[Talk-GB] Stitching Aerial Photographs

Matt Williams lists at milliams.com
Sun Sep 20 22:57:54 BST 2009


2009/9/20 John Robert Peterson <jrp.crs at gmail.com>:
> ok -- now we are getting somewhere -- this is awesome.
>
> I would sugest rectifying against the gps trace data, not the map layer,
> because the positions on the map later are derived, introducing slight
> inacuracies.

That would be a good idea. I'm never quite sure what map features I
can rely on to be accurate. Having gps traces as a layer would make
things more accurate and reliable.

> Do we have anything that will draw map tiles of the trace data? (I'd like
> this for another project anyway: checking whether traces exist for an area
> when out with a mobile device)
>
> now hugin/panarama tools/sift has a strong concept of control points, so
> does warper -- is there any way that we could get them to use the same
> points, and bolt the 2 together automatically?
>
> nearly a week ago I posted the following, any further thoughts on it:

<I'll respond to this in another email>


Now that I'm getting more used to the idiosyncrasies of Hugin and co.,
I've worked out a workflow that creates usable images. It seems that
Hugin is _very_ sensitive to trying to stitch together two images that
weren't taken from the same point in space (or for our purposes,
anything more than the distance travelled by a plane in 30 seconds or
so). This means that an image of a village taken from the north (i.e.
camera facing slightly south) cannot be reconciled with an image of
the same features taken from the south (camera facing slightly south).

However, there is still stitching we can do. John's method for many
parts (or at least the bits I've analysed so far) seems to have been
to strafe the camera from near-vertical to ~45° with overlap between
the images. The images within each of these sequences can be stitched
together (i.e. 804-804, 805-808, 809-812 etc.) and those composite
images rectified. Perhaps it's worth skipping the more oblique images
from the stitching to avoid them being on the same 'layer' as the good
vertical ones.

My workflow in Hugin to create each composition is:
1. Find a series of images which belong to one sweep of the camera
(e.g. 805-808)
2. New Hugin project - Go to Images tab
3. 'Add Individual Images' and select the images you need
4. Select them all and click 'Create control points' button at the
bottom (you need panotools-swift-c for this)
5. Once it's created the points, go to Optimiser tab, Choose
'Positions (incremental, starting from anchor)' and optimise and
accept.
6. Then Optimise again but with 'Everything' selected in the drop-down
7. Stitcher tab: Projection = 'Mercator', click 'Calculate field of
view' then 'Calculate optimal size'
8. Choose TIFF compression LZW and 'Stitch now!'

This should create a nice composite image ready for rectifying.

-- 
Matt Williams
http://milliams.com




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