Where is GPS location calculeted?

I asked once before, here maybe, and was told it was in the device, the
car radio or the phone, I guess.

But recently I read it was in the satellites.

Either one seems hard to imagine.

I'm sorry that I have no links, but I don't.

On Mon, 24 Jan 2022 15:47:28 -0500, micky wrote:

> Where is GPS location calculeted?
>
> I asked once before, here maybe, and was told it was in the device, the
> car radio or the phone, I guess.
>
> But recently I read it was in the satellites.
>
> Either one seems hard to imagine.
>
> I'm sorry that I have no links, but I don't.

On the phone.
<https://www.google.com/search?q=how+is+locaiton+calculated+gps+on+a+phone>

First hit:
*How does GPS work on your phone*
<https://helpdeskgeek.com/featured-posts/hdg-explains-how-does-gps-work/>

"Here's how this GPS process works:

Four satellites transmit the same timestamp to your phone at 5:12:14 PM.
Your phone receives that timestamp at 5:12:15 from satellite 1.
It receives the timestamp at 5:12:16 from satellite 2.
Finally, at 5:12:17, it receives the timestamp from satellite 3.
This tells your GPS receiver that it took 1 second for the radio signal to
reach it from satellite 1, 2 seconds from satellite 2, and 3 seconds from
satellite 3.

The speed of light is a known constant of 299,792,458 meters per second.

Using simple math, the receiver can calculate that its distance is roughly
300 thousand meters from satellite 1, 600 thousand meters from satellite 2,
and 900 thousand meters from satellite 3.

Using a lookup table from a GPS satellite database, your phone's GPS
receiver knows the approximate current location over the earth of all three
satellites, which provides longitude and latitude coordinates of all three.

With that information, your phone is able to calculate your own longitude
and latitude coordinates on the Earth.

Using your known coordinates, your GPS receiver can then use the distance
measured between itself and a fourth satellite to determine what altitude
above the Earth you're located."

In article <nv3uugl44jrv3u47fsmqjlihq0vbi50qh1@4ax.com>, micky
<NONONOmisc07@fmguy.com> wrote:

> Where is GPS location calculeted?

in the gps device itself, which in this case, is a phone.

Andy Burnelli <spam@nospam.com> wrote:
> On Mon, 24 Jan 2022 15:47:28 -0500, micky wrote:
>
>> Where is GPS location calculeted?
>>
>> I asked once before, here maybe, and was told it was in the device, the
>> car radio or the phone, I guess.
>>
>> But recently I read it was in the satellites.
>>
>> Either one seems hard to imagine.
>>
>> I'm sorry that I have no links, but I don't.
>
> On the phone.
> <https://www.google.com/search?q=how+is+locaiton+calculated+gps+on+a+phone>
>
> First hit:
> *How does GPS work on your phone*
> <https://helpdeskgeek.com/featured-posts/hdg-explains-how-does-gps-work/>
>
> "Here's how this GPS process works:
>
> Four satellites transmit the same timestamp to your phone at 5:12:14 PM.
> Your phone receives that timestamp at 5:12:15 from satellite 1.
> It receives the timestamp at 5:12:16 from satellite 2.
> Finally, at 5:12:17, it receives the timestamp from satellite 3.
> This tells your GPS receiver that it took 1 second for the radio signal to
> reach it from satellite 1, 2 seconds from satellite 2, and 3 seconds from
> satellite 3.
>
> The speed of light is a known constant of 299,792,458 meters per second.
>
> Using simple math, the receiver can calculate that its distance is roughly
> 300 thousand meters from satellite 1, 600 thousand meters from satellite 2,
> and 900 thousand meters from satellite 3.
>
> Using a lookup table from a GPS satellite database, your phone's GPS
> receiver knows the approximate current location over the earth of all three
> satellites, which provides longitude and latitude coordinates of all three.
>
> With that information, your phone is able to calculate your own longitude
> and latitude coordinates on the Earth.
>
> Using your known coordinates, your GPS receiver can then use the distance
> measured between itself and a fourth satellite to determine what altitude
> above the Earth you're located."

