[Openstreetmap-dev] Re: [Openstreetmap] Landsat update
nick at hogweed.org
Sat Apr 30 18:33:39 BST 2005
> re nick,
> On Sat, Apr 30, 2005 at 01:03:00PM +0100, Nick Whitelegg wrote:
> > I'm wondering if it's to do with the inaccuracy of WGS-84 at the UK's
> > latitude
> no, apart from the usual GPS error factor it shouldn't make a difference.
> > i.e. it uses Airy coordinates even though the GPS uses WGS84.
> > to screen display, the error becomes worse!
> what's being passed in here is OSGB projected numbers, yes?
> i now feel quite confused as to what you're reprojecting where :)
In freemap, the track and point coordinates are stored internally using OSGB
gridrefs (yes, maybe not the best design decision but anyway...) and
displayed without need for complex transformation. They were originally
surveyed with a GPS (WGS84).
The SRTM data are stored as lat/lon and transformed to screen points using the
same transformation that I used to transform the GPS points.
> agreement with this. quick consultation with Schuyler, couple of
> suggestions as to what the problem is:
> - with SRTM at 90M and Landsat at 15 or 30, it's possible that the
> appearance of accuracy over the SRTM layer is a lucky coincidence?
> or do your other experiences bely that?
Haven't had any inaccuracies with the SRTM which are as noticeable as the
Landsat... at a guess I would say the Landsat is perhaps a little under a
quarter of a mile (perhaps 300m) out.
> - the projection error could well be on the landsat WMS side;
> "misregistered" or slightly misrectified tiles. Can you try this out
> against a different set of known good data and check the fit?
Haven't got any other sources of aerial data, but have tried three areas (west
Sussex, Southampton Water and the New Forest) and consistently get about the
Not really used irc before I have to admit.... but am quite happy to keep
talking via email.
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