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Map Accuracy and WGS84 Datum Shift
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MrT
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PostPosted: Sun Jan 04, 2004 8:26 pm    Post subject: Map Accuracy and WGS84 Datum Shift Reply with quote

Memory Map Navigator 2004 says that a discrepancy between the GPS co-ordinate and the map co-ordinate for the same place is likely to be caused by the GPS unit not being set to WGS84. I am suffering from a mismatch between the GPS output and the map by 30metres North and 20metres East in the London area.

I have a TomTom BT GPS and I cannot find any documentation referring to WGS84 for this device. I assume that this NMEA device works with WGS84?

If I go into other maps such as MS Autoroute 2004, TomTom Navigator2, and Multimap on the Web or if I take the known co-ordinates of any known point in London, these match with each other, but do not exactly match with the same co-ordinates on Memory Map. When I have been on a long trip of over 200 miles using the TomTom GPS logging feature (captures raw NMEA data) and later turn the log into a pushpin overlay on Autoroute, it exactly matches the roads and turns I have made (within the margin of GPS error).

If I edit the map properties in Memory Map and put in a WGS84 Datum Shift of 30m North and 20m East, Memory Map seems to match up with the other maps and my GPS. This shift is not unique to one map as I am on the boundary of Regions 2 & 4 and it occurs on both the 2003 & 2004 maps.

With so many other maps agreeing, the discrepancy seems to lie within Memory Map and it appears to be easily fixed by editing the datum shift, but I would like to know why this fix is necessary or if I am fixing something that is a result of me not setting up or using the product correctly?
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matt_e
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PostPosted: Sun Jan 04, 2004 11:01 pm    Post subject: Reply with quote

Hello Mr T. I posted something along these lines a number of months ago. I had a similar discrepency on maps around the Hampstead Heath (NW3) area of London. I never got to the bottom of it and it remains a mystery as to why my Memory Map OS maps are 'out' in my local area.

Matt
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DavidW
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PostPosted: Mon Jan 05, 2004 10:59 pm    Post subject: Reply with quote

Though I have never seen developer level documentation on the Navigation Library that's loaded in many devices using the most recent SiRF firmware builds (from, I believe 2.30 onwards), I don't think there's any way to configure a SiRF based GPS consistently to return anything other than WGS84 position information in NMEA mode.

The same is not true of other devices - for example Garmin GPSes send NMEA data on whatever datum they're configured to, and I'm pretty sure I'm right in remembering that the datum used by an Evermore chipset device (such as is used in Haicom GPSes where the model ends with an E) is also configurable in NMEA mode.


I'm 99.9% sure I'm correct in saying that the TomTom Bluetooth GPS is SiRF based. I don't think this is a datum problem with your GPS receiver.


It is worth remembering that you shouldn't expect metre perfect accuracy from GPS. I can't remember the guaranteed performance of the Standard Positioning Service (barring the kind of fault mentioned elsewhere in the forums with PRN23 sticking out duff data for a while on New Year's Day), but typical performance is much better than the specification anyway. An error of 20 metres in the position your GPS is giving is not unusual, however.

What is unusual is that applying a constant correctly seems to give rather better results! I don't have a copy of Memory Map - but I'd be inclined to contact Memory Map's support directly, giving full details of your setup (particularly version numbers of the Memory Map software and, if they have separate version numbers, the map data) and see what their views are.


One thing you could do is convert your raw WGS84 position (which you can find with software like Winfast Navigator - if you've got software that does position averaging like one program mentioned recently in the forums for geocaching and similar use, use that instead) to the OSGM02 geoid using the convertor at http://www.gps.gov.uk. You will need to register (which is free); once you've done that you can either do an online conversion or download some conversion software.

There's more information about Ordnance Survey datums and geoids on that site, but unless you're after survey type accuracy (which your GPS receiver isn't actually capable of), the OSGM02 position given should match up with an OS map. Similarly, the ETRS89 position you're asked for is, for your purposes, the same as the WGS84 position from your GPS (WGS84 keeps moving relative to the earth's surface due to the movement of tectonic plates - ETRS89 is essentially WGS84 frozen at a particular time).

