PostgreSQL/PostGIS And SFCGAL

[jlpoole]

There exists an extension to PostGIS, postgis_sfcgal, which is not explicitly supported by Gentoo. The postgis_sfcgal extension provides access to a system's installation of SFCGAL, "Simple Features for CGAL," where "CGAL" is the Computational Geometry Algorithms Library. Thus, SFCGAL provides a bridge that allows the use of advanced geometric operations compliant with the Simple Features standard on top of CGAL. In other words, SFCGAL is a library which provides advanced 2D and 3D spatial functions. Gentoo does not offer SFCGAL as a package, though Bug 653086 "sci-geosciences/sfcgal - new package" tracks a request.

By "explicit support" I mean that there is not a USE flag or any mention of this extension. Yet, the PostGIS configuration file for building PostGIS recognizes (see Chapter 8. SFCGAL Functions Reference" of the PostGIS 3.5.1dev Manual ) and will install support for SFCGAL if SFCGAL is present on the system. See also https://postgis.net/docs/manual-3.0/reference.html#reference_sfcgal.

I thought I would want to be able to define a 3 dimensional object in space on a map and then perform queries against the object to determine if a 3 dimensional positions, e.g. latitude, longitude, and elevation, fall within object. I'm tracking aircraft. It turns out in PostGIS, you can have 3 dimensional objects such as a six-sided box, but if your try to query whether an object is "within" the box, you are really performing a 2 dimensional query or a query of whether an object is within a face of the box. Since my application concerns aircraft and defining whether an aircraft is within a three dimensional "approach" to a runway. I don't want to be concerned with jets flying 5,000 or more feet above, I only want to identify those which are within certain altitudes and which the "Glide Slope" of the approach path. I learned that PostGIS differentiates between box-like structures containing six faces and "solids" and that SFCGAL is what I would need to define a solid area and then perform queries if an aircraft's track goes through the solid. As I went down this trail I learned the following:

You can have an instance of Gentoo's Postgis and have it extended to use the SFCGAL libraries by doing the following:

1) Download and install SFCGAL. I use /usr/local/src to stage foreign projects.