Yes it works like that, except that the timestamps and receive times
are measured in 1/10000000000 of a second and as your phone does not
know the correct time to that accuracy, it actually requires FOUR
satellites to determine the position from 4 time differences to an
unknown base time. This position then already includes the height.

Rob <nomail@example.com> wrote:
> Yes it works like that, except that the timestamps and receive times
> are measured in 1/10000000000 of a second

Too many zeroes there, it should be 1/10000000 second (100 nanoseconds).

This corresponds to an accuracy of 30 meters.

On Mon, 24 Jan 2022 22:01:26 +0100, Rob wrote:

> This corresponds to an accuracy of 30 meters.

I was reading the article with respect to what you said prior to the zeroes
correction, where it seems it uses three spheres for "_triangulation_"
and then a fourth sphere for "_trilateration_" to calculate your altitude.
<https://helpdeskgeek.com/featured-posts/hdg-explains-how-does-gps-work/>

Is that right?

As for the zeroes, this is what the article says about the time stamps.
"Your phone's clock is not an atomic clock, so its time isn't synced
with the atomic clocks of the satellites in orbit. However, this doesn't
matter when it comes to calculating location using signals from
those satellites.

This is because your phone's GPS receiver is focusing on the data it
receives from the satellites, and a database of known satellite locations
over the Earth. Since all of the satellites do have an atomic clock,
the current time on each satellite is exactly the same at any given moment.

However, due to distance from the satellite, and the fact that the radio
signals travel at the speed of light, the differences between each
received timestamp reveals the distance between your phone
and each of the satellites."

micky,

> Where is GPS location calculeted?
>
> I asked once before, here maybe, and was told it was in the device, the
> car radio or the phone, I guess.
>
> But recently I read it was in the satellites.

Maybe you should ask yourself "how does GPS work?" (and google for it).

The short answer is that the satellites send out signals with which the GPS
device can figure out where its located.

IOW, Your GPS device does all the calculations to determine its own
position.

Regards,
Rudy Wieser

Am 24.01.22 um 21:47 schrieb micky:

> Where is GPS location calculeted?
>
> I asked once before, here maybe, and was told it was in the device, the
> car radio or the phone, I guess.

In the device (the phone, or whatever it is).

The time stamps sent by the satellites arrive at the receiver with a
slight offset depending on the distance to the respective satellite.
Together with the orbital parameters of the satellites, this is enough
to deduce the position of the receiver.

> But recently I read it was in the satellites.

The satellites only broadcast an extremly precise timestamp from their
on-board atomic clock together with their exact orbital parameters and
some correction data uploaded by the control center.

They have no means to receive requests from a consumer device or send
individual data to it.

Andy Burnelli <spam@nospam.com> wrote:
> On Mon, 24 Jan 2022 22:01:26 +0100, Rob wrote:
>
>> This corresponds to an accuracy of 30 meters.
>
> I was reading the article with respect to what you said prior to the zeroes
> correction, where it seems it uses three spheres for "_triangulation_"
> and then a fourth sphere for "_trilateration_" to calculate your altitude.
> <https://helpdeskgeek.com/featured-posts/hdg-explains-how-does-gps-work/>
>
> Is that right?
>
> As for the zeroes, this is what the article says about the time stamps.
> "Your phone's clock is not an atomic clock, so its time isn't synced
> with the atomic clocks of the satellites in orbit. However, this doesn't
> matter when it comes to calculating location using signals from
> those satellites.
>
> This is because your phone's GPS receiver is focusing on the data it
> receives from the satellites, and a database of known satellite locations
> over the Earth. Since all of the satellites do have an atomic clock,
> the current time on each satellite is exactly the same at any given moment.
>
> However, due to distance from the satellite, and the fact that the radio
> signals travel at the speed of light, the differences between each
> received timestamp reveals the distance between your phone
> and each of the satellites."

Yes that is correct, it is trilateration, but triangulation is probably
a more meaningful word for many readers. The GPS receiver does not
measure angles so it really is not triangulation.

The text quoted before appeared to assume that the phone knows that
a signal with timestamp X and received at X+1 has taken 1 second to
send from the satellite to the phone. But it doesn't, since it does
not know the correct time accurately enough to measure that time
difference.

