Accuracy of nuvi 855. Some observations.
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  14 years
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I have two maps in my nuvi 855, the garmin North American map and the OSM UK map. Both are stored in the main memory. Since I was playing with the UK map I had turned the NA map off. However, I left the unit in navigation mode, with the "show track" option turned on. Even with the map turned off, the display still showed the shoreline of Lake Ontario and the Hwy 401 expressway running just north of the lake.
My 855 typically has an accuracy of between 6 and 9 metres (20 to 30 feet). It never seemed to matter what accuracy the unit claimed to have at any specific time. I was always on the appropriate road, always close to the streets I was crossing and the directions were always on time. Well, sometimes the timings on turns were not quite as good as I expected, but not enough to bother about.
How things changed, when the map was turned off. Off course I had no turn by turn data, but I could see how my actual track was varying from the historical tracks. I was all over the place 
Typically within the accuracy noted above but sometimes way off, upto a 100 feet. As most of you know, the 855 does not turn completely off unless you remove the battery. My home location was a complete mess of tracks, ranging from my garage to about a 200 diameter circle.
Obviously the 855 uses an algorithm that uses the acquired gps coordinates and compares those with the nearest roads. Some computations are then made to determine where your vehicle should be displayed on the map. This may explain that when I am in my driveway, the display shows my vehicle either in the intersection just north of me, or the road just south of me. (Both about 100 feet away.)
Now for navigation by car, the nuvi 855 algorithm is quite adequate, even though it does have some problems. It puts your vehicle in a (usually) safe position.
However, when walking or boating in limited visibility,
the position on the display cannot be fully trusted. I would certainly not navigate close to shorelines or walk in uneven country relying on the 855 for my safety. Remember, it's really designed for autos, not for boating or other activities.
Yes
...Obviously the 855 uses an algorithm that uses the acquired gps coordinates and compares those with the nearest roads. Some computations are then made to determine where your vehicle should be displayed on the map. This may explain that when I am in my driveway, the display shows my vehicle either in the intersection just north of me, or the road just south of me. (Both about 100 feet away.)...
Yes, nuvis when used with City Navigator and not in off-road mode, will pin your location to a street if nearby. It's great most of the time, but now and then when you're on a major highway with a slight construction lane diversion, you find your location being bounced from being on the highway to being on the adjacent parallel frontage road.
855
...Obviously the 855 uses an algorithm that uses the acquired gps coordinates and compares those with the nearest roads. Some computations are then made to determine where your vehicle should be displayed on the map. This may explain that when I am in my driveway, the display shows my vehicle either in the intersection just north of me, or the road just south of me. (Both about 100 feet away.)...
Yes, nuvis when used with City Navigator and not in off-road mode, will pin your location to a street if nearby. It's great most of the time, but now and then when you're on a major highway with a slight construction lane diversion, you find your location being bounced from being on the highway to being on the adjacent parallel frontage road.
As with any electronic "tool", there are limitations on what the unit can do. One needs to realize the unit is trying to intrepret your intentions based on the limited information it has.
That's why when I'm in unfamiliar territory, I always review street maps before I travel the route.
Always on the Road Knowing where I've Been