[OSM-talk] Aerial photo offsets

[mick]

On Sun, 6 Nov 2011 19:38:06 +0900
Andrew Errington <a.errington at lancaster.ac.uk> wrote:

> Hi everyone,
> 
> Forgive me if this has been covered elsewhere, but I am wondering what can be 
> done about the need for aerial photo offsets.  There's very little in the 
> wiki[1]
> 
> In Korea (as elsewhere, I assume) the aerial photos are not always accurately 
> aligned with reality.  Across the country different offsets are needed 
> (sometimes no offset).  What I do is look for a set of GPS traces from a very 
> visible landmark and then use them to align the aerials.  If there are none I 
> will often find a visible landmark, such as a park, or a running track in a 
> stadium, and make several GPS traces of my own on different days, so that I 
> can use them to line up the aerials.  I know that one trace is insufficient.
> 
When I first got my GPS (Garmin GPS 60) I traced the fence lines at home. I found that the accuracy was not very good - between +- 12 to 30 metres so I traced the GPS unit on the top of each post so I could accurately repeat the logging then took readings each day, at different times, and plotted them in a CAD program with a circle centred on the reading with a radius of the error range. At the end of the month almost all the circles were reduced to under 1/2 a metre.

I guess everyone knows this trick.

mick