[talk-ph] Does WMS/Landsat have an offset?

[D Tucny]

2009/4/10 Ed Garcia <eppgarcia at gmail.com>

> Related my earlier reply post on coastlines, here is an example showing
> that the landsat image is probably offset or "not calibrated"?
>
> here is a screen capture of JOSM showing Aslom Island south of Mindoro:
>
> http://www.waypoints.ph/images/aslomisland.jpg
>
> The blue trace is what OSM/Mapnik is currently showing as Aslom Island
> (quite a big error),  And, the green raster image is the landsat image
> showing Aslom Island.  Now, the yellow trace is an actual GPS track on a
> boat that was actually made to go around the island with its outrigger
> (katig) just about 5 meters from the land edge to trace the shape of the
> island.  The GPS has the 12 satellites in view (which means very very low
> position error).
>
> As can be seen, the offset is about 50 meters between the landsat image and
> the GPS tracks.  This is about the same offset observed when we tracked the
> mangrove river in Sariaya that I mentioned in my earlier post.
>
> Does this indicate that the landsat image is generally offset or not
> calibrated?
>

Landsat is generally 'good enough' at lower zoom levels, but, at that level
it's pretty much at it's limit... It's not often perfect, but, it does vary
between being pretty close and pretty far off the mark... Around here the
offset looks to be about 40m to the north east based on offset of bridges
and riverbank compared to GPS tracks and yahoo (which matches the GPS tracks
pretty well, at least for roads that existed back then...)

d
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