How accurate are the 50k maps?

Dullthud · Occasional Visitor Joined: Mar 05, 2011 Posts: 8

Hello all.

I've only recently got my Satmap as I was tempted by Amazon's bundle with full UK 50k maps. I'm steadily getting to grips with it but I was wondering how accurate the maps are in relation to the actual lat/long position of the unit.

Basically, I appear on the map to be half a block to the south west of where I really am. I've only been around East London with the unit so far so I'm not sure if this is consistent over the rest of the country. Is that just the limits of the mapping and am I just being fussy?

Take the example of the post office on Dalston Lane, E8, in London. I found a few local places on Google Earth to test the method of saving points to the unit through Satsync. A truly wonderful function by the way. I was very happy to see them materialise on the map of my unit, but on zooming in I found they weren't quite where I knew them to be. The Google earth marker for Dalston Lane post office is smack bang on the roof. N51 32.968/W0 03.524 is given as it's location. However, the downloaded POI on the map on the unit is shown to be round the corner half way across a major road junction, probably somewhere in the region of 150ft away.

I decided to test the unit by taking a walk down there. I stood outside the post office and made another marker. I think I can assume that the GPS part of the unit is functioning properly. The new marker nearly overlapped the one downloaded from Google earth. I guess it was a few feet different, possibly because I couldn't easily get onto the roof. Anyway, Google was astonishingly accurate. If I wanted to shell the place Google would probably get the job done. But if I used the mapping in my Satmap it would probably remain unharmed.

I have no particular grievance with the post office on Dalston Lane so such action may prove unnecessary but should I expect more from the mapping which would have been rather pricey had I not been gifted it with the Amazon offer? Or is the fact that it's in an urban area a bit too much to ask from a 50k map? Do they take it a bit more seriously if you're stumbling around a rainswept hillside in the dark rather than trying to post a letter?

Any thoughts gladly received.

Dt

Darren · Posted: Mon Mar 07, 2011 10:12 am Post subject:

Sounds like your trying to compare POI data with GPS and map accuracy.

Often, POIs for such things as Post offices etc are based on PostCodes which ARE NOT in any way an accurate marker. They're compiled en-massed and are not accurate geocoded references.

Does the GPS accurately reflect your current location and does it match on the map? If so, then it's working fine.
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Darren Griffin

Dullthud · Occasional Visitor Joined: Mar 05, 2011 Posts: 8

Hello Darren.

Thanks for responding.

No. what I'm seeing is not where I am. If I stand on a street corner the unit doesn't say I'm at that corner. It will say I'm across the road somewhere to the SW. If I move the pointer to where I know I am on the map in the planning screen it says that point is about 100 or so feet away.

Should I be expecting greater accuracy than that or is that all I can expect with 50k maps?

Thanks again.

Dt

Darren · Posted: Mon Mar 07, 2011 12:48 pm Post subject:

Hmm, it goes without saying that 1:50k resolution can never be definitive in terms of accuracy but it should be capable of putting you on the correct corner or at least within 20-30ft of it at most.

Don't forget that the GPS signal itself isn't pinpoint accurate and so taking it's error of +/- 10m or so and any errors in a low resolution map such as 1:50k could add up to 100ft on some rare occasions.

You'd need to test over a number of hours in one location to remove GPS error and also compare the result in a number of places before we could draw any firm conclusions as to if this is a simple case of map not being wholly accurate or if it's something else.
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Darren Griffin

Bosun · Posted: Mon Mar 07, 2011 1:14 pm Post subject:

If you go into "Settings" on thw Active 10 and scroll down to "2 and 4 Data Box Layout" there is an option to display the accuracy of the GPS signal.

It may be as well to have a look at this before you do anything else as the accuracy may only be to 100 feet,so that is a circle of 200 feet diameter.

As you are in an urban area this may be the cause.

Ian

Dullthud · Occasional Visitor Joined: Mar 05, 2011 Posts: 8

Darren and Bosun.

Thank you both. I put the accuracy on the display field and went for a walk. It stayed between 12ft and 19ft but the snail trail was offset SW by about 150ft. The shape was exactly right for the route I took but it just didn't match up on the map. I guess I'll get in touch with Satmap and see what they have to say.

Thanks again.

Dt

JimmyTheHand · Frequent Visitor Joined: Apr 16, 2005 Posts: 386

What display format are you using? There are some differences in the world models used to calculate latitude/longitude - for using a OS 1:50,000/1:25,000 try use OSGB and see if that makes the map match the location
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J.

Bosun · Posted: Tue Mar 08, 2011 11:03 am Post subject:

Good point Jimmy,never thought of that.

Ian

Dullthud · Occasional Visitor Joined: Mar 05, 2011 Posts: 8

Jimmy.

I have it set on decimal degrees. I'll try putting it back onto OSGB and see if that makes any difference.

I saved yesterdays little walk as a kmz which I loaded onto Google Earth and it matched perfectly with where I actually went. Certainly within the stated accuracy of the unit as I was doing it. It was actually quite impressive, so I guess the unit is doing its thing correctly and it's the map that's out of whack.

I send off an e-mail to Satmap as well and I'll let you know what the outcome is.

Thanks all.

Dt

Dullthud · Occasional Visitor Joined: Mar 05, 2011 Posts: 8

Oh well. So much for that idea. I had a little wander about in my lunch hour but it failed to get a gps signal in the fifteen minutes I was out so I gave up.

Maybe I have a lemon.

Dt

JimmyTheHand · Frequent Visitor Joined: Apr 16, 2005 Posts: 386

Dullthud · Occasional Visitor Joined: Mar 05, 2011 Posts: 8

Jimmy.

Thanks. I think you might be right. I was moving about. I did get it to lock on later and I'd changed it to OSGB to see if that helped with the position slip. It took five minutes to get a lock and the position was no better on the OS grid.

When I got home there was a reply from customer services at Satmap and it was a bit depressing. They said the inaccuracy was down to "the cartographical, generalization and simplification of ordinance [sic] survey maps"

I've since been right across London with it and measured the slip on Google earth with loaded kmz files as accurately as I could and it was consistently 190 odd feet out. That is just not good enough and not even a cheapo automotive satnav would get away with that level of innacuracy with its free built in map so I can't see what Satmap are thinking with the money they expect for their OS ones.

I've come to the conclusion it's just an overpriced and essentially rather poor piece of kit and it's going back.

Thanks for all the input though people. I tried.

Dt

davidor · Regular Visitor Joined: Nov 14, 2008 Posts: 117

I think it would be worth ringing and speaking to Satmap and see if they will retune it. I am very impressed by how spot-on my location is on the OS maps. I can usually tell which side of a hedge I am on if walking alongside one. It can drift a bit, but usually within the accuracy range of the gps shift.

It will NOT lock on if moving, but I soon get a lock if I put in othe roof of the car say while I put my boots on. It will also get a lock indoors when I put it on charge, ehich my Garmin would never do.

So try speaking to them, as there must be something wrong if it is tracking 50 metres out.

Bosun · Posted: Wed Mar 09, 2011 11:04 am Post subject:

BigPerk · Posted: Wed Mar 09, 2011 11:44 am Post subject:

I agree as well, from when I had one - once they see there is a real problem (ie it is 190ft out ALL the time) they will act quickly. After all, the Active 10s are used by mountain rescue teams, amongst others - imagine the enormous hazards of that inaccuracy in those conditions! (it just would be TOTALLY unusable.)
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David
(Navigon 70 Live, Nuvi 360)

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