Finding Intersections with Python

Where roads meet

We need to be able to build a table of all the places roads meet.

• This is not a page about the general case of geometric intersections. Road intersections.
• And we don't want to find the places that roads CROSS (over- or under-crossings). Just where they join.

Both of these points are salient because they mean dealing with dirty input data. Always the case.

• Roads should always meet at a vertex
• Roads that meet should always have a common vertex, not just be "close".
• Roads that do not meet (crossings) should NOT meet these conditions.
  • Any gaps should indicate no connection.
  • Any place roads touch but not at a vertex should indicate a crossing.

But none of this is ever true! Alas, we always get data that looks good on a map but is not routable.

To this end, I want a tool that:

• Generates a point layer of intersections
• Generates a point layer of error cases

The tools

I have a version of this program already written in C# using ArcObjects, and it's a BIG MESS. Worst of all, every time I do a build in a new release of ArcGIS, it breaks!

I want something easy to maintain, easy to update, easy to run, and reasonably fast.

To accomplish this I am reading Python Geospatial Development by Erik Westra. Packt Pubs

At this point, I'd prefer NOT to use PostGIS because I might end up giving the tool away to ArcGIS users. I think this means I will be using Shapely.

I want to be able to use File Geodatabases, so I will be using a custom built GDAL with Python bindings.

GDAL

Here is a page where I talk about Building GDAL on Linux.

Shapely

Here is the home of Shapely, where it is described as "PostGIS-ish operations outside a database context for Pythoneers and Pythonistas." Sounds like what I need.

The steps

Starting off with the simplest version, ignoring all the data cleaning.