[Openstreetmap-dev] Re: [Openstreetmap] Landsat update

Nick Whitelegg nick at hogweed.org
Sat Apr 30 13:03:00 BST 2005


On Saturday 30 Apr 2005 12:29, you wrote:
> hi nick,
>
> nice work.
>
> On Sat, Apr 30, 2005 at 11:05:31AM +0100, Nick Whitelegg wrote:
> > There must be, however, some sort of projection issue. The coastal path
> > is slightly in the water!
>
> This looks like a similar degree of offset that Jo had while working with
> garmin GPS collected data and heath's bristol survey. I can't remember
> what the problem was exactly.. jo? do you? Perhaps this is related.
>
> Cheers,
>
> Saul.

Jo, Saul,

I'm wondering if it's to do with the inaccuracy of WGS-84 at the UK's latitude 
and longitude? My GPS uses WGS84, so that's the same projection as the 
Landsat data. However, I did discover a possible reason. The code I used to 
generate maps mistakenly used a routine to convert the *Airy (OSGB)* 
latitude/longitude of track points and waypoints to x,y coordinates for 
screen display. 

i.e. it uses Airy coordinates even though the GPS uses WGS84.

However, if I correct the code so that it converts *WGS84* track and waypoints 
to screen display, the error becomes worse!

I think it must be a projection issue rather than inaccuracy in my GPS, 
because the SRTM height data and my GPS tracks correlate very well. For 
example, tracks which follow a narrow valley in real life follow the same 
valley when overlaid on SRTM data, e.g. the path to the centre-south of the 
map at

http://www.free-map.org.uk/drawmap.php?e=490806&n=130314&mapview=10


Nick






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