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Which do I believe?


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OK, this has been bugging me for a while now and although not a life or death problem it would be nice to hear some views/explanations.

If I plan a route on my home laptop using Map Source(?) in combination with TOPO Germany it will show me say a distance of 20km. This has been marked out using the pen tool over nothing other than forest tracks. I realise that I'm setting lots of points as opposed to using waypoints at junctions etc but I prefer this option as (a) others can see the planned route on the unit plus I know exactly (or should) the total distance of the route I have penned out and transferred to the unit - Garmin Map60CSx.

 

Now, when I follow that route the unit records a track on top of my route making it easy for me to follow. Upon return home however, and downloading the track to the laptop it shows me a different distance covered - say 21.5 km.

 

Not a huge difference I know but more of a difference than can be explained if I was zigzagging across a motorway for instance which is why I mentioned forest tracks earlier on.

 

So which do I believe the distance planned at home using the application or the track distance recorded by the unit and why the discrepancy. A couple of hundred metres wouldn't cause me to post this question, but 1.5 km although not a marathon is nothing to sneeze at.

 

Would appreciate any views on the matter - thanks.

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OK, this has been bugging me for a while now and although not a life or death problem it would be nice to hear some views/explanations.

If I plan a route on my home laptop using Map Source(?) in combination with TOPO Germany it will show me say a distance of 20km. This has been marked out using the pen tool over nothing other than forest tracks. I realise that I'm setting lots of points as opposed to using waypoints at junctions etc but I prefer this option as (a) others can see the planned route on the unit plus I know exactly (or should) the total distance of the route I have penned out and transferred to the unit - Garmin Map60CSx.

 

Now, when I follow that route the unit records a track on top of my route making it easy for me to follow. Upon return home however, and downloading the track to the laptop it shows me a different distance covered - say 21.5 km.

 

Not a huge difference I know but more of a difference than can be explained if I was zigzagging across a motorway for instance which is why I mentioned forest tracks earlier on.

 

So which do I believe the distance planned at home using the application or the track distance recorded by the unit and why the discrepancy. A couple of hundred metres wouldn't cause me to post this question, but 1.5 km although not a marathon is nothing to sneeze at.

 

Would appreciate any views on the matter - thanks.

 

Just bumping this post as it may have been overlooked. Surely I can't be the only one to have noticed this?

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OK, this has been bugging me for a while now and although not a life or death problem it would be nice to hear some views/explanations.

If I plan a route on my home laptop using Map Source(?) in combination with TOPO Germany it will show me say a distance of 20km. This has been marked out using the pen tool over nothing other than forest tracks. I realise that I'm setting lots of points as opposed to using waypoints at junctions etc but I prefer this option as (a) others can see the planned route on the unit plus I know exactly (or should) the total distance of the route I have penned out and transferred to the unit - Garmin Map60CSx.

 

Now, when I follow that route the unit records a track on top of my route making it easy for me to follow. Upon return home however, and downloading the track to the laptop it shows me a different distance covered - say 21.5 km.

 

Not a huge difference I know but more of a difference than can be explained if I was zigzagging across a motorway for instance which is why I mentioned forest tracks earlier on.

 

So which do I believe the distance planned at home using the application or the track distance recorded by the unit and why the discrepancy. A couple of hundred metres wouldn't cause me to post this question, but 1.5 km although not a marathon is nothing to sneeze at.

 

Would appreciate any views on the matter - thanks.

 

Just bumping this post as it may have been overlooked. Surely I can't be the only one to have noticed this?

 

I haven't tried mapping a route in mapsource and comparing it to a track in the gps. It seems as though you are talking about travels via auto (not on foot)? If so, did you try resetting your trip odometer and seeing what that says compared to the GPS/Mapsource? Just a thought

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Thanks for the reply but no, I am not referring to travel by auto. To briefly recap - using Map Source/TOPO Germany I mark out a route on my laptop very carefully using the pen tool in order that I can accurately mark out the walking route I wish to take in the forest. I could just use waypoints set at junctions I know but prefer the pen tool to mark out a route because that way I can supposedly accurately read the distance of the route plus I can see the routes profile.