Does your conversion agree with Memory Map's at locations in your area? If not, the problem is that down at the ten metre level, some of the assumptions I've made (about WGS84 and ETRS89 being directly equivalent for your purposes) may actually be breaking down - but the biggest source of error is still almost certainly the inaccuracy of the Standard Positioning Service that your GPS receiver is using.



David
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MrT
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PostPosted: Tue Jan 06, 2004 8:47 am    Post subject: More Reply with quote

Thanks for that reply, I will take a look at the site mentioned. I have asked Memory Map for help, but they usually take a few days to respond. I am using the lastest version of software and the 2004 maps. The same error occurs on last years software and map although I did not notice it until recently when I added the GPS unit.

I understand how accurate the GPS system I use is and if you looked at a map with the GPS track on it you would except a certain amount of scatter from the route. However on Memory Map, the recorded track appears consistently as shifted (always by the same amount) on the screen (PDA & PC) to the left and down. The same route data plotted on AutoRoute follows the road.

The GPS track also follows the road on TomTom2, but I believe TomTom automatically adjusts your position to put you on the nearest road as occaisionally it has put me on a parallel road.

Now 30m does not sound much out, but if you in navigation mode and arriving at a waypoint on a path with a turn to the right, Memory Map thinks that you are at a point 30m to the right of the waypoint. Instead of telling you to turn right (90 degrees) on the correct path to the next waypoint it is telling you to take a path to the left (270 degrees) to get to the waypoint you are at.

Using the datam shift overcomes the problem so I can use the software as intended and it is good, but when something is not how I expect, I like to try and work out why and fix it.
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matt_e
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PostPosted: Tue Jan 06, 2004 9:51 am    Post subject: Reply with quote

I shall also follow up on the information in your detailed response David thank you. Mr T has jogged my memory now of the problem I was experiencing. It was in this post:
http://www.pocketgpsworld.com/modules.php?name=Forums&file=viewtopic&t=3241&highlight=&sid=2928ba181a94fa3d0ff403f981115b31
I did make a gif of the discrepency but seem to have lost it. I would use a track log from a car journey. It would follow the road precisely until I got within an area of North London at which point it would be a set distance out (30m NE or so sounds about right). When I viewed the track on an aerial photograph it was accurate but not on the OS map.

To check this I droppped a number of waypoints (mark) in places locally. Again there was a consistent discrepency between what the aerial photo showed and what the OS50 memory map showed. I did the same thing on OS50 maps in other areas I own and they coincided. So I had to conclude that there is a discrepency on the OS50 memory map for my local area.

Matt
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MrT
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PostPosted: Tue Jan 06, 2004 11:40 am    Post subject: London Reply with quote

I have put some old GPS logs onto my map and you are spot on, it is a North London error. I have a trip to Norwich with over 10,000 GPS points and apart from a 2004 map errors (such as the A11 is Dual Carriageway at Snetterton, but it is shown as Dual on the 250K:1 map) the only deviations are in North London so if I set my Datum on region 4, to be correct in North London, it is out at Norwich. Maybe I will set my datum on Region2 map and leave 4 as standard.
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matt_e
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PostPosted: Tue Jan 06, 2004 12:26 pm    Post subject: Reply with quote

fabulous. it feels great i can share my problem Wink

Question is what to do? Are there discrepencies anywhere else in the country? I don't really understand how this can happen as I believe the edges of my map are accurate it just the bit in the middle (N.London) that's not. Is it worth trying to gather details from others before sending off to Memory Map?
Matt
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MrT
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PostPosted: Tue Jan 06, 2004 2:31 pm    Post subject: North London Reply with quote

I have been trying some experimentation on deviation (purely geographic)and plotting routes.

Clearly on the map the grid references are perfectly linear on the screen. If you plot a straight route keeping to the same latitude or longitude the route is perfectly straight on the screen. Oddly enough I thought at the Greenwich Meridian the grid ran vertically along the Meridian. It doesn't, a path due North keeping to Zero Longitude leans to the left. Must be something to do with magentic north or the map projections used.

