple of feet or less. I would use this averaging method when placing a geocache, giving me high confidence that I would be publishing very accurate coordinates for my cache.

Thanks Alan. Benchmark hunting & geocaching have been hobbies of mine for many years. I don't think I ever located a cache using GPS alone. Usually, it gets you into the immediate area where you can use clues & visual markings.

Your point about listing accurate coordinates when placing your caches is well taken though. Doing so reduces the error of the GPS used by the geocacher rather than compounding it.

A simpler but less precise way to check the accuracy of your GPSr is to use Google Earth. Enter the coordinates of the benchmark you're using, place a pin and then enter the coordinates displayed on your GPSr and place another pin. The distance between the pins is the GPS position error.

There are inherent errors in Google Earth however, especially in mountainous areas. Google Earth uses these Classic Horizontal Control benchmarks to align their grid so the error here should be minimal.

Yes, position averaging does increase the accuracy but this feature is usually only available in handhelds.

almost forgot I have one of them in the camper

getting back to the car mostly
park somewhere in the outback, turn on the gps, save position
three/four days later turn on the gps, navigate to car

didn't need to be accurate,
did need to be precise, consistently wrong was ok

That is a great post, Ralph. I have three Garmins that have GPS and Galileo. They are:

Garmin Zumo XT

Garmin DriveLuxe 51

Garmin DriveSmart 65 with Amazon Alexa

Right now I have all of them inside, connected to external power and displaying the coordinates.

The Coordinate Format is h ddd.dddddd°

All three of them, when they are next to one another, seem to be receiving signals from most of the same satellites. The coordinates agree to four places beyond the decimal point. (The fifth and sixth digits vary.)

I recall some years back discussion on here about which GPS receiver chipsets were being used. If I recall correctly, the SiRFstarIII was desirable at one time. Perhaps the Qualcomm SiRFstar V 5e performs better.

I may try this outside later to see if they get signals from additional satellites outside.

That is a great post, Bdhsfz6!

Thank you.

I may have to look for one of these on my bicycle ride today.

For home/commercial use we are limited to 12-bit chips that read the GPS signals which gives no more than 10 feet accuracy.

Military and possible some other professional occupations have a 16-bit chip that provides accuracy down to a couple inches or so. 16-bit is 16 times higher resolution than 12-bit.

If Garmin ever produces GPS receivers that can access the European GPS satellites as well, I don't believe they are as limited.

Most smartphones can read both the US and European GPS satellites.

The already do. That is what Galileo is.

https://support.garmin.com/en-US/?faq=YXd153clB76LEPOaWA2799

On my nuvi 2689LMT I was once warned "Wrong way on a one-way street". In actuality it was a 2-way highway with a divider so accuracy is good enough for my purposes.

Just for fun today, I downloaded an Android app from the Google Play store onto my Google Pixel 6 called GPS Test. It also uses the term Accuracy (± ft.).

In the Satellite Filter area it lists the following:

GPS

GLONASS

GALILEO

BEIDOU

QZSS

SBAS

? UNKNOWN

I tried disabling all but one at a time. I get signal from GPS, GLONASS and GALILEO. I get nothing from BeiDou, QZSS, SBAS and ? UNKNOWN.

It looks like my Google Pixel 6 lists BDS (B1I+B1c+B2a). So, if I travel to China that might help.

QZSS is Japanese, so it should help in the Asia-Oceania region.

SBAS I still need to learn a bit more about this.

? UNKNOWN I am not sure if they just left that there for any future systems.

What is interesting to me is my smartphone is getting signals from more satellites than my Garmin Zumo XT. The Zumo XT shows: 10 Foot Accuracy, but the Google Pixel 6 shows Accuracy to 15± ft.

As with WAAS, BAS is not itself a complete navigation system, but rather an augmentation system which enables increased accuracy by broadcasting correction adjustments.

It is a somewhat generic term, and in fact WAAS within the coverage area is considered itself to be one of several SBAS systems.

The "SB" part stands for "Satellite Based". These are wide-are improvement schemes, as distinct from the very localized ones which are used to tune up GPS accuracy for a single airport or such.

See https://www.gps.gov/systems/augmentations/. Take note that the topic of GPS "accuracy" is very complex. Explore the site https://www.gps.gov/ should you desire better understanding.