So instead it listens for FOUR different satellite signals and measures
the DIFFERENCE in time delay between them. It does not know its
own time beforehand, but from the time differences it can calculate its
position in 3 dimensions and the accurate real time. This is a little
harder to imagine than what was pictured in the quoted text so that
is probably why it was simplified.

Rob wrote:
> Rob wrote:
>> Yes it works like that, except that the timestamps and receive times
>> are measured in 1/10000000000 of a second
>
> Too many zeroes there, it should be 1/10000000 second (100 nanoseconds).
> This corresponds to an accuracy of 30 meters.

https://www.gps.gov/systems/gps/performance/accuracy/

-p

On 1/24/2022 3:47 PM, micky wrote:
> Where is GPS location calculeted?
>
> I asked once before, here maybe, and was told it was in the device, the
> car radio or the phone, I guess.
>
> But recently I read it was in the satellites.
>
> Either one seems hard to imagine.
>
> I'm sorry that I have no links, but I don't.

It is calculated in the phone -- the chip (or module) that does it is
advanced but is just a commodity item now and will just cost a few dollars
in quantities. The GPS satellite knows nothing about you and your phone (or
nav unit in your car, or watch, etc), each satellite is in basic terms a
really really precise clock which transmits its 'name' (ID code) and its
time down to Earth. The satellite sends but does not 'care' if you are
listening or not and your phone etc does not 'talk' to the satellite.

Given the timings of received time signals from a handful of satellites and
a knowledge of where they are supposed to be at any precise instant gives
the distance from satellite to the phone to be measured and allows precise
calculation of coordinates on the planet's surface. The calculations look
insanely complicated, at least to me, but the little chip has no problem
with it.

--
Bodger's Dictum: Artifical intelligence
can never overcome natural stupidity.

On Mon, 24 Jan 2022 22:38:19 +0100, Rob wrote:

> This is a little
> harder to imagine than what was pictured in the quoted text so that
> is probably why it was simplified.

I agree with that statement that they may have simplified the process.

The thing we have to understand is how the phone calculates the time
difference when the phone can't use its own crappy clock to do that.

a. The atomic clock time stamps all _started_ at the exact same time.
b. And, of course, they were received at _different_ times by the phone.
c. And, of course, the phone _knows_ where each satellite is supposed to be.

From that, somehow, the phone calculates an _accurate_ time difference
(at least accurate enough to position us within a few meters of exact).

If someone could expound on how the phone calculates an accurate time
difference from the four satellites without using its own clock, that would
be wonderful for the rest of us (who don't delve normally into such stuff).

On Mon, 24 Jan 2022 22:46:53 +0100, Piet wrote:

>> This corresponds to an accuracy of 30 meters.
>
> https://www.gps.gov/systems/gps/performance/accuracy/

From that reference:
"GPS-enabled smartphones are typically accurate to within a
4.9 m (16 ft.) radius under open sky"

On 24/01/2022 21.47, micky wrote:
> Where is GPS location calculeted?
>
> I asked once before, here maybe, and was told it was in the device, the
> car radio or the phone, I guess.
>
> But recently I read it was in the satellites.

Then you can discard that source where you read that as unreliable for
anything >:-p

--
Cheers, Carlos.

On 2022-01-24 12:47 p.m., micky wrote:
> Where is GPS location calculeted?
>
> I asked once before, here maybe, and was told it was in the device, the
> car radio or the phone, I guess.
>
> But recently I read it was in the satellites.
>
> Either one seems hard to imagine.
>
> I'm sorry that I have no links, but I don't.

Whoever wrote it was calculated in the satellites was hopelessly ignorant.

Andy Burnelli <spam@nospam.com> wrote:
> On Mon, 24 Jan 2022 22:46:53 +0100, Piet wrote:
>
>>> This corresponds to an accuracy of 30 meters.
>>
>> https://www.gps.gov/systems/gps/performance/accuracy/
>
> From that reference:
> "GPS-enabled smartphones are typically accurate to within a
> 4.9 m (16 ft.) radius under open sky"

Yes, I meant to mention only a ballpark figure. Many GPS receivers
use a 10 MHz clock for the time which implies a 100ns resolution,
and signals travel 30m in that time. But the calculation of the
position is more involved and it can be a bit better.

