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<font face="Arial, Helvetica">Hello Steve,<br><br>
<x-tab> </x-tab>Sorry to
rain on the parade yet again but I find this matter of image alignment to
be puzzling and concerning.<br><br>
<x-tab> </x-tab>One of the
first things I learned when embarking upon GIS/GPS mapping was that
accurate georeferencing of all layers, but especially the base layers
(imagery in this case) was sacrosanct. If things are not in their
correct point in space, what use is that to the end user?
Especially in rugged terrain, with difficult access and rapidly changing
stream flows, it is important to know where a trail or road really
is. Why try to cross a raging torrent when you don't need
to?<br><br>
<x-tab> </x-tab>Having
untrained users realign the imagery willy-nilly is amazing to me.
What faith can anyone have in the new tracings if the earth is literally
moving every time a new user opens up the file? Accurate map datums
and projections were created for a reason.<br><br>
<x-tab> </x-tab>How is it
that, "</font>...the DigitalGlobe 2015-05-03 (DG) images have had
minimal georectification.." This is bizarre, this is not GIS,
this is merely sketching. Why is such imagery being offered and
accepted? I know that this is a major emergency but then all the
more need for quality data.<br><br>
<x-tab> </x-tab>However, I
am newly arrived, and it seems that most people are content with a world
that can be up to 200 m out of whack. I'm not sure if I can
contribute much under the circumstances other than this gloomy
criticism. Sorry, will try not to dampen the enthusiasm
further.<br><br>
<x-tab> </x-tab><x-tab>
</x-tab>Thanks for your
patience, Cheers . . . . . . . . Spring<br><br>
<br><br>
At 06-05-2015 11:59 Wednesday, Steve Bower wrote:<br>
<blockquote type=cite class=cite cite="">Ross - If you haven't already,
see the recent threads on "data alignment to satellite imagery"
and "imagery alignment", in the archives for May:<br>
<a href="https://lists.openstreetmap.org/pipermail/hot/2015-May/thread.html">
https://lists.openstreetmap.org/pipermail/hot/2015-May/thread.html</a>Â
<br><br>
Note some links pointed out there by althio:<br>
Â
<a href="http://wiki.openstreetmap.org/wiki/Using_Imagery">
http://wiki.openstreetmap.org/wiki/Using_Imagery</a>Â <br>
Â
<a href="http://learnosm.org/en/editing/correcting-imagery-offset/%C2%A0" eudora="autourl">
http://learnosm.org/en/editing/correcting-imagery-offset/Â </a><br><br>
Because the DigitalGlobe 2015-05-03 (DG) images have had minimal
georectification (needed mainly for elevation distortion), they may be
offset by 100m or more. On one tile (5.5km wide) I saw offsets relative
to Bing of 125m to the west and, elsewhere, 85m to the east. The offsets
may vary considerable even in nearby areas, especially in steep terrain.Â
<br><br>
You should align your work with Bing imagery. Thus to digitize from the
DG imagery you should first adjust the DG imagery to the Bing imagery,
and re-adjust it as you move from place to place. As you noted, adjusting
in one area makes it worse in others, so you have to keep re-adjusting as
you go. You should be able to compare the Bing and DG imagery to confirm
where a feature visible on DG is located on the Bing imagery (if Bing is
clear enough). I try to adjust based on buildings, or road
intersections/curves (keeping in mind that roads are sometimes
relocated), or even less permanent features (rivers generally are not
good, they move around to much). It's a time-consuming process, but
needed to correctly locate features.<br><br>
It's not essential that everything be within a few meters of its true
location, but features should be mapped correctly relative to
one-another.<br><br>
The links above provide guidance on how to align imagery to correct
locations. It's easy in JOSM with the Imagery Offset tool (on the
toolbar).<br><br>
Steve<br><br>
On Wed, May 6, 2015 at 1:53 PM, Steve Bower
<<a href="mailto:sbower@gmavt.net">sbower@gmavt.net</a>>
wrote:<br>
<dl>
<dd>I don't think Chad's IDP guidance document (though very helpful)
addresses the issue of spatial accuracy of the DG imagery, raised by
Ross. I'm going to post that as a separate issue with more
detail.<br><br>
<dd>On Wed, May 6, 2015 at 4:35 AM, Heather Leson
<<a href="mailto:heather.leson@hotosm.org">heather.leson@hotosm.org</a>
> wrote:<br>
<dl>
<dd>HI Ross, sorry for my delayed response. It is best if you ask your
questions on the main
<a href="mailto:Hot@openstreetmap.org">Hot@openstreetmap.org</a> mailing
list. <br><br>
<dd>Chad provided this guidance document on IDPsÂ
<a href="http://hotosm.github.io/tracing-guides/guide/Nepal.html#IDP%20Collection%20Guidance">
http://hotosm.github.io/tracing-guides/guide/Nepal.html#IDP%20Collection%20Guidance</a>
<br><br>
<dd>Hope this helps<br><br>
<dd>Heather <br><br>
<dd>On Tue, May 5, 2015 at 12:40 AM, Ross Taylor
<<a href="mailto:ross@byrdtechnology.com">ross@byrdtechnology.com</a>
> wrote:<br>
<dl>
<dd>Hi, I am seeing many more IDP sites using DigitlaGlobe imagery vs
Bing. I can toggle between the two image sets, but they are significantly
nonaligned. I created a landuse=brownfield tagged area which aligns with
Bing, but if I mark and tag the individual IDP sites showing up in
DigitalGlobe imagery, the brownfield and idp are not going to be
aligned. I want to help out as much as possible and would like the data
to be correct. Please advise, thanks!<br><br>
<dd>Note: I tried to adjust alignment but it fixes one area and creates
more offset in other areas.<br><br>
<dd>-Ross<br><br>
<dd>Sent from mobile<br><br>
</dl><br><br>
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