 

Once I set off on the pre-planned walking route the GPS unit also simultaneously records the track that I am following. In my case I have set the route to the colour Red and the track is then recorded in Black. By ensuring that the Black recorded track by the unit overlays the Red pre-planned walking route I know that I am following the correct course/route. This is basic navigation surely?

 

Anyway, to repeat my initial question. The pre-planned route created on the laptop and downloaded to the GPS unit shows a distance of 20km. Although the track that the unit records is directly over the pre-planned route i.e. I am on course, back at home the recorded track registers as 20 plus X km's. The question why the difference and which distance is the correct one?

 

Edit - and no I haven't tried resetting the odometer to zero or even made a comparison to it. The odo is one of the things that I most definitely never reset as I wish to easily record the total distance travelled on foot over the lifetime of the unit. I'm trying to beat the record set by Forest Gump :-)

Edited by Alter Nereus
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What interval are you using to record track points?

Are you using the WGS84 datum?

Do you have "lock on roads" set to "on" maybe?

 

Oh, and you can reset the Trip Data while keeping the unit's overall odometer.

 

Auto for track points

Yes WGS84 datum being used

Lock on roads set to off. Don't quite follow the reasoning behind this question. If I was asking the unit to take me from A > B then of course it would lock onto roads if this function was set on. However, I am not using the unit to take me from A > B from within the unit itself, rather I am asking it to follow a previously uploaded route. So surely the unit when turned on to that route and the Start Navigation button activated would follow the uploaded route - the lock onto road function plays no part in this I would have thought.

And yes, I am aware of the fact that I can use Reset to reset everything BUT the odometer - I do this regularly.

Edited by Alter Nereus
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Auto for track points

Yes WGS84 datum being used

Lock on roads set to off. Don't quite follow the reasoning behind this question. If I was asking the unit to take me from A > B then of course it would lock onto roads if this function was set on. However, I am not using the unit to take me from A > B from within the unit itself, rather I am asking it to follow a previously uploaded route. So surely the unit when turned on to that route and the Start Navigation button activated would follow the uploaded route - the lock onto road function plays no part in this I would have thought.

And yes, I am aware of the fact that I can use Reset to reset everything BUT the odometer - I do this regularly.

Sorry, I missed the point about following the route... I can't recall, and my 60 CSx is at home, is there a more-frequent track point interval? That may be where your difference lies- the track is calculating from point to point, and if you're not exactly on the route, it could add up.

 

That's just a guess- maybe someone else can point us in the right direction.

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I would guess that the actual distance is between your penned out route and what the track log recorded. When you made the route unless zoomed in really tight and used a lot of points then you chopped off some curves making the distance shorter. Also unless the terrain is completely flat then your route isn't going to include the extra distance of going up/down hills.

 

The track log will include the extra distance traveled going up/down hills. However if your sat reception was poor in areas because of trees/terrain then you could have erroneous points adding distance you didn't travel. The accuracy of the track log will vary depending on your speed. In general the faster you go the more accurate the track log will be, the slower you go the more impact the accuracy of the gps position will have, for instance if you are standing perfectly still, your position will drift around the spot you are standing and the distance of the track will slowly increase.

 

You will also get a third distance from the trip odometer. The trip odometer is probably more accurate than the track log since it uses doppler shift of the gps signals to measure speed & distance instead of a calculated gps position.

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I would guess that the actual distance is between your penned out route and what the track log recorded. When you made the route unless zoomed in really tight and used a lot of points then you chopped off some curves making the distance shorter. Also unless the terrain is completely flat then your route isn't going to include the extra distance of going up/down hills.

 

The track log will include the extra distance traveled going up/down hills. However if your sat reception was poor in areas because of trees/terrain then you could have erroneous points adding distance you didn't travel. The accuracy of the track log will vary depending on your speed. In general the faster you go the more accurate the track log will be, the slower you go the more impact the accuracy of the gps position will have, for instance if you are standing perfectly still, your position will drift around the spot you are standing and the distance of the track will slowly increase.

 

You will also get a third distance from the trip odometer. The trip odometer is probably more accurate than the track log since it uses doppler shift of the gps signals to measure speed & distance instead of a calculated gps position.

