[OSM-newbies] Trust Yahoo or OSM?

[James Ewen]

On Sun, Sep 13, 2009 at 9:39 AM, Habib Habib <metni009 at gmail.com> wrote:

> I am starting to edit maps by using Yahoo images. The area that I'm mapping
> has already many buildings layed out. However, they seem displaced a bit and
> do not match the map correctly. My question is, should I trust Yahoo and
> correct the positions of all buildings, or instead trust OSM?

The aerial photos can be displaced slightly. Potlatch has a built in
facility for moving the background photo to align it to the real
world.

The word is to trust the GPS traces over the aerial photography.
Consumer grade GPS units do still have a margin of error, but it
usually is not very much if you have a good view of the sky.

The way that I would look at getting an accurate alignment of a photo
would be to find a survey monument that has a highly accurate lat/long
associated with it, and align that spot on the photo to the lat/long
provided.

The issue with doing that is that the information for the survey
marker may not fit within the bounds of legal requirements for use in
the OSM project.

In the real world, I have located survey markers as part of a
Geocaching hunt, and my consumer grade GPS was never more than a
couple feet off from where the survey monument was located. I'm happy
with having a map that "may" have a margin of error of a couple feet.

James
VE6SRV