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I’ve been banging my head against a wall for a few days on this one, first struggling with ImageMagick, then switching to gdal to get full-resolution geoTIFFs from Cartagen Knitter. I had a ‘duh’ moment just now when I realized that gdalwarp can only do polynomial or thin plate spline warps, neither of which are what I want – that is, perspectival warping. I want to map 4 corner ground control points or GCPs to four latitude/longitude positions. Back to ImageMagick…
I know there are 5 GCPs in the image above – it was the same deal with just 4… it can’t do more than a shear unless you either add the minimum 6 GCPs for a single polynomial warp, or go for a thin plate spline (TPS) distort. A good way to think about TPS is as if the image were a sheet of thin metal (the reason it’s called a TPS) and that you’re bending it in the z-dimension, aplanar. This causes funny curved edges and is not what I’m looking for.
OK, one more note for future reference: see this page for a discussion of different warping techniques and also for the minimum number of GCPs required for different-order polynomial warps.
To offer OSM-JSON along with of OSM-XML, we added a route to accept a “.format” suffix, and split up the render call based on the params[:format] part of the route:
We know we just released Cartagen 0.5 a couple weeks ago, but after testing it extensively in the wild, we really wanted a fast, low-resource release so that users of netbooks, older computers, and older browsers could use Cartagen too.
So, as well as including some general cleanup, this version hums along on a variety of machines. Please send feedback on speed/load times/CPU usage, etc; we tested it on an 800mhz G4 iMac and while it wasn’t super responsive, it did load and was somewhat usable in Firefox 3.5. For reference, Hulu.com’s Flash player does not run on that machine. On more powerful machines (4x3ghz Intel Xeon, 2500×1600 monitor), it can load over 10,000 objects in the viewport without hiccuping. For most users, a typical browser window size yields a responsive and reasonably low-resource experience.
The map is built on Cartagen, a mapping framework for viewing and geographic data in a dynamic, personally relevant way. Cartagen uses the GSS (Geo Style Sheet) format, which allows users to design maps with CSS-like styles. Learn more at Cartagen.org and the Cartagen wiki, or download the source at Google Code.
Go to Cartagen.org to view anywhere in the world through this stylesheet.
Quick update: some users (read: most) are having trouble zooming in and out! This is because of a known last-minute bug in the 0.6 release of Cartagen, and only affects maps embedded in other pages. It does not affect cartagen.org. For a quick fix, to zoom you can also hold down the ‘z’ key and drag up and down on the map. Followup update: zooming with the scroll wheel should work now. Thanks for your patience!
This is the first release we consider to be useable by the general public; as such we are releasing a video to demonstrate how to download Cartagen, get a data set, and write your own GSS stylesheet in just a few minutes:
Special thanks to Ben Weissmann who’s joined the Cartagen team and has been instrumental in bringing it this far.