CNBC posted (on my Apple plus subscription) an interesting video on GPS systems world wide with some emphasis on the dangers of China. Not sure if this link will be available to any, but if it is.. worth listening too.
https://www.cnbc.com/live-tv/?__source=applenews%7Cwatchlive...

Tried this link on my MacBook and it would not play.. so a wasted post, sorry.

Thanks for the advice. This post has been educational. The best I get on my nuvi 1690 is about 18 ft.

I did enter my ZIP code, which is 55068. Several Classical Horizontal locations show up. I might take an e-bike ride to PID: DE9109.

44 40 39.90388(N) 093 08 12.71818(W)

44.677751, -93.136866

https://www.google.com/maps/place/ 44%C2%B040'39.9%22N+93%C2%B008'12.7%22W/ @44.6777549,-93.1394411,17z/data=!3m1!4b1!4m4!3m3!8m2!3d44.6777511!4d-93.1368662?entry=ttu

https://boulter.com/gps/?c=44.677751%2C+-93.136866#44.677751...

I did this today. I went to DE9109, which is in Empire Township, near Farmington MN 55024. My Garmin Zumo XT was receiving a number of satellite signals and said it was at "10 Foot Accuracy". It displayed:

Location

N 44° 40' 39.86"

W 093° 08' 12.12"

I also checked my Google Pixel 6 which has a GPS app.

It showed:

44° 40' 40.001" N

93° 08' 12.504" W

Does anybody here have a good calculator to measure the distance between these coordinates found and the coordinates for DE9109?

There is a calculator at https://www.meridianoutpost.com/resources/etools/calculators.... Note as well that same page links to a converter for degrees minutes seconds to decimal degrees. The link is under the picture of the chart and protractor. For what may be small distances, like your case, the number of significant digits in the calculation may be a factor.

Should you want to compare, another can be found at https://latlongdata.com/distance-calculator/.

Before putting absolute faith into these calculators, you should also read the cautionary text at http://www.csgnetwork.com/gpsdistcalc.html

What I get is summarized below:

DE9109
POSITION- 44 40 39.90388(N) 093 08 12.71818(W)
Or 44.67775108° N x 93.13686616° W

Garmin Zumo XT
N 44° 40' 39.86" (44.67773889°)
W 093° 08' 12.12" (93.13670000°)
Zumo about 42.2 feet from DE9109

Google Pixel 6
44° 40' 40.001" N (44.67777806°)
93° 08' 12.504" W (93.13680667°)
Pixel 6 about 15.8 feet from DE9109

Thank you for the reply.

but much easier way to see the difference is to enter the first set of coordinates in the Google Earth search box and place a pin. Then enter the second set and place another pin. Use the measuring tool to determine the offset error from another pin placed at the DE9109 coordinates.

Using this method yields:

Zumo XT - 44.69' from DE9109

Pixel 6 - 18.75' from DE9109

I was looking at equipment used for professional applications. I was wondering what those devices do to make them more accurate than consumer grade devices. This gives some information on how professional and consumer GPS receivers differ.

https://www.subseatechnologies.com/media/files/files/80667c8....

google maps
copy n n paste the two coordinates as start and finish in the directions tab
13 feet on the road

right click either point, "measure distance" click the other point 31 feet

I'm guessing the red topped stake is the marker

I wonder what the Google Pixel 6 has or does that allows it to get more accurate coordinates than the consumer Garmins?

Does it use GPS signal data from both carrier phase
as well as code phase?

Is the Google Pixel update rate for frequent than my Garmins?

Does it use a wider RF bandwidth?

When you have Google Location Accuracy turned on, your phone uses these sources to get the most accurate location:

GPS
Wi-Fi
Mobile networks
Sensors (such as accelerometer)

Some info is also collected by Google.

When you turn off Google Location Accuracy, your phone uses GPS and sensors, like an accelerometer, to determine location. GPS can be slower and less accurate than other sources.

When Google Location Accuracy is off, GPS, Wi-Fi, network, and sensor data are not used. or collected by Google.

Last week, far from home (~800 miles) running Google Maps my Pixel 6A asked me to take a photo to enhance accuracy. There were stores nearby. Of course I declined: who cares?

I'd still like to know the metric for accuracy. When the computation says 10 foot accuracy is that with a specified confidence interval? Something else?

