[HOT] Imagery alignment

Helen Tait helent55 at gmail.com
Mon May 4 07:36:07 UTC 2015

This is in reply to a previous question today (sorry I have not worked out
how to reply to specific post).
Archival imagery (such as Bing) has usually been processed and
georeferenced/orthorectified (although to variable degrees depending on the
source) prior to publishing. The post earthquake imagery provided by DG
will have had minimal such processing undertaken on it as this takes time
and effort and obviously DG want to just get the imagery out there for
people to use ASAP.  Alignment of this imagery to existing map layers will
not be a straightforward shift in one direction due to the effect of
elevation/terrain and satellite capture angle.  My advice for this type of
imagery would be to continually adjust the imagery to available/reliable
map information as necessary - in one area of the image you may only need
to adjust by say 10m to get a decent alignment - other areas may require a
100m shift. For the archival (e.g. Bing) imagery there can still be some
alignment issues - this imagery will may have had systematic corrections
but no manual input using ground control points (which is required for
really accurate orthorectification).
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