Ive read FAQ - Now, just how accurate is GPS, is mine bad?

[jonokimber]

Hello Ladies and Gents, First post on this forum for me, its already been informative, but after reading the FAQ's on accuracy I need a real world answer for my situation. I'm not sure whether what I'm seeing is to be expected but here goes...

With my Bluetooth Fortuna GPSmart connected to my HP iPAQ 2210 in my car with a 3D fix on about 8 satellites running pocket streets my current position is always shown about 20 metres (maybe 30) south. I went for a quick drive and when heading north there seemed to be a lag, but when travelling east or west i appeared to be travelling either above or below the road I was on, hence I dont suffer from random inaccuracy, its very accurate, but offset in the same direction all the time.

Now then, if this is normal, how will a more advanced software handle this? Will i keep getting an 'off course' error, or is there a way to fix it, or is my unit dodgy, or what!!!

I have some actual figures of OS grid values and Lat Long values, also a couple of jpg''s of an OS map, and a screen shot of Autoroute with the Lat Long value showing i live on a different street! If anyone wants to see these i can email them?

Anyway, just interested, will take it for an extended radge tomorrow, might go to an exposed area with a very recognisable junction, and get the OS map online out for a comparison.

Cheers

Jon

[Dave]

Ugh, firstly I wouldn't use Pocket Streets for acuracy, with a 2 second update, you will get lag. An 8 sat fix is very respectible, you can see anything up to 12, but on an average/good day you'll see a 7-9 sat fix.

If you stop, where is your position on the map, is it 20-30m out still ? There's two things that can be causing this, the map datum, or calibration. I'm not sure if the Fortuna BT GPS allows for datum configuration, but if it does, make sure you're using WGS84 and not OSGB or another map datum, as this will throw you out 20m+.

The other thing is calibration, it's quite possible as Pocket Streets really isn't used for navigation much, it's mainly used as a mapping program, that Microsoft do have the co-ordinates out somewhat. I used several other programs, took a screen dump of an autoroute map and used this map in another program, calibrated it as the co-ordinates said on screen in AutoRoute, but this made me about 10m out.

Personally I would ditch Pocket Streets, it's great to be able to carry custom maps on the Pocket PC, but it really isn't that great for GPS navigation. TomTom do have a demo of RoutePlanner or CityMaps I believe still on their website, you might want to give these a try. If you want to use it for full street navigation, then purchasing TomTom Navigator, CoPilot Live or PowerLOC Destinator would be the best choice.

[jonokimber]

I changed the map datum in my GPSmart to WG84. I did this in the Setup | Map datum menu. There were plenty to choose from. I havent tried it yet but i'm confident, as I read of the same problem in another forum (sorry for straying to another forum, i didnt register, honest ).

Anyway, I had the same offset problem with TomTom II as I did with pocket streets, so the map datum better be the problem or i'm stuffed!

[Dave]

You'll be fine, I'd bet my hat on it, that it's a map datum problem. Otherwise I'll eat my hat. On second thoughts, I don't have a hat

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