[Talk-GB] Surrey County Air Survey

[Jonathan Bennett]

On 01/05/2010 13:04, James Rutter wrote:
> Guys, there's been some suggestions on the list that there are
> discrepancies between the new air survey and previous GPS traces and
> also suggestions that the image needs 'reprojecting' etc.

James, you've got the wrong end of the stick on this. I'll explain, but
since you're clearly talking about the message I posted, could you not
have just replied to that?

You will be aware that the very, very accurate coordinates included in
the imagery are in OSGB. You will also be aware that OSM's tools only
work with WGS84 coordinates, and in the case of slippy maps, using a
spherical Mercator projection. Therefore it was necessary for Andy to
reproject the images and slice them into tiles so that they could be
viewed in Potlatch, JOSM and Merkaartor.

This reprojection would not have been perfect, since digital computers
aren't capable of infinite precision, and there's bugs of some sort in
most software. That's where I was suggesting the error had crept in,
rather than in the source material. In any case, as I mentioned the
amount of error we're talking about appears to be around 5 metres at
most, so it's not a problem except if, as will probably happen at some
point, the imagery gets used for micro-mapping.

The imagery is fantastic. I've already been able to draw some complex
road junctions far better than I could with traces alone. You and all
the other people who have made this available have done an amazing job,
and it will put OSM data in Surrey level with, and in some places ahead
of Ordnance Survey. Let's just not pretend that it's impossible for
there to be positional errors in it.

-- 
Jonathan (Jonobennett)