Andy Burnelli <spam@nospam.com> wrote:
> If someone could expound on how the phone calculates an accurate time
> difference from the four satellites without using its own clock, that would
> be wonderful for the rest of us (who don't delve normally into such stuff).

The GPS receiver has multiple independent channels that can listen
for a satellite at the same time. Each of them can report the time
difference to the (arbitrary) local clock.
There also is a low-speed data transmission from all satellites, that
the GPS receiver can decode and store. It contains the Almanac, a
description of the orbit parameters of all satellites.
(this Almanac is determined by a control station and regularly uploaded
to the satellites, which then transmit it all the time)

From the 4 received timestamps and the 4 calculated satellite positions
the receiver can calculate the position by solving a set of 4
linear equations with 4 unknowns.
The result is the position in X Y and Z w.r.t. the center of the
earth (called ECEF for Earth Centered Earth Fixed), plus the accurate
time. 4 output variables.

Then, this ECEF info has to be transformed into a latitude, longitude
and height. The first two are relatively simple (when you have come
this far) but the height is nontrivial because the earth is not
an exact sphere. So knowing your exact position does not tell you
precisely how high you are above sealevel, and even less how high
you are above the ground.
This is one of the reasons why the height information usually sucks.
(the other one being that due to the geometry of the satellite
positions around you the measurement errors affect the height more
than the horizontal position)

On Tue, 25 Jan 2022 10:46:22 +0100, Rob wrote:

>> "GPS-enabled smartphones are typically accurate to within a
>> 4.9 m (16 ft.) radius under open sky"
>
> Yes, I meant to mention only a ballpark figure. Many GPS receivers
> use a 10 MHz clock for the time which implies a 100ns resolution,
> and signals travel 30m in that time. But the calculation of the
> position is more involved and it can be a bit better.

Thanks for clarifying as I don't delve into the particulars of GPS so much
as I use the calculated location (mostly for backcountry hiking purposes).

For backcountry hiking, on already inaccurate topographic geoPDF maps, the
approximately five meters (about 15 feet) on average, is good enough for me
(where I use free USGS 1:24000 geoPDF topo maps on my phone with the GPS).
<https://i.postimg.cc/9fXYpgSt/approximatepath.jpg>

The phone has no problem showing your position on _any_ geoPDF maps you use.
<https://play.google.com/store/apps/details?id=com.Avenza>
<https://play.google.com/store/apps/details?id=ca.abbro.androidmap>

I'm curious how anyone else who uses their phone for GPS location finds a
need for greater accuracy than five meters gives us already?
<https://i.postimg.cc/rFghS6ZB/drinkingwater.jpg>

What do people do, on their phones, which needs greater accuracy than that?

"Rob" <nomail@example.com> wrote in message
news:slrnsuvhne.5gu.nomail@xs9.xs4all.nl...
> Andy Burnelli <spam@nospam.com> wrote:
>> On Mon, 24 Jan 2022 22:46:53 +0100, Piet wrote:
>>
>>>> This corresponds to an accuracy of 30 meters.
>>>
>>> https://www.gps.gov/systems/gps/performance/accuracy/
>>
>> From that reference:
>> "GPS-enabled smartphones are typically accurate to within a
>> 4.9 m (16 ft.) radius under open sky"
>
> Yes, I meant to mention only a ballpark figure. Many GPS receivers
> use a 10 MHz clock for the time which implies a 100ns resolution,
> and signals travel 30m in that time. But the calculation of the
> position is more involved and it can be a bit better.

Yes, apps such as GPS Status can quote a horizontal degree of precision
which is the diameter of the circle within which your phone is known to lie.
The smaller the circle, the more accurately your position is known. I've
seen diameters of as little as 3 metres quoted, though usually it's 5-20
metres.

When I record a track, I sometimes see a *systematic* (ie not random) error
in position, relative to a background map. The track follows the curves in
the map perfectly but it offset from it by a small constant distance. Then
suddenly the track snaps back onto the line of the road. I've always what
can cause a systematic offset in x or y position that lasts maybe 1-2
minutes before righting itself.

