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User Name Thread Name Subject Posted
Rob Naylor Tech: GPS quits when close so I never arrive (52* d) RE: Tech: GPS quits when close so I never arrive 09 Jan 18

BobL: The problem is that GPS has its own co-ordinate system, which doesn't exactly match the one used by maps. This is not helpful when filling stations are shown in the wrong street and you're running on empty. This happens when a place's lat & long were measured from a map rather than read from a satnav.

GPS is associated, in its native mode, with the WGS84 Geodetic Datum, which is the same datum used by Google Maps. Google chose that datum specifically because GPS coordinates are calculated in it when operating "normally".

However, most GNSS systems marketed *as* GNSS systems (GPS is a GNSS - or Global Navigation Satellite System system - of which there are several) are capable of allowing their output to be transformed into local datum AND projection (eg UK National Grid projection on OSGB36 Datum). So in UK, eg, I can easily set up my GPS system to output coordinates that match my OS map.

Satnav on phones will be using Google Maps , which are in WGS84 anyway, so it's unlikely that the filling station will be shown in the "wrong" street due to datums being different.

The vast majority of errors or misdirection seen in satnav systems are not down to the GNSS/ GPS side ( ie the actual satellite positioning systems) but to how the mapping and direction algorithms are handled in the element of the system that the satellite positions are fed into.

Postcodes and Zip codes are one source of ambiguity, as mentioned already by several people above.

Update frequency of the mapping is another. There is a whole area around Dorchester in UK that my VW Nav system, updated in 2017, doesn't recognise, and I know that these roads have been there for at least 5 years, so its not the GPS that's "wrong" but lousy updating of maps by VW.

A third source of problems are route-selection algorithms, which is the hardest part of any satnav system to get right, and to keep updated, as transport systems evolve ( ie a road being designated one way). I've seen large container trucks stuck down tiny rural roads near where I live "because the satnav told me to take this route"....well yes, but, Mr driver, "are you aware that there are specific satnav map sets for freight hauliers, using algorithms which factor in truck size and avoid narrow roads, low bridges etc?" It's just that many haulage companies use standard satnav systems rather than freight haulage systems, because they're a lot cheaper. Until your truck gets stuck under a low railway bridge!

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