[Talk-GB] Stratford imagery

Tim Waters chippy2005 at gmail.com
Tue Dec 15 10:11:14 GMT 2009


2009/12/10 Peter Miller <peter.miller at itoworld.com>:
>
> On 10 Dec 2009, at 14:18, Andy Allan wrote:
>
>> Hi Peter
>>
>> On Wed, Dec 9, 2009 at 9:20 PM, Peter Miller <peter.miller at itoworld.com
>> > wrote:
>>
>>> Thanks Andy, your wiki page is very useful and it is also good to
>>> see the
>>> imagery in Potlatch. Is the location of image encoded in the image
>>> somehow,
>>> or how else does PotLatch know where to load it?
>>
>> They are made into 256x256 tiles, with the same z/x/y.png notation as
>> the main tileserver. It's the same way that the out-of-copyright maps
>> work too. The !'s in the url on the wikipage are placeholders for
>> potlatch to put in the tile numbers. For example,
>>
>> http://gravitystorm.dev.openstreetmap.org/imagery/stratford/14/8113/5397.png
>
> Thanks. That is very neat.
>
>>
>>> When I tried warping the images I did feel that the algorithm used
>>> by warper
>>> was possibly a bit suspect - the warping suddenly went completely
>>> wrong as
>>> one added more control points in a rather counter-intuitive way.
>>
>> I'd discussed this with a few people over beers, and the problem
>> appears to be that the warper's algorithm is assuming that you start
>> with a fairly "flat" image, and doesn't take into account that the
>> plane of the image might not be horizontal. It's for warping maps,
>> after all, and we're semi-abusing it by warping photos instead. An
>> extra control point or two can send it off wildly (as it tries to
>> figure out how exactly a map sheet would end up stretched so) and of
>> course it gets worse the further from vertical the photo is.
>>
>
> So there is certainly a nice job for someone to sort out a suitable
> algorithm that will work in practical situations when one doesn't
> necessarily even have height data and certainly has distortions of
> various sorts. The idea of associating 'ways' on the image to 'ways'
> on the map sounds very neat assuming that one has a skeleton road
> structure in place (which one could of course get from old OS maps if
> necessary). I think it would then work a treat - certainly much better
> than Warper currently manages.

I also agree would be great to be able to use linear features to
rectifty, it's on my list!

But I just thought I'd make you aware of the "advanced options"
feature on Warper - by default, the algorithm changes based on the
number of control points - you can override this behaviour.

[You can also have a go at using the "Thin Plate Spline" method, which
is best with lots of points - although ymmv and there may be a bug
(mainly because no one uses it!). But essentially it gives a better
warp when theres more points and treats the points as fixed - worth
trying it offline with the command line GDAL tools if you have the
time. ]

**For all methods, the more points, the better.**

Orthorectification is another thing though - thank god the photo's
were in a relatively flat area!

More generally as you know, in the world of aerial photography,
there's usually extensive overlap (up to 60%?) for photos over areas
and so you really shouldn't worry if 20% of the oblique image is
unusable. We should try to get this large redundancy when flying again
:)


Cheers,

Tim




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