We use an app called Viewranger which can record a track and can display it
and your current position on a background map. We've bought all the Ordnance
Survey maps of the UK at 1:25000 and 1:50000 scale. Those are available
offline because they are stored in the phone. With internet you can also
plot on Open Streetmap anywhere in the world at a much larger scale which
shows and names roads in a town and shows buildings.

Sadly Viewranger was bought out by another company who have their own (much
inferior) application which requires online access for OS maps, and which
cannot use OSM for greater detail. And the new company will actually
forcibly disable Viewranger in February. Such is progress. The Viewranger
users are furious because people have paid a lot of money in the past for
the right to use the OS maps and expected it to be forever more, not
disabled at the whim of the (new) owner of Viewranger.

OK, so there's Google Maps, but those maps are just toys and don't contain
all the geographical symbols that walkers use for navigation.

On 25/01/2022 22.04, NY wrote:

....

> We use an app called Viewranger which can record a track and can display
> it and your current position on a background map. We've bought all the
> Ordnance Survey maps of the UK at 1:25000 and 1:50000 scale. Those are
> available offline because they are stored in the phone. With internet
> you can also plot on Open Streetmap anywhere in the world at a much
> larger scale which shows and names roads in a town and shows buildings.
>
> Sadly Viewranger was bought out by another company who have their own
> (much inferior) application which requires online access for OS maps,
> and which cannot use OSM for greater detail. And the new company will
> actually forcibly disable Viewranger in February. Such is progress. The
> Viewranger users are furious because people have paid a lot of money in
> the past for the right to use the OS maps and expected it to be forever
> more, not disabled at the whim of the (new) owner of Viewranger.

Oh, crumbs.

Thanks for the info, I have Viewranger installed but used it little. I
was considering registering one day.

It says it is called now Outdooractive, and wants me to update or change
app.

>
> OK, so there's Google Maps, but those maps are just toys and don't
> contain all the geographical symbols that walkers use for navigation.

--
Cheers, Carlos.

On Tue, 25 Jan 2022 21:04:59 -0000, NY wrote:

> Yes, apps such as GPS Status can quote a horizontal degree of precision
> which is the diameter of the circle within which your phone is known to lie.
> The smaller the circle, the more accurately your position is known. I've
> seen diameters of as little as 3 metres quoted, though usually it's 5-20
> metres.

With respect to showing your position on a topographic map, usually the map
itself gives you additional clues of any minor inaccuracy of a few meters.
> When I record a track, I sometimes see a *systematic* (ie not random) error
> in position, relative to a background map. The track follows the curves in
> the map perfectly but it offset from it by a small constant distance. Then
> suddenly the track snaps back onto the line of the road. I've always what
> can cause a systematic offset in x or y position that lasts maybe 1-2
> minutes before righting itself.

Speaking of the "curves in a map", I've found in my area a marked
simplification of the topographic curves of the OSM maps compared to that of
the USGS 1:24,000 maps which are freely available for the entire country.

For other countries, you may be stuck with OSM topo maps, but in the USA, I
can't fathom any reason to use them given the free USGS maps are far better.
> We use an app called Viewranger which can record a track and can display it
> and your current position on a background map. We've bought all the Ordnance
> Survey maps of the UK at 1:25000 and 1:50000 scale. Those are available
> offline because they are stored in the phone. With internet you can also
> plot on Open Streetmap anywhere in the world at a much larger scale which
> shows and names roads in a town and shows buildings.

For others who want to save tracks and show their position on any
topographic geoPDF map that they download (or create) of any hiking area,
these two free apps work the best based on my tests run last month.
<https://play.google.com/store/apps/details?id=com.Avenza>
<https://play.google.com/store/apps/details?id=ca.abbro.androidmap>

You can take any park map that is available online for example, and if it's
a geoPDF, it works exactly as you'd expect it to for saving tracks &
plotting them and showing your position on the map relative to the route.

> Sadly Viewranger was bought out by another company who have their own (much
> inferior) application which requires online access for OS maps, and which
> cannot use OSM for greater detail. And the new company will actually
> forcibly disable Viewranger in February. Such is progress. The Viewranger
> users are furious because people have paid a lot of money in the past for
> the right to use the OS maps and expected it to be forever more, not
> disabled at the whim of the (new) owner of Viewranger.

