Point vs MultiPoint
I got some feedback about yesterdays blog post. To be clear, I’m not suggesting that multipart geometry types are not needed. Rather, I think it’s the single part types that may be superfluous.
A couple of you suggest that single points and sets of points are different from the higher dimensional (curve and surface) cases. What exactly sets these geometry types apart and is the argument for setting them apart a geometrical one? Consider the two WKT strings below and the geometric point sets they describe.
POINT(0 0) MULTIPOINT((0 0))
You’re banking on the distances from either of these objects to any other object, or a 50-meter buffer zone around either object, being the same. In a Simple Features world, you’re also counting on these objects being equal (See section 126.96.36.199, Named Spatial Relationship predicates based on the DE-9IM, in OGC 99-049). Applications using Shapely, to name one category of users, are certainly banking on it.
>>> from shapely.geometry import Point, MultiPoint >>> coords = (0.0, 0.0) >>> Point(coords).equals(MultiPoint([coords])) True
If a single point with coordinates x,y is equivalent to a set of points with a single member (same coordinates x,y), why do we trouble ourselves with different representations for these things? I’m sure historical reasons are a big part of the answer. Not just “Feature B followed Feature A” historical reasons, but point-in-time historical reasons. I’ve a half-baked thesis that the late 90s and early 00s are partly characterized by information modeling practices that tended to produce the greatest possible number of classes rather than the least sufficient number of classes.
Just thinking out loud here. And GeoJSON is the context for my thinking: how it came to be and how things might have been done differently. And how things might be done differently the next time around.