Shapely from the outside
Right before the the start of my holiday, I stumbled onto a post from a GIS blog I'd never seen before. It's an older post that compares Shapely to OGR's Python module and GeoScript. As comments are disabled on that post, I'll make a few here. At the time the post was written, Shapely 1.2.8 was released and the manual was fully mature. Greg Corradini writes:
Shapely is a little bit different than OGR and GeoScript. One big difference is that you can’t read and write to file formats with it. The idea, at least the idea that I’ve taken from it’s website, is that Shapely is a “swiss-army-knife” spatial analysis tool — it does some things really really fast and pythonic but it isn’t going to fix a flat tire. Or something like that.
Shapely is more limited than a Swiss Army knife. It's just a wrapper for the GEOS library, using idiommatic Python. It tries to do one thing – or rather, one fairly large set of related things – very well. It doesn't read or write GIS data formats, so if it's a Swiss Army knife it's one without can and bottle openers.
The coolo-neato things about Shapely include the ability to serialize/deserialize to/from data formats that implement a GeoJson-like interface and to/from numpy arrays. There’s also a couple helpful methods — linemerge and polygonize — that it wraps from the GEOS library.
I'm glad to see that somebody appreciates the GeoJSON-like Python geo interface.
These methods aren’t as easy to reach in GeoScript or OGR built with GEOS support off the proverbial ‘shelf’ because you’d have to call Java libraries in the case of GeoScript and C++ libraries using Python ctypes. Shapely makes using these methods easy, even though you could develop your own wrappers to get at them in GeoScript and OGR with GEOS support.
I hadn't thought of these functions as killer features. I suspect they're probably available in osgeo.ogr by now.
I want to say one last thing about Shapely, though it might reflect some naivete on my part about the functionality of this tool. It seems to me that you can do about 80% of what Shapely does if you build OGR against GEOS and use those bindings. Although it might be more pythonic, optimized in certain ways and syntactically simpler, I’m still unsure who the users are and why. I think the TileStache project uses this library on the backend. I’m interested to find more examples.
I used to work in a GIS shop where at least half of the work was taking some data in one obscure format, doing something relatively simple to it (reprojection, for example), and writing it out in some other arcane format. Shapely by itself is practically useless for that kind of work, and I understand that it's purpose can be baffling to the programming GIS analyst. I think I might have found it baffling before my career change.
Finally, Corradini writes:
Since Shapely can’t read or write geometries we have to lean on an existing interop library. Example 3 will use OGR for reads and writes.
Or better yet: Fiona.
It's an interesting post, and the way it weighs a few different software packages is rare these days. It also has me thinking about whether I should be benchmarking Shapely
and Fiona versus GeoScript. Assuming the
testit module is supported by
Jython and produces directly comparable numbers (because I won't be able to benchmark my C Python modules in the same process), it should be pretty easy to do.
Re: Shapely from the outside
Author: Tom Kralidis
We use Shapely inpycsw
to do low level geometry operations (for spatial filters) as well as manage the geometry lifecycle of metadata records. IMHO, it's a portable approach to acheiving this functionality without having to carry OGR, etc.