I ran across this on Quora this afternoon:

Bill Hazelton
·
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Peregrinating Geospatial Professor & Practitioner8mo
How accurate is the Global Positioning System (GPS) for determining longitude and latitude?
That depends on how you work it.

The consumer level devices us the C/A (Coarse/Acquisition) code-based signal to determine location, and they are good to about 5 m in the horizontal (latitude and longitude) and 15 m in the vertical (height, but not allowing for the separation between the ellipsoid and the geoid).

If you add a correction signal to this, using something like WAAS, you can get down to about 1.5 m in horizontal (it doesn’t make much difference in the vertical). It also helps if you are receiving on both frequency bands and can therefore correct for ionospheric signal delays.

If you are in the military and have a military-grade receiver that can use the P and W code signals, you can get down to about 1 m in horizontal location.

All of the above should give you the above-mentioned precisions of location on a continuous basis, once they lock on to the satellites and download the ephemeris (this can take up to about 30 seconds). Assuming you maintain reception of the signal, this should be good most of the time, although this can vary with the geometry of the satellites at any given time. If your geometry is degraded, perhaps because you can get a signal from some direction because of trees, then your location will also be downgraded. Effectively, the above precisions of location are the best situation, and they can be degraded because of poor geometry.

If you move away from the idea of a stand-alone receiver and consider using two receivers that can communicate in either real time or through later post-processing, and can use better orbital data obtainable at a later time, then you can get to the centimeter level. If you can set up a real-time link, such as using RTK (real-time kinematic) or a virtual reference network (VRN), you can get results down to 0.05 m in 3-D. This can also be done in a matter of seconds if you can maintain lock on the satellites on both receivers. You need survey-grade receivers for this, and these usually cost about $US10,000 and up per unit. For RTK you will usually need 2 of these units and perhaps a radio system on top of that for communications. VRN systems commonly connect through your phone to the receiver.

If you want to do a static session, in many places around the world you can post-process your data with that from a network of fixed high-precision stations that have their data available with perhaps an hour or so’s delay. Here you can get down to 0.01 m with a 2- to 4-hour session. You need survey=grade receivers, but usually only one. You may need to wait a day or so to get better orbit data. Methods like PPP allow you do get centimeter level results, but after a delay of up to a day or two for good orbital data.

If you place a receiver in a secure location and let it run continuously, your precision of location will keep getting better. The CORS (Continuously Operating Reference Stations) can measure their location to a millimeter (0.001 m) or better over time, and can also measure the tectonic movement of the US continental masses. You can use receivers to monitor structures for movement to millimeter levels using this kind of approach.

If you want to get location continuously as you move, such as for obtaining imagery or measuring gravity on a moving platform (like a ship or aircraft), you can use a pair of receivers and using various processing ‘tricks’ you can track your platform to about 0.01 m as it moves. In many cases, these systems are connected to an Inertial Measurement Unit (IMU), which together with the GPS unit will provide 3-D location to about 0.005 m and 3-D orientation to about 30 arc seconds, at about 100 Hertz (i.e., 100 times a second) while traveling at aircraft speeds.

Because satellite geometry is a critical factor in the precision of your location, the more satellites you can use, the better. If you can also see and measure using other constellations, e.g., GLONASS, Galileo, BeiDou, QZSS and Navik, you can improve the precision of your position, especially in areas with limited sky visibility. Collectively, these constellations are called GNSS, the Global Navigation Satellite System. GPS is one constellation of (currently) four such global systems. QZSS is purely regional (around Japan) and Navik is currently building out (slowly) from a regional system (India) to a global one.

One more important point. In measurement science (which is what we are dealing with here), “accuracy” means ‘how close are we to the truth.’ The trouble is, we don’t know the true location, because the only way we can know it is to measure it, and measurement is necessarily imperfect. So ‘accuracy’ is a bit meaningless. Instead, we take about something we can determine, precision. “Precision” means ‘how close my measurements are to each other.’ ‘High precision’ means a very tight grouping of measurements. So all the values for how good your position is given above are all precisions, simply because we can’t know the true location unless we measure it, and even if we do decide the point has a very specific latitude and longitude, over time it will move because everything on Earth does, and that latitude and longitude will change. Further, the latitude and longitude grid also get updated on a regular basis, and all four global GNSS constellations use slightly different latitude and longitude grids.

