Interior points for country boundaries
There are some important use cases for this capability:
 Constructing a "proxy point" for a polygon to use in drilldown spatial query. This has all kinds of applications:
 recovering attribution after polygon coverage generalization
 determining parentage during polygon overlay
 faster and more robust spatial join between polygonal datasets
 Cartographic rendering including:
 Creating leader lines
 Placing labels for polygons (for which it is a quick solution but not necessarily a quality one. More on this in a later post)
JTS ScanLine Interior Point Algorithm
 Determine a Yordinate which is distinct to every polygon vertex Yordinate and close to the centre of the vertical extent
 Draw a scan line across the polygon and determine the segments of intersection
 Choose the interior point as the midpoint of the widest intersection segment
Locating a polygon interior point along a scanline
The current code has been in the JTS codebase since the release of the very first version back in 2001. It is elegantly simple, but is quite nonoptimal, since it uses the overlay intersection algorithm. This is overkill for the computation of the scanline intersection segments. It also has a couple of serious drawbacks: slow performance, and the requirement that the input polygon be valid. These are not just theoretical concerns. They have been noticed in the user community, and have caused client projects to have to resort to awkward workarounds. It's even documented as a known limitation in PostGIS.
Thanks to Crunchy Data recognizing the importance of geospatial, we've been able to look into fixing this. It turns out that a relatively simple change makes a big improvement. The scanline intersections can be computed via a lineartime scan of the polygon edges. This works even for invalid input (except for a few pathological situations).
Interior points of invalid polygons
(LH invalid polygon shows suboptimal point placement)
Best of all, it's much faster  providing performance comparable to the (less useful) centroid computation. The performance results speak for themselves:
Dataset  # polys  # points  Time  Prev time 
Improvement 

World countries 
244

366,951  25 ms  686 ms 
x 27

Land Cover  64,090  366,951  78 ms  6.35 s 
x 81

More Ideas
Some further improvements that could be investigated:
 Use the centroid to provide the Yordinate. This is probably better in some situations, and worse in others. But perhaps there's a fast way to choose the best one?
 Use multiple scanlines (both vertical and horizontal)
 Provide better handling of short/zerowidth scanline intersections
 Support clipping the interior point to a rectangle. This would provide better results for cartographic labelling
[1] JTS was based on the original OGC SFS specification (version 1.1). The spec actually does include a method Surface.pointOnSurface. The reason for the different choice of name is lost to time, but possibly the reasoning was that the JTS function is more general, since it handles all types of geometry. (One of the design principles of JTS is Geometric Uniformity, AKA No Surprises. Wherever possible spatial operations are generalized to apply to all geometric types. There is almost always a sensible generalization that can be defined, and it's often quite useful.)
[1a] Also, the acronym IPA is much better than the one for PointOnSurface.
[2] Apparently Oracle has decided to provide blazingly fast performance by simply returning a point on the boundary of the polygon. Thus proving the maxim that for every problem there is an solution which is simple, fast, and dead wrong.
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