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<div class="moz-cite-prefix">On 2019-01-12 06:33, Eugene Alvin
Villar wrote:<br>
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<div dir="ltr">On Sat, Jan 12, 2019 at 8:26 AM John Willis
<<a href="mailto:johnw@mac.com" moz-do-not-send="true">johnw@mac.com</a>>
wrote:<br>
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The issue I am facing is that, even after some adjustment of
the angle of bing imagery, there seems to be some
distortion. things don’t line up well between the Bing and
ortho maps in some places, and are much closer in others. a
*lot* of the mapping aligns with the bing imagery, but there
are areas of obvious 2-3m distortion in places (the road is
wavy), but other areas of newer/clearer imagry align with
the ortho imagry.<br>
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Bing is sometimes (often? I almost never use it) inconsistent
(offset) with itself at different zoom levels.<br>
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<div>I also encounter this "wavy" roads in imagery. I think
they are the result of improper orthorectification by the
imagery provider. Satellite imagery is often off-nadir (not
photographed straight down) so providers correct for
differences in terrain elevation by rectifying them based on
available elevation models. Unfortunately, many elevation
models like SRTM cannot distinguish between buildings and
hills and so roads are often distorted around tall buildings
in many parts of the world.<br>
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I don't think that orthorectification uses elevation maps as
correction. Rather, they combine shots from different angles to
compute the position of objects and that computes their elevation. A
nadir photograph of a building is uninteresting because it only
shows the roof. Side shots show the walls and allow computation of
their height and ground location. Aerial imagery shows only one of
the many shots they have.<br>
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<div>I don't have any good solution for this aside from trying
to get access to better imagery so I just try to map things
as best as I can. It may also help to avoid micromapping
unless you are sure that the imagery is really good.<br>
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The orthorectification we can use in Wallonia (PICC) is really
perfect. Of the lines I see it computes in places I know, almost
only the center of the road can be not wrong but debatable.
Sometimes lanes that have been changed to parking or things like
that are not up to date, but that's manual fixing, not
orthorectification.<br>
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All the best,
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<td>André.</td>
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