Once you burrow into geodesy, you start to realize that figuring out the Earth is a huge job, and often counter-intuitive! Good thing we have geodesists to do it all for us!

I was looking at the Minnesota Department of Transportation website this weekend. There is a geodetic tations near me in Eagan MN (Dakota County). If I am interpreting this one correctly, though, it is "Unpositioned (scaled or handheld GPS)."

MNDOT Station Name PARK MN037
GSID 7511

https://www.olmweb.dot.state.mn.us/Geod/PDF%20Single%20Sheet...

https://mndot.maps.arcgis.com/apps/webappviewer/index.html?i...

https://wiki.openstreetmap.org/wiki/Test_the_accuracy_of_GPS....

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Contents
1 Accuracy and Integrity
2 Accuracy of Data
3 Testing the Device Oneself
4 Measuring the Reference Point
4.1 Simple Method
4.2 Complicated Method
5 Reference Points for Aerial Images
5.1 Reference Points of Others
5.2 Determine the Reference Point on Your own
5.3 Choosing OSM nodes and POI as reference points
5.3.1 Surface
5.3.2 Protruding
5.3.3 Synthetic
6 Survey Points in OSM
7 See Also
Accuracy and Integrity
Accuracy is the deviation of measurement data of real actuality. At GPS-devices it is the deviation of the coordinate shown by the device from the 'correct' coordinate. Required are both coordinates being recorded in the same system (WGS-84 at OSM), respectively the accurate conversion of the coordinate when the reference value comes from another system.
Availability is the complete and unmodified arrival of the data on the recipient.
Reliability is a percentage describing the expected length of a system being able to execute an intended function with the required accuracy.
Integrity is the ability of a system to inform the user early enough when not to being used anymore (wrong or missing measured values).
Accuracy of Data

error picture
The accuracy depends on:

quality of satellites
disturbance at transmission until reception
properties of reception devices (hard- and software)
The accuracy of the device depends on:

quality of GPS-chip
quality of electric signal processing
quality of software
The producers of consumer-devices mostly don't give any information on the used hard- and software. Details on signal correction ("WAAS" etc.) are often indefinite to misleading. Also, the displayed "accuracy" of some devices is not defined and therefore rather useless.

Testing the Device Oneself

reference point
For that purpose you need an exactly known reference point on the free field. If you are lucky there is a measurement point with all around free sight to the sky and you get the exact coordinates. Or you ask a measurement engineer at a building lot whether he could determine a measurement point with all around sight to the sky for you, for example at a country road or a country lane (mostly he is gladly to do that). Then you can measure that point several times with your device and compare the middled result with the official coordinate.

Measuring the Reference Point
A single measurement of a point through a consumer-GPS-device is imprecise. The deviations add up to several dozen meters. Through multiple measurements (scatter-plots) and EGNOS improvements are possible. The best results are gained through differential-GPS-devices, but they are substantially more expensive.

Exact reference points can aso be created with long-term measurements. Therefore you need an exact determinable point on free field with free sight to sky:

Flaches Feld für Referenzmessung.jpg

The horizon should be lower than 10..20° and especially free to the South so that the EGNOS-satellites are better visible.

Satellite PRN ID Position Direction right-pointing Elev. in Germany Comment
Inmarsat AOR-E 120 33 Atlantic 15,5° W 195,5° 25..30°
ARTEMIS 124 37 Africa 21,5° E 158,5°
Inmarsat IOR-W 126 39 Africa 25,0° E 155° currently testing system
Sirius 5 5,0° E 175° since end of 2011
Simple Method
If your device has a mode for just saving a point every minutes (some devices do that automatically if they are not moved), then you can simply lay the device on the reference point and get it back after 10 hours (as long as the battery works). From the logfile you calculate the mean value. With that you middled out any satellite- and weather-anomalies and get (on the free field) a submeter-exact position.

Complicated Method
If your device has no minute-mode you can get a similarly exact result by measuring several points at several moments and then calculating the average value.

Mark a view points on a free area (or a plan of it) that are a few dozen meters away from each other and can be found again without a doubt at any moment. Suitable "callibration areas" are: big parking areas with drains, beach at a danish North Sea coast with residual bunkers, sports fields with solid markers, for example tennis, etc.

Measure the distances between all points (triangles) with a measuring tape or laser distance measurer.

