What is the center line of a complex polygon? (cont. 2)

I glossed over the difficulties of finding the centerline of a complex polygon in the last couple of posts, and didn’t realize the disservice until we got to the nitty-gritty of finding the centerline of “bumpy” streams, for which our solution, which is arguably a discretized version of the traditional medial axis, is quite sensitive to noise and “bumpiness”. With a little more googling, my … Continue reading What is the center line of a complex polygon? (cont. 2)

Postgis for breakfast: ST_Donut

This post typed into my iPod as an homage. Assisted today by my collegue, J. Stein. Moderate obfuscation of locations is an important technique for the protection of data, say something sensitive like the nesting locations of the very rare and strange fuzzy-bellied gnat catcher. We still want to display the data, but want to make it slightly wrong. A naive approach would place it … Continue reading Postgis for breakfast: ST_Donut

OGC Web Services and Security

I’ve been starting to inform myself on authentication/authorization schemes for OGC Web Services (OWS). This is because, while I’m pretty pleased with most of the functionality basic mapping of our internal GeoExt/GeoServer/PostGIS stack, user editing is the next natural step. As it is now, users can access a variety of really detailed useful layers, print to PDF, and for the most part the interface is … Continue reading OGC Web Services and Security

Looping Trails– alternate routing for recreational use (cont.)

I’ve been thinking more about the recreational looping problem, given a start location and a given range of travel (e.g. 0-3 miles). A way to approach this is to build up loops from a set of nodes to which has been applied a traveling salesman algorithm. To test this on a piece of paper, before digging under the hood of e.g. pgRouting or OpenTripPlanner, I … Continue reading Looping Trails– alternate routing for recreational use (cont.)

What is the center line of a complex polygon? (cont.)

If we try to extract the centerline of a polygon using Voronoi polygons, like in my previous post, it works pretty well for hydrologic cases, like extracting a stream centerline from stream banks, e.g.: We’ll use this to extract flow lines, in order to build out a better hydrologic network, but also use it to update property boundaries based on deed descriptions which may alternately … Continue reading What is the center line of a complex polygon? (cont.)

Motivations behind Open Source

Waxing philosophical tonight.  Apologies in advance. Paul Ramsey had a great opening keynote opening keynote at FOSS4G on the business model/economics (punchline: business tactic) behind Open Source software, and Free and Open Source companies.  It was funny and convincing, began with absurdity, and made me laugh, as did his PostGIS for Power Users.  His presentation style is at once casual, clever, and passionate.  What caught … Continue reading Motivations behind Open Source

Copying GWC Directory efficiently– Linux

There has been discussion recently amongst some of the web cache builders of switching from file-based tile caching to SQLite or other database driven systems.  I haven’t kept close tabs– perhaps this is already implemented for some.  For GeoWebCache, we’re working with files (lots of them), so copying efficiently if we need to move our cache is important.  The tar command helps here, e.g. (run … Continue reading Copying GWC Directory efficiently– Linux

Cleaning PostGIS Geometries — Revised

In a previous post: “Another short post. Got self intersecting geometries? Want to fix them inside PostGIS? Me too. Fortunately, someone has already written the code, so you and I don’t have to: http://linfiniti.com/2010/07/cleaning-geometries-inside-postgis/ Now, let’s see how long it takes to run on my 760,000 record landcover table… .”   As usual, there’s a better way.  With PostGIS 2.0, you can (will be able … Continue reading Cleaning PostGIS Geometries — Revised