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) _______________________________________________ Talk-GB mailing list [email protected] http://lists.openstreetmap.org/listinfo/talk-gb
