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Probably the key difference between the OGC SWE and conventional earth observations approaches is that SWE is based on the Observations and Measurements (O&M) model, which ties observed values to a _feature_ rather than a location. The reasoning behind this follows more or less directly from the General Feature Model (GFM) as described in the OGC Abstract Model, and also in ISO 19101 and ISO 19109. In the GFM the "feature" is the central (meta-)concept, carrying all characteristics of the domain of discourse. In contrast, geometric objects (points, lines, polygons, etc) are abstractions that may describe some aspects of a feature (e.g. location, shape) but are *not* features in their own right. ISO 19107 (the "Spatial Schema") makes it clear that a geometry may only carry spatial of geometric properties. As an example in plain language, a *point* does not have a temperature/pressure/chemistry, etc, the *material at a point* does! In the O&M model this all is explicit, through the so-called "feature of interest" which is the feature instance whose properties are under observation. Note, however, that it is often important to distinguish between the "proximate" feature of interest (pixel/scene, station, sounding, well, transect etc) and the "ultimate" or "project" feature of interest (ground cover, water body, aquifer, rock unit, atmosphere, etc). The former usually embody the sampling strategy, while the latter embody domain semantics. In conventional earth-observation science, the existence of a sampling feature is often elided: its position used as a proxy. This is also followed through in the ISO "Coverages" model, which assigns properties by location. There is an implicit "feature" encompassing the entire coverage domain, with the coverage range describing the distribution of the property value within the bounds of this feature. This implicit feature, if described at all, is in the coverage "metadata" somewhere. This is all fine in practice, and because O&M describes a *conceptual* model there are many potentially conformant serializations or implementations. The OGC SWE suite of standards are relatively explicit about it. But IMHO the key point is to recognise that there *is* a feature of interest, not just points, lines and polygons. Simon
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