[Talk-GB] Stratford imagery

Peter Miller peter.miller at itoworld.com
Thu Dec 10 14:40:26 GMT 2009

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.

>> At present I feel that it was a great experiment, but that we  
>> either need
>> much more vertical images which will be hard to achieve without a  
>> plane
>> without specialist equipment or we need a smarter warper.
> There were plenty of photos that were vertical enough, just
> interspersed amongst many more that weren't. It's good that we know
> now that it's more helpful to wait until the plane is banking around
> rather than trying to get as many photos as possible, since the angled
> ones just make the processing more of a faff. In saying that, a warper
> aware of the (approximate) angle of capture would help too.

I was wondering if 'type approval' for powered hang-gliders allows for  
bolt-on vertical cameras. Thinking about it I am going to worry a bit  
more about hang glider pilots dropping phones and cameras etc next  
time they are wandering about overhead!

>> We also need to compare the cost of our own imagery with the cost of
>> purchasing satellite imagery and I feel that purchased imagery will  
>> often
>> win (from $14 per sq km). The West Midlands guys are getting  
>> organised to
>> host purchased photography for that area.
> Awesome. If anyone needs help processing imagery that's been
> purchased, let me know.

The thread is here in case you want to get involved:-



> Cheers,
> Andy

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