Distance discrepancies between mapping/planning software

[Ningishzidda]

The distance displayed in ‘Dist to Dest’ is what the mapping knows from far more accurate measurement methods than a handheld or cycle Garmin.
The resultant distance displayed on the Garmin screen and on the Garmin’s recording will be a distance built up from a ‘corner cutting’ and ‘zig-zagging’ line of points where the unit thought it was.

The DIY designer might have a ‘total distance’ of 202 km on the plan, BUT what the Garmin records might be anything from 190 to 214 ISH???

Its pot luck if the recorded distance is OVER the 200 required distance.

The graciousness of the DIY Org 'should' accept a -5% distance seeing the ‘general route’ has been followed and any underdistance result is fault of the recording device and not intentional.

Enquire with your DIY Org which mapping package you should use.

[frankly frankie]

... BUT what the Garmin records might be anything from 190 to 214 ISH???

I'd be surprised if the variance was anything like as much as that.  And opinions differ as to whether 'under' is more likely than 'over'.  I think 'over' is the more likely error, I think there is more distance to be gained from the drunkards walk effect (and from dithering whilst stationary at controls) than there is distance to be lost by corner-cutting.

[Ningishzidda]

I would agree that 'over' would be more likely along the flat, open straight roads of North Cheshire. 

[bikenrrd]

Read this paper for the real answer:

http://faculty.washington.edu/joelzy/howLongIsTheCoastOfBritain.pdf

[fuaran]

In my experience, the recorded GPS track is usually a couple of percent under distance, at least for 'auto' or 'smart' recording.
If you record 1 point per second, it will be a lot closer (for an fairly modern, reliable GPS device).

I don't think dithering while stationary adds much, and that can be filtered out afterwards.