GPS Problems

Nige composed the following ...

It's quite a while since I had a new one, I don't have any at all now, but I seem to remember having to synchronise (or some other technical term) the map to the signal. Ours was a company sourced Autoroute map and (I think) Garmin based GPS receiver, all done on laptop .. so may be well out of date.

Garmin site has loads of manuals etc to download ...

earthquake caused some problems

I can't comment on the latest ones with pre-installed maps but if you are relying on the OS grid reference as a means of determining accuracy, check your datum defaults.

I can't remember by how much but the default(?) WGS84 datum gives a considerable error.

Assuming UK use, you want Ordnance Survey GB as the Map Datum, British Grid as Position Format. Look in the Navigation sub-menu of the main menu.

David

So Dougal was, like

Yup, had same problem when I got my Garmin a few years ago. Out by about ¾ mile compared to an OS map. The units are all set to WGS84 as the default datum. Change that to something like OSGB and it'll be spot on.

That's correct but the latest marine charts use WGS84 so if your GPS is primarily intended for marine use it'll be pre-set to that. Change the datum as suggested and it'll be fine. TonyB

"Nige" wrote

know the earthquake caused some problems

What GPSr is it?

The earthquake has changed a few earth parameters, but with a commercial GPSr, you probably wont notice it.

If it is not one of the newer Garmin GPSrs like the 2610, which can't change the datum from WGS84 (default) to OSGB36, then this could be set incorrectly.

If you have changed the unit of display from Lat/Long to BNG, then you also have to change the datum on the older models. Some newer ones have these two options merged into one, which would only cause you problems if using an OS sea going map and OSGB36/Lat/Long.

Restricting the GPSr from 'seeing' the satellites can also reduce the accuracy - like having a heated windscreen.

But an error of 2 miles indicates a major problem as the causes mentioned above will only give error of up to about 300 yards. Setup errors will not give errors of 2 miles.

Reg.

Also it wasn't clear if the OP was using his GPS in conjunction with a piece of route-finding software, if so then he'd need to set it to use one of the WGS datums. When I am using my ETrex with OS maps I switch to OSGB datum, when using it with my PDA-based route finding software I switch to WGS72 or sommat.

Have a look on

useful information there, especially the following link and sub-pages;

So TonyB was, like

That was what I said, wasn't it?

So TonyB was, like

:-)

I, "Reg" wrote

Setup

Sorry Nige, I've just thought of the possible cause of large errors. If you are displaying Lat/Long, you are probably using the wrong format.

There are a few ways of giving a Lat or Long.

52 degrees, 30 minutes, 15 seconds, or 52 degrees, 30.25 minutes - i.e. a decimal

I guess that this could give the errors that you are seeing. As I use BNG, I'm not aware of the magnitude of error caused by getting the format wrong.

Reg.

As I acknowledged. I was just trying to explain why they are not always set for land use. So there. TonyB

My Garmin Emap is set to WGS84 and has not shown any positional errors in 3 years. I can plot my back garden on an OS25 map to within 2 metres, when using an external aerial.I would suspect the integrity of the loaded maps if there is much of a difference between WGS84 and OSGB.

Mark

2002 KLR650 C7, 1989 KMX200, 1981 PE175 X, now sold,but without memories 1971 Range Rover ( mostly in bits! ) remove_nojunk to reply off group