Why does my GPS do this?

[Psamathe]

Hi

Worst case accuracy of the GPS signal is 7.8m, but this is in space. Atmospheric and other effects at bike level change this.
Military systems make an ionospheric correction for greater accuracy.

@OP crosswind, maybe?

Regards
tim-b

I have a gps which runs more or less full time on my boat. It's a quality garmin unit with a garmin external antenna. It's accurate as you say most of the time but every now and then will give fixes up to 1/4 mile away. I know because it turns up in the record. That's why sailors consider gps an aid to navigation rather than a report of their position. Fit your bike with a depth sounder and radar while using the latest charts for optimal accuracy.

Some years ago we were off Brighton (making our way back to the Hamble) and our Furuno GPS with external antenna put us down in the middle of France!! Stayed like that for a few hours and then put us back in the Channel in our correct location. We switched the thing off/reset it but it was convinced we were in central France. Might have been a GPS "glitch" or maybe a problem with the system. Not dangerous (for us) because, as you say, it is an "aid to navigation".

Ian

[nez]

Gross differences are no problem. Not knowing if the Owers are north or south could be an issue.

[Mick F]

Mick F, what map are you currently using in your Montana and do you use in the car? I remember reading you sold the Garmin Discovery map because it became fussy as you zoomed in.

At present, generally the Garmin City Navigator, but I do have a "copy" of GB Discoverer. (Don't ask! )

For use in the car, there's a Nuvi system installed in Monty.

[Godlykepower]

I had another issue with the damn thing today.

I plugged it into a laptop to plot a route, Garmin Express automatically opened and tells me there is an update available, some 'time zone' thing. I update it, it takes about 30 seconds and off I go with plotting a route in Garmin Connect.

I've been looking forward to today's ride all week and this lovely weather is a nice bonus, so I did a nice 50 mile round trip to Saffron Walden and back, taking in several villages in the loop.

After an aborted start with rectifying a loose bolt on my mudguards (which I ended up having to remove the front - don't ask) I finally get off.

I fire up the saved routes options and load the route I'd created earlier. The unit says "loading file" then "done" once complete. I hit "ride" to start and it says "route calculation error."

Try as I might, I cannot get the bloody route to load, so back inside I go. Delete the route, recreate new route. Same story.
Several tries, always same result - route calculation error.

All of a sudden it does decide to calculate a shorter route I created! I finally set off a couple of hours later on this new, shorter route, but all the directions are missing. It's not telling me to "turn right on Green Street" anymore, but to "turn right and head South" and when I do that it says "travelling South" not "travelling on B1039" like it should. The map looks all cockeyed, so I go home, knowing that it would bother me the rest of the ride if I left it as it was.

Upon arriving home, it turns out that during the update I did earlier, it had somehow managed to delete all the map options from the unit except the Basemap.

I had to re-download the whole lot which took 3 hours!

Works fine now and everything is as it should be on a short created route and test ride round the block, but I'm thinking that this unit is more trouble than its worth.

[cycloret]

Just struck me, I can see the track from today to show you what I mean.

174.jpg

Is this the sort of distance you are off?

Please confirm Mick, is your Montana's screenshot here of the Garmin's City Navigator Map, Discovery Map or Open Street Map you downloaded from the site you posted about elsewhere?

[Mick F]

The shot is of GB Discoverer about three miles from here.

[Mick F]

PS:
Gunnislake Newbridge Cornwall/Devon border.

I have all three maps in my Montana.