[OSM-talk] coastline accuracy?

[mle]

Hi Folks

I'm new to mapping, and looking at the newbie list archives I could not 
see anything that addressed my question, so here I jump into the mainstream.

I recently spent some time on a small island in the Mediterranean. As 
the area was not well covered in Openstreetmap, I took GPS traces of 
everywhere I went, and used those to add some tracks and roads to the 
mapping using Potlatch.

I was staying very near the east coast, and noticed that tracks near the 
coast appeared too far inland (further west than expected) when the GPS 
tracks were plotted.  They also seemed too far inland when looking at 
the Yahoo background layer in Potlatch.

But when I travelled many already mapped roads, my tracks overlaid these 
accurately.  Also, repeated journeys roads were tightly overlaid, so I 
think the GPS unit accuracy was good.

Some of the existing roads were apparently in the sea, and my GPS tracks 
from these roads joined and overlaid the existing mapping almost 
exactly.  Everything seems shifted approximately to the south-west 
relative to the coastal outline.

So what's going on here?  Are the coastline traces somehow displaced, 
and if so, how can this be corrected ?

regards
Martyn