Discussion
Well maybe not an error, just that GPS based sat navs appears to have decided my house has moved.
Over the years several cars with their various sat navs have allowed me to define where my home is and then have been able to navigate to within a couple of metres of my drive. I should point out I can find my own house, but the sat nav is useful in allowing me to know my ETA.
Since the start of 2014 my sat nav thinks my home is about 150 metres from where I live. And it's not just me; delivery drivers, who previously would come straight to my front door, are getting misdirected too.
I understand the general way GPS works, but am surprised that such a large error has appeared; I thought the error for civilian systems was only as few metres. Or is there something afoot in the US department of defense?
Over the years several cars with their various sat navs have allowed me to define where my home is and then have been able to navigate to within a couple of metres of my drive. I should point out I can find my own house, but the sat nav is useful in allowing me to know my ETA.
Since the start of 2014 my sat nav thinks my home is about 150 metres from where I live. And it's not just me; delivery drivers, who previously would come straight to my front door, are getting misdirected too.
I understand the general way GPS works, but am surprised that such a large error has appeared; I thought the error for civilian systems was only as few metres. Or is there something afoot in the US department of defense?
What you maybe seeing is the accuracy of the location of the roads change overtime. I have both Sat nav and hand held GPS and noticed that the latitude and Longitude of the Sat Nav doesn't agree with the GPS when set on World Geodetic System 1984 (current GPS main datum) or Ordnance Survey 1936 datum (what all UK maps uses).
As long as the position of the Sat Nav agrees with the position with in the road network and where north is it doesn't matter.
With good access to the sky (not middle of tall bunch of buildings) most modern GPS should be accurate to 10-20m after 2mins of satellite data reading.
As long as the position of the Sat Nav agrees with the position with in the road network and where north is it doesn't matter.
With good access to the sky (not middle of tall bunch of buildings) most modern GPS should be accurate to 10-20m after 2mins of satellite data reading.
Part of my job is using software that tracks people's phones via the GPS in the handset. (Its a business, duty of care thing rather than stalking stuff I hasten to add!)
It can be quite interesting to see how the GPS data is displayed on our system, as it uses both a GPS fix to display a handset to within a few metres, or if there's poor GPS reception it will triangulate the position via the phone network GPRS, which is very inaccurate but gives an 'area' the handset could be in.
There have been times when a handset is amazingly travelling at a few hundred miles an hour, or they are miles out to sea despite being tucked up in bed. There's also an error which can occur which shows the handset somewhere off the coast of Ghana, but I think this is related to a kind of default point, I think that area is the origin point of all the Lat/Longs perhaps as its on the equator.
Strong solar flares can provide dodgy readings as well.
It can be quite interesting to see how the GPS data is displayed on our system, as it uses both a GPS fix to display a handset to within a few metres, or if there's poor GPS reception it will triangulate the position via the phone network GPRS, which is very inaccurate but gives an 'area' the handset could be in.
There have been times when a handset is amazingly travelling at a few hundred miles an hour, or they are miles out to sea despite being tucked up in bed. There's also an error which can occur which shows the handset somewhere off the coast of Ghana, but I think this is related to a kind of default point, I think that area is the origin point of all the Lat/Longs perhaps as its on the equator.
Strong solar flares can provide dodgy readings as well.
Dr Jekyll said:
Slightly OT here. But can I assume the authorities have some kind of hand held gadget that helps them follow suspects via their mobile phone? Essentially using the phone as a homing beacon.
I've always assumed they do but never seen a reference to it.
Hmm. If they told you, they'd have to...I've always assumed they do but never seen a reference to it.
Dr Jekyll said:
Slightly OT here. But can I assume the authorities have some kind of hand held gadget that helps them follow suspects via their mobile phone? Essentially using the phone as a homing beacon.
I've always assumed they do but never seen a reference to it.
yes but it's usually intelligence types with access, typically the police use triangulation from networks for run of the mill stuff, they can also send a silent text to the phone which will reply with gps co-ordinates.I've always assumed they do but never seen a reference to it.
Quite a few years ago I found it necessary to learn about GPS as part of my job as we were using it to measure fall-of-shot coordinates in some artillery rocket trials.
Depending on which coordinate mapping system you choose and the Geoid (mathematical model of the Earth's shape) the Eastings and Northings coordinates of a particular location can be 100s of metres different. The US GPS system uses the WGS84 (World Geographical Survey 1984) as its basis with its own Geoid and the UTM (Universal Transverse Mercator grid based system, not Lat/Long) based on it is zoned with the Prime Meridian (central line of longitude for local UTM maps) for the UK goes through 0° Longitude.
The UK's cartographical OSGB system uses a variant of the UTM system but with the Prime Meridian moved 2° West so that it lies down the centre of the country, which improves the overall accuracy of the maps as this degrades as you move away from the central meridian. OSGB also uses a subtly different Geoid (Airey IIRC). All this means you can sometimes see an error between a map and your GPS depending on how you have set it up or its defaults if you haven't.
Ordnance Survey have defined a GPS coordinate system for the UK though which is separate to the cartographic system and is congruent with WGS84 so provided this is being used by the digital map maker you shouldn't see any differences when using a GPS.
The upshot of all thus is that there are a number of reasons why you might see apparent errors in GPS vs map data which can be due to the vagaries of the various systems and models in use. However, for 99.9% of the time all this should be invisible to the user, so I have no idea why your house appears to have moved.
Depending on which coordinate mapping system you choose and the Geoid (mathematical model of the Earth's shape) the Eastings and Northings coordinates of a particular location can be 100s of metres different. The US GPS system uses the WGS84 (World Geographical Survey 1984) as its basis with its own Geoid and the UTM (Universal Transverse Mercator grid based system, not Lat/Long) based on it is zoned with the Prime Meridian (central line of longitude for local UTM maps) for the UK goes through 0° Longitude.
The UK's cartographical OSGB system uses a variant of the UTM system but with the Prime Meridian moved 2° West so that it lies down the centre of the country, which improves the overall accuracy of the maps as this degrades as you move away from the central meridian. OSGB also uses a subtly different Geoid (Airey IIRC). All this means you can sometimes see an error between a map and your GPS depending on how you have set it up or its defaults if you haven't.
Ordnance Survey have defined a GPS coordinate system for the UK though which is separate to the cartographic system and is congruent with WGS84 so provided this is being used by the digital map maker you shouldn't see any differences when using a GPS.
The upshot of all thus is that there are a number of reasons why you might see apparent errors in GPS vs map data which can be due to the vagaries of the various systems and models in use. However, for 99.9% of the time all this should be invisible to the user, so I have no idea why your house appears to have moved.
Datum (which people are talking about) shift between data sets can be within UK about 130m when comparing OS data set and the World Geodetic System 1984 (GPS WGS84 system) but world wide data the shift can be 2km! My Sat Nav data isnt on any known datum that I could work out! Contential drift would be very small off the top of my head European Terrestial Reference Frame is about 3cm / year movement when compared to WGS84.
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