[OSM-newbies] Trust GPS or Yahoo aerial imagery?

[Chris Fleming]

Gervase Markham wrote:
> Richard Fairhurst wrote:
>   
>> To answer the original question, though, trust your GPS over the  
>> imagery. The imagery may not always be rectified, whereas as long as  
>> your GPS has produced a good-quality track - i.e. no "concrete  
>> canyon" distortions - it should always be accurate.
>>     
>
> OK. So if that's the answer (and it's sort of what I expected) then the 
> follow-up question is: do we have any procedure for fixing the 
> rectification, or letting Yahoo know their images are out?
>   
I don't think that there's an option to do this in the flash editor; 
potlatch. However, if you're feeling brave then the java editor JOSM 
will allow the layers to be moved. You will need the wms and the ywms 
plugins.

My experience is that when the signal strength is good, then my tracks 
will be spot on and agree exactly with the yahoo mapping. The GPS 
usually keeps track of a the accuracy in a parameter called hdop, you 
may be able to see this if you look at the gpx tracklog. Values less 
than 10 should be okay, if it's below 4 then there is a good chance you 
can rely on it. 

If we view GPS tracks as being accurate to 10m anyway so if the 
difference is small then I would fret too much. In my view it's more 
important that the relative positions are correct rather than everything 
being absolutely correct. OSM works on a refinement policy enter the 
data to the best of your knowledge at the time and either your self or 
someone else can improve it in the future.

Cheers
Chris

-- 
http://www.chrisfleming.org/