<div><div>As far as I can tell from looking at GPS tacks and Landsat images of the UK, the Landsat images are not as accurate as the GPS tracks, but are still quite good for purpose.<br><br>Compare a Landsat image of a Motorway with a single GPS track for the same motorway. The Landsat might be 20-30m off, but then you may not know which lane or side of the motorway was being driven when the GPS tracks were laid. Given that motorway can be easily 30m wide its hard to judge that one may be more accurate than the other.
<br><br>Add to this the fact that the motorway path may be curved. If not enough nodes are drawn then the error introduced due to the segment being a straight line can also easily be 20m or more.<br><br>If the Landsat is clear enough to pick out the route of a road then I don't think any harm is done by mapping it even in the absence of any GPS tracks. If GPS tracks are later added the path can be reviewed and refined in the true collaborative Wiki manner.
<br><br> By comparison, does anyone know how accurate the roads on Google maps are?<br><br>Etienne<br></div><br><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
<div dir="ltr" align="left" lang="en-us"><font face="Tahoma" size="2"><b>From:</b> Mikel Maron [mailto:<a href="mailto:mikel_maron@yahoo.com" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">mikel_maron@yahoo.com
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<br><b>Sent:</b> 16 December 2005 12:25<br><b>To:</b>
<a href="mailto:openstreetmap@vr.ucl.ac.uk" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">openstreetmap@vr.ucl.ac.uk</a><br><b>Subject:</b> Re: Landsat ? (was:Re:
[Openstreetmap] New roads appearing withoutGPS tracks)<br></font><br></div>
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<div style="margin-left: 40px;">From: Tom Carden
<<a href="mailto:tom@tom-carden.co.uk" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">tom@tom-carden.co.uk</a>><br><br>The vast majority of OSM data so far has
been entered with the<br>assistance/sanity check that landsat offers
us. We'd have to junk the<br>lot if there was ever a successful
challenge on derived landsat data,<br>but I don't think there ever will
be.<br></div><br>
<div><a href="http://bat.vr.ucl.ac.uk/cgi-bin/mailman/listinfo/openstreetmap" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)"></a>Landsat 7 is in the public domain. There is no restriction on
derived works.<br><br>However I would be cautious, when relying on Landsat for
drawing vectors. Only usable as a very rough sanity check (ie, generally in the
right part of the world). Until there are other imagery sources integrated, GPS
traces are the primary vehicle for deriving vectors. Don't think there's any
disagreement on this, just want to make the point.<br><br>There are serious
rectification errors in the JPL Landsat data. For instance, the shoreline here
in Brighton is off by 20-30m.<br>Some discussion on this in the following
WorldWind thread (post #8)<br><span> <a href="http://forum.worldwind.arc.nasa.gov/index.php?showtopic=1512" target="_blank" onclick="return top.js.OpenExtLink(window,event,this)">http://forum.worldwind.arc.nasa.gov/index.php?showtopic=1512
</a></span><br><br>-Mikel<br><br><br></div></div></div>
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