I just looked on my phone for what related apps I use for off road hiking.
GPS Waypoint Finder === points to the next waypoint
GitHub Trail Sense === handy tool for backtrack, weather, sunrise, etc.
GitHub GPS CoPilot === handy tool for basic backcountry calculations
Azimuth Compass === nothing but a big fat very simple to read compass
SatStat === all around GPS radio debugging tool suite with maps to towers
Heading Calculator === calculation tool for angles, distances & elevation
Avenza Maps === Load any custom geoPDF maps for offroad navigation
Paper Maps === Load any custom geoPDF maps for offroad navigation
Custom Maps === Load any custom geoPDF maps for offroad navigation
Offline Maps === free USGS topo maps with routing, tracks & position
US Topo Maps === free USGS topo maps with routing, tracks & position
LocusMap & BRouter === free topo maps with routing, tracks & position
OSMAnd~ === free OSM topo maps with routing, tracks & position
GPStoSMS === emergency text of your location to an emergency group
Stellarium === I'm still testing which is the best offline start chart
SkyMap === I'm still testing which is the best offline start chart
Star Chart === I'm still testing which is the best offline start chart
Magic Earth === free world map which is useful when discussing countries

Note that _everything_ in that list should be free ad free google free & gsf
free and they should all work offline (once properly set up) and they all
should work with your external sdcard so you can easily populate other
devices.
> OK, so there's Google Maps, but those maps are just toys and don't contain
> all the geographical symbols that walkers use for navigation.

The huge flaw in the Google topo maps is that they don't get down to the
"ground level" when you need them to do that. The satellite maps are nice
though as not many outfits provide satellite geoPDF maps for free.

On Mon, 24 Jan 2022 22:21:39 +0100, R.Wieser wrote:

> Your GPS device does all the calculations to determine its own
> position.

To that point, Rudy is correct.

And, to that point, there's even _more_ that the phone does by way of
GPS-related calculations _on_ the phone itself (all by its itty bitty self).

I just looked at my backcountry folder to see what GPS calculations the apps
do, and it turns out it's more (much more) than just calculating position.
GPS Waypoint Finder === points to the next waypoint
GitHub Trail Sense === handy tool for backtrack, weather, sunrise, etc.
GitHub GPS CoPilot === handy tool for basic backcountry calculations
Azimuth Compass === nothing but a big fat very simple to read compass
SatStat === all around GPS radio debugging tool suite with maps to towers
Heading Calculator === calculation tool for angles, distances & elevation
Avenza Maps === Load any custom geoPDF maps for offroad navigation
Paper Maps === Load any custom geoPDF maps for offroad navigation
Custom Maps === Load any custom geoPDF maps for offroad navigation
Offline Maps === free USGS topo maps with routing, tracks & position
US Topo Maps === free USGS topo maps with routing, tracks & position
LocusMap & BRouter === free topo maps with routing, tracks & position
OSMAnd~ === free OSM topo maps with routing, tracks & position
GPStoSMS === emergency text of your location to an emergency group
Stellarium === I'm still testing which is the best offline start chart
SkyMap === I'm still testing which is the best offline start chart
Star Chart === I'm still testing which is the best offline start chart
Magic Earth === free world map which is useful when discussing countries

Note that _everything_ in that list should be free ad free google free & gsf
free and they should all work offline (once properly set up) and they all
should work with your external sdcard so you can easily populate other
devices.
--
The huge flaw in the Google topo maps is that they don't get down to the
"ground level" when you need them to do that. The satellite maps are nice
though as not many outfits provide satellite geoPDF maps for free.