 

Thanks for that - some very interesting points mentioned especially regarding the Doppler shift effect. I'm not altogether convinced by the point you make in your opening paragraph because once the route has been drawn and planned using MapSource/TOPO Germany one is actually able to view a route profile showing terrain height along the route: My own conclusion from this therefore is that the software must also take this into account when calculating the overall distance. Regarding the standing still part - yes I agree fully and for this reason I tend to 'Pause' navigation when taking a break etc.

 

I'm going to try a little experiment this coming weekend by visiting an old disused airfield in my locality. This airfield, and more importantly the runway' appear in MapSource/TOPO Germany and so I shall carefully draw a track keeping tight along one of the runway edges. I shall then walk the same route which will always be a straight line and a steady height of course without pause and compare the distances as shown on the software during the pre-planning, the difference in odometer readings from start to finish and of course from the trip odometer itself. After those have been noted I shall download the completed track to the laptop and note what distance is being shown from the downloaded track. If the differences between the best and worst readings are only meters/yards then I won't think any more about it of course. On the other hand if there is a significant difference between any readings I shall have to rethink the entire thing again.

 

Thanks once again for your constructive observations and comments.

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Right then - update.

The weather is so nice today and with the belief that the old military runway (now used for social purposes) will be much busier over the weekend, I tried my little experiement today.

First of all I mapped out a route on my laptop using MapSource/TOPO Germany. This route consisted of 3 straight lines covering three sides of the runway and when fully mapped out was shown to be 2.3 km in length. Bearing in mind that the same maps are used on both the laptop AND the GPS unit the visual effect apart from zoom is the same.

At the end of the walk the Main Accumulated Odometer indicated I had travelled 2.26 km and the Trip Odometer also indicated 2.26 km. Unusual was that although the route mapped out on the laptop indicated 2.3 km, when looking at the Profile of the route it indicated 2.26 km to coincide with both Odometers. I put this small discrepancy down to a rounding up of figures in the route planning part of the application.

 

Now then, whilst both Odometers indicated a distance covered of 2.26 km, when the active track was uploaded to the laptop after the trip the distance covered was shown as being 2.3 km once again. This would seem to confirm that there is a rounding up/down in the part of the MapSource/TOPO Germany software that logs the routes in the planning stage where the Profile page shows the route down to 2 decimal place.

 

Prompted by someone earlier having mentioned something about curves in the route etc I delved further into this and looked at the MapSource/TOPO Germany mapping software a bit closer. I am no mapping expert and have absolutely no idea how digital maps are created but it would appear that anything representing a curve on a paper map and in the real world is made up of a series of straight lines on a digital map. I looked at a paper map of the same area through a magnifying loop and noticed that all curves,whether on roads or tracks are smoothly illustrated. Zooming in to max (20 I think it was) shows these same curves are made up of a series of very small straight lines so small one doesn't notice them until zoomed close in on the digital map. If this indeed the case then when penning a route using a mouse then each and every curve in a planned route will have a few metres/yards chopped off as a consequence of the straight lines - chopping off the corners so to speak and making the actual curved walked that few metres/yards longer. Is this what they mean by vector maps?

 

Anyway, with a route covering 20 km and comprising hundreds of curves then these extra few metres are going to add up and I believe this is where the discrepancy lies which prompted my initial question. I draw the conclusion therefore, that planning a route that only consists of straight lines A > B > C etc (hardly applicable in the real world) will give an accurate distance to be covered when viewed in any topographical software and both the software and GPS unit will be in tandem. On the other hand, where the route is over a long distance and comprises many non straight line courses (the real world) then the distance to be covered as shown in the software should only be taken as a guidline and that the reading from the GPS will be the more accurate of the two.

 

Another consequence of this of course is the ETA to destination. The unit, via the satellites, is equating one's position in relation to the map and route that one has previously uploaded to the GPS unit. The unit itself has no idea that in reality each and every curve is that few metres/yards longer than what the digital map represents and therefore calculates one's ETA based on the digital map. If any of this is relevant to non topographical maps e.g. road maps I have no idea and it would be interesting to hear from someone who has planned a route and driven it and then compared the GPS's odometer to the cars own odometer.

Edited by Alter Nereus
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