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

[Maning Sambale]

Some pointers:
1) Are you sure you've set you GPS unit projection and datum to Geog
lon/lat WGS-84?  I found gps units set to PRS92 with the same offset.

2) You can actually adjust the position of the wms layer in josm.  There
is an icon with arrows in four cardinal directions.  I think you can
also do that in potlatch.

3) Landsat has various offset errors depending on the location
(sometimes by just 10 meters or so)


On Fri, 2009-04-10 at 21:50 +0800, D Tucny wrote:
> 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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