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Pocket GPS World :: View topic - [QUES] MemoryMap and TomTom MKII Elevation Error...
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[QUES] MemoryMap and TomTom MKII Elevation Error...

 
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ChristopherPovey
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Joined: Apr 29, 2005
Posts: 24

PostPosted: Mon Jul 06, 2009 8:51 pm    Post subject: [QUES] MemoryMap and TomTom MKII Elevation Error... Reply with quote

I have the following:

MemoryMap OS2004
TomTom Wireless GPS MkII
HP iPAQ hx2410

I have had all of these a while, but recently I have started to use this kit to record cycle rides that I do. However I have noticed an issue. The track that is drawn as I cycle along is accurate to the bridal way or road, but the elevation data is way out. Originally I thought it was around +100ft out. On my last ride though I did a dis-used railway line which involved cycling from point A to B then back to A along the same route. Looking at the MemoryMap elevation it is a mirror image about half way along the graph (as I would expect). The GPS elevation is all over the place. If I take three points along the route then outbound elevations are: 270ft, 185ft, and 267ft. The same points inbound are 291ft, 383ft, and 279ft. Giving an difference of 21ft, 198ft, and 12ft. As this is the same return path I would expect the height data to be the same at the same points on the map!

The TomTom MkII is in a side pocket of a handle bar mounted bag. The PDA is inside the bag the otherside of the pocket.

What would cause this? Is it the settings in MemoryMap? I have it set as follows for a track:

Min Update Interval: 5 Seconds.
Min Update Distance: 5m.
Track Partition Interval: 60 min.
Track Interval: 5 min.

Thanks.
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jolly47roger
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Joined: Jan 11, 2008
Posts: 116

PostPosted: Tue Jul 07, 2009 12:42 pm    Post subject: Reply with quote

Memory-map uses its own elevation data - so settings there will have no effect.

GPS units always report an error depending on the quality of the signal - more under trees or in steep valleys (or railway cuttings!). Whereas the latitude and longitude error is, nowadays, only a few metres the altitude error can be an order of magnitude more. My IPAQ with a SIRFIII chip reports large variations in altitude - I use the 'smooth altitude' feature in GPSU to get a more useful report. It may help to position the GPS unit higher where metal handlebars and your body don't 'shade' it - eg behind the saddle.
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ChristopherPovey
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Joined: Apr 29, 2005
Posts: 24

PostPosted: Tue Jul 07, 2009 8:54 pm    Post subject: Reply with quote

Cheers. I'll give that a try. I transferred the track to Google Earth and it is out from the path in some places, mainly where there were trees about.
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