Measurement with GPS
Determine the positions of these points with your GPS: Fasten the device motionless for at least one minute at each point. Repeat the measurement ten times in the interval of at least 90 minutes to have different satellite constellations and possibly different atmospharical conditions. So for such an attempt you need 15 hours.

Calculate the average of each scatter-plot:

Sum LAT / number of points in scatter-plot = LAT[average]
Sum LON / number of points in scatter-plot = LON[average]
Some GPS-devices have a built-in function for that. Alternatively, log in save as spreadsheet and let it calculate automatically.

If you want to know it exactly, do 20 measurements on respectively only half of the points. That excludes a possible regular mistake whereby all measurement values could be shifted in the same direction through long-term atmospheric disturbances.

Correction of Measured Values
At the start of a measurement and sometimes at the end the device draws "wrong" points (for example residual movements, power-up-mistakes, satellite-search-mistakes). These can be determined well through statistic-programs and/or graphical displays. Such systematic mistakes can then be deleted from the series of measurement.
top right: movement mistake at the beginning
top right:
movement mistake at the beginning

Calculation
Calculate the distances out of coordinate differences between a two reference points (or tap them in an editor)
Determine the difference between measured and calculated distance
Calculate the average difference: count all differences together and divide them through their quantity.
The result is a proportion for the maximal total accuracy of the device and environmental influences under good conditions. If the environmental influences decline the accuracy also declines. See Accuracy of GPS data ten times per minute in an interval of 90 minutes. Calculate the average of the scatter-plot.

You can also measure several points and middle the middle of every scatter-plot to an even more exact reference point:

Sum LAT[Middle] / number of observations = LAT[total]
Sum LON[Middle] / number of observations = LON[total]
Reference Points for Aerial Images
To check the georeferencing of aerial images, respectively to correctly shift not exactly georeferenced aerial images in JOSM, you need exactly measured reference points on the terrain. Requirement: the point needs to be exactly recognisable on the aerial image.

Reference Points of Others
Measurement points are ideal. You get them at the local government, at the building authority, at the land surveying office or any surveyor at your place (just ask). Such points are accurately on a centimeter-scale. You need the coordinate in WGS-84 (or you have to translate convert it) and also the height in WGS-84 (here the conversion is more complex).

Determine the Reference Point on Your own
To position satellite images exactly you can calibrate reference points on your own. Look for points in the terrain that

a) have as free sight as possible to the sky
b) are highly visible at the aerial image
Measure the point with the GPS-device as described above.

Well suited are manhole covers, road markings and traffic islands, because they are well visible on the aerial image.

Choosing OSM nodes and POI as reference points
It is even better if you choose an object that exists on the ground and is of common public interest regardless of this measurement. You may also add the below keys on the said object as well (accuracy=*, lat=*, lon=*). Ensure that dense vegetation does not cover the object, or at least it should be clearly visible in months without foliage.

Surface
Midpoint:

man_made=manhole
emergency=fire_hydrant + fire_hydrant:type=underground
Corners:

leisure=pitch
playground=sandpit
amenity=parking_space, especially with ones having extra marking like parking_space=disabled or parking_space=charging
Protruding
One needs to watch out to properly intersect the lean and the shadow on aerial imagery to arrive at the on the ground base point of the given object. Here are some low rise points where you can usually put your instrument on top at the midpoint:

man_made=street_cabinet
amenity=drinking_water
emergency=fire_hydrant pillars
advertising=* boards or small columns
amenity=bench
leisure=picnic_table
amenity=waste_basket
playground=* equipment
sport=table_tennis table
Special measurement apparatus needed (for measuring and averaging out 3 or 4 points around the object):

highway=street_lamp
power=pole
Synthetic
We usually add tagless empty nodes as part of ways when micromapping. These points are not POI (points of public interest), but if you can find literally nothing else from above, you may also choose some of these. They can also be clearly inferred on the ground, hence we may also add measurements to their midpoints:

midpoint of centerline at top and bottom end of highway=steps where it joins a footway
midpoint of centerline at the two ends of a marked pedestrian highway=crossing where it joins the footway/sidewalk - this is usually the place where you can also add a kerb=* (a measurement would also be possible in the middle of the road if it has no connected path ways, but that is a lot more difficult to determine on the ground)
midpoint of preferably linear traffic_calming=*
some points within a linear barrier=* like barrier=cable_barrier, barrier=guard_rail, barrier=handrail
some midpoints of the centerline of footway=sidewalk
Survey Points in OSM
Load the ground control point in the OSM-DB with an editor and name it with:

man_made=survey_point
survey=ground_control_point
source=*
survey (for normal GPS-devices)
egnos (for EGNS-corrected signals)
on_top (for elevated recipients/antennas)
differential (for differantial GPS)
official (for purchased points)
lat=* (Width decimal in WGS-84)
lon=* (Length decimal in WGS-84)
alt:wgs84=* (Height decimal in WGS-84)
note=* 'position will change'
description=* '1. Sieldeckel im Immenredder westlich der Kreuzung Grützmühlenweg/Immenstieg'
name=* GCP_Immenredder_001 (the name starts with GCP (for GroundControlPoint)
type=* manhole_cover (just as example)
See Also
Accuracy of GPS data
Genauigkeit von Koordinaten
Umrechnung von GK in WGS-84
Reliability of OSM Coordinates
Koordinate
v · d · eGPS related articles
Main
GPS with good internal links, GNSS. Another formats: NMEA, FIT (Garmin),
GPX tracks: create
List of GPS trace file formats, GPX, Why not GPX?, GPX viewer and recorder (Windows™ only), OpenGpxTracker (iOS™ only), Gps Event Sync, GPX extractor, OsmAnd, MAPS.ME, GpsMid (J2ME), Gpsd
GPX tracks: convert
Convert GPS tracks, Converting/NMEA to GPX, Edit GPS tracks with big software list, GPSBabel,
GPX tracks: upload
Upload GPS tracks, Batch Upload of GPX (various tools), Upload (stub), Category:Tracelog hosting
Visualise
Online: Track drawing websites, Openlayers Track example, PhpMyGPX, offline: GpsPrune, GPX Viewer, JOSM, Florb, GpsMaster
Hardware (receivers)
GPS device reviews (with devices table), receive/lend: GPStogo
Accuracy
Accuracy of GNSS data, RTKLIB, RTKLIB-compatible GPS devices, Open-source hardware, KartaView/OBD2, Test the accuracy of GPS devices
Other
Comparison of Android applications
Category: GPS
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This page was last edited on 7 May 2021, at 23:45.
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I was reading about refresh rates today. I saw that the Garmin Aera 560 claims a "Fast 5 Hz GPS refresh rate." I also entered that as a search term on Garmin.com and found this:

"GPS Refresh Rate/Frequency for Overlander, Tread, and zūmo XT Devices
The GPS refresh rate/frequency is listed below.

Tread®, Tread® - Base Edition, and zūmo® XT GPS navigators are 1Hz GPS.

Tread® - Overland Edition, Tread® XL - Overland Edition, Tread® - SxS Edition, Tread® XL - Baja Chase Edition, Tread® XL - Baja Race Edition, and zūmo® XT2 GPS navigators are Dual Frequency GNSS (L1 + L5) and 10Hz. (GPS/Galileo/Beidou).

Overlander® GPS navigator is 1Hz L1 GNSS."

I do have a Garmin Zumo XT with a refresh rate of 1Hz. I have considered a Garmin Zumo XT2 that has a refresh rate of 10Hz.

Does that necessarily equate to a more accurate reading?

Delete-Duplicate

Delete-Duplicate

You need to understand “refresh rate” and its ramifications. You mention the Aera 560 which is a device for aircraft navigation. A plane at 160 mph is traveling at about 240 feet/sec. So a refresh rate of 5 Hz means it updates 5 times per second, and the reality is that 160 mph plane may have travelled 48 feet every update. But a car at 60 mph is traveling 88 feet/sec, the approximate distance across a multi lane highway intersection.

Truthfully, IMO this topic is poorly titled when it says “accuracy”. It would be better titled “potential accuracy” meaning it might attain some measurement under ideal conditions. For example, my nuvi 2689 might be able to yield a certain distance representing “accuracy” given a set of conditions, but that number represents a best case scenario. If I’m in Manhattan where reflections of signals may be substantial, my “potential accuracy” will be far less than if I’m standing on top of the Empire State building or traveling the New Jersey Turnpike. If I have 4 satellites vs. 7 my potential accuracy is different.