[Talk-GB] Stratford imagery

Andy Allan gravitystorm at gmail.com
Thu Dec 10 14:18:28 GMT 2009


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

> 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.

> 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.

> 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.

Cheers,
Andy




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