NY <me@privacy.invalid> wrote:
> "Rob" <nomail@example.com> wrote in message
> news:slrnsuvhne.5gu.nomail@xs9.xs4all.nl...
>> Andy Burnelli <spam@nospam.com> wrote:
>>> On Mon, 24 Jan 2022 22:46:53 +0100, Piet wrote:
>>>
>>>>> This corresponds to an accuracy of 30 meters.
>>>>
>>>> https://www.gps.gov/systems/gps/performance/accuracy/
>>>
>>> From that reference:
>>> "GPS-enabled smartphones are typically accurate to within a
>>> 4.9 m (16 ft.) radius under open sky"
>>
>> Yes, I meant to mention only a ballpark figure. Many GPS receivers
>> use a 10 MHz clock for the time which implies a 100ns resolution,
>> and signals travel 30m in that time. But the calculation of the
>> position is more involved and it can be a bit better.
>
> Yes, apps such as GPS Status can quote a horizontal degree of precision
> which is the diameter of the circle within which your phone is known to lie.
> The smaller the circle, the more accurately your position is known. I've
> seen diameters of as little as 3 metres quoted, though usually it's 5-20
> metres.
>
> When I record a track, I sometimes see a *systematic* (ie not random) error
> in position, relative to a background map. The track follows the curves in
> the map perfectly but it offset from it by a small constant distance. Then
> suddenly the track snaps back onto the line of the road. I've always what
> can cause a systematic offset in x or y position that lasts maybe 1-2
> minutes before righting itself.

What you mention there is accuracy relative to a known position on a
map. That is a whole different can of worms because the actual grid
lines on a map (longitude and latitude) are often not the mathematically
calculated values, but conform to some locally legislated standard.
There are at least 100 of those around the world. The app has to
re-calculate the actual GPS position to that "map datum" as it is called.

When you additionally have "snap to road" function in the app, of course
that is fake accuracy. When the GPS yiels a position to 3m accuracy
that is actually quite good, but of course it is not enough to show
you exactly on the road. So when you drive systematically 3m alongside
a road, the app considers that "driving on the road", and adjusts
the calculated position to one that is on the road. That is not
GPS accuracy, that is fake ("snap to grid"). The GPS is not that
accurate, the 5-20 meter figure is right.

"Carlos E.R." <robin_listas@es.invalid> wrote in message
news:0066ci-nvi.ln1@Telcontar.valinor...
>> Sadly Viewranger was bought out by another company who have their own
>> (much inferior) application which requires online access for OS maps, and
>> which cannot use OSM for greater detail. And the new company will
>> actually forcibly disable Viewranger in February. Such is progress. The
>> Viewranger users are furious because people have paid a lot of money in
>> the past for the right to use the OS maps and expected it to be forever
>> more, not disabled at the whim of the (new) owner of Viewranger.
>
> Oh, crumbs.
>
> Thanks for the info, I have Viewranger installed but used it little. I was
> considering registering one day.
>
> It says it is called now Outdooractive, and wants me to update or change
> app.

Yes, I first had the introductory nagging message yesterday. I knew the
change was coming because my wife (whose email was used to create the
Viewranger account many years ago, before we even had Android phones) had
emails a few months ago saying that OutdoorActive had taken over Viewranger,
and later to say that they planned to close down Viewranger by forcibly
disabling it on devices where it was installed. We've decided to keep one
iPad tablet with auto-updating disabled in an attempt to stop this
happening. We are assessing other options. The general consensus on
Viewranger forums is that Outdoor Active is not yet good enough to be
released - it crashes too easily and omits important features that
Viewranger has. There are other similar programs (I'm not sure what: my wife
has done most of the research) which at least allow tracks to be overlaid on
Ordnance Survey maps, but the best one has a licensing restriction that an
up-front cost for the maps only allows you to access them online; if you
also want offline access (for when you're out of range of wifi or mobile
internet) you need to keep paying a subscription rather than a single
up-front cost. But then that's the way that software licensing
seems to be going :-(

If I have any further info, I'll post to this thread or at least to a new
thread in this group.

On Wed, 26 Jan 2022 10:45:42 +0100, Rob wrote:

> when you drive systematically 3m alongside
> a road, the app considers that "driving on the road", and adjusts
> the calculated position to one that is on the road.

Do we know for certain that this "snap to track" is truly the case with any
of the vehicular mapping apps?

I'm not saying it's not the case, as it sure _seems_ to be the case based on
what I see with Google Maps, MapFactor Navigator, and OSMAnd~ road maps.

But do we know for sure that snap-to-track is indeed what they're doing?