Anyway it seems that the image of certain places are slightly out of position compared to where they are expected to be.

Trying various places around London, the maximum "out of placeness" compared to AutoRoute (not very scientific) appears to be centred around Archway at nearly 50m and by 10km away the map is accurate. I could not find anywhere outside the M25 that does not match exactly to MS Autoroute maps.

Anyway I have reached the limits of my geographic knowledge so I might as well start a conspiracy theory that there is a secret establishment located at
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matt_e
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PostPosted: Tue Jan 06, 2004 5:04 pm    Post subject: Reply with quote

I have lost my old tracklog to show this. However here are my waypoint examples. The quality of the jpgs is not good but I think the discrepency can be seen.

In the first one there is a waypoint in the road shaped like a loop right at the bottom of the loop. Also there is a waypoint at the uppermost and left of the road that goes into 'Parliament Hill'.



When you switch to the photo you see the position shifts some way to the North East - consistently for both waypoints.



Having taken my GPS out I know the photo is accurate and the OS map is not accurate.

I hope I am allowed to show these small and low res snippets in the forum without breaching copyright law.

Matt
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MrT
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PostPosted: Thu Jan 08, 2004 8:33 am    Post subject: London Reply with quote

Interesting as well that the Memory map A-Z London is also spot on.

Also it makes me wonder when you upgrade to the 2004 OS maps then see on them roads marked as "Due to Open in 2002".
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matt_e
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PostPosted: Thu Jan 08, 2004 5:09 pm    Post subject: Reply with quote

This can turn out to be an obsession Wink

In case anyone wants to try on their own maps the marks for the above are TQ 28105 86415 and TQ 27415 85935. I have measured the discrepency as 55m. In actual fact as the photo is I believe accurate I suppose the discrepency is in a SSW direction.

I have tried on a number of different maps now and there are some acceptable discrepencies (mainly 20m or even up to 30m). I accept that this is not a precise science but 55m seems a bit much really.

Anyway I've now emailed Memory Map to see what they think.

Matt
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MrT
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PostPosted: Sat Jan 10, 2004 8:01 pm    Post subject: Very Odd Reply with quote

Today I was driving round Bushey and I tracked my path round many streets. Looking at my path on the OS 2004 region 2, the track follows the roads but is to one side or the other at places, at other places is on the road. Not a constant shift but varying which one would assume is GPS inaccuracy.

Change the OS map for Memory Map Greater London A-Z and the track is bang on for the whole trip, even on the correct side of the road (left) for the wider roads.

This points out to me that there is nothing wrong with the Memory Map software, but the OS maps are not accurate within 25meters.
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matt_e
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PostPosted: Sat Jan 10, 2004 8:32 pm    Post subject: Reply with quote

I certainly don't have the knowledge to know whether you are right or not although I would be very interested in the answer.

I had a slightly esoteric discussion (disagreement) with someone some time ago because they said that OS maps were a scale copy of reality. My view was that they are actually an interpretation of what is there and use symbols and some non-scale components (e.g. road widths) to empahasise things that are important to humans to navigate by eye (and compass).

Maybe they are good enough (optimally designed) for that purpose but don't work so well for GPS 'position on map' accuracy. Certainly in my very limited experience of GPS/PDA topographical map navigation, I have not relied entirely on the 'on map' location pointer but have used it in conjunction with identifying surrounding objects on the map like churches, streams, bridges etc.

Maybe on a OS50 map you are right that the absolute position can't be relied on . Maybe it can on the OS25? Strange that the A-Z is so accurate.

Maybe I am spouting rubbish. If anyone knowledgeable on this subject can enlighten me (us) plesae do so.

Matt Confused
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matt_e
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PostPosted: Thu Feb 19, 2004 8:23 pm    Post subject: Reply with quote

Mr T - did you ever get a response from Memory Map?

I haven't heard a thing since they said it was going to the support team!

Matt
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MrT
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PostPosted: Mon Feb 23, 2004 8:10 pm    Post subject: Support Reply with quote

My query on this was forwarded to the development team. So both the development and support teams must be working on it!
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