jerstlouis
commented
1 year ago My understanding is that the way DE-9IM is applied to Topic 21 is that an operation with DGGS zone is simply replaced by the geometry of that zone, which is normally a 2D polygon or a 3D or 4D prism. Then everything works as usual.
The valid operations between two zones are those valid for e.g., polygon - polygon.
According to part 8.3 in the document OGC Abstract Specification Topic 21 v2.0 - Part 1, the elements of Topological Query functions are based on the DE-9IM model. However, we are considering if the DE-9IM model can be fitted well to the DGGS since the model only measures the relationships between any two of point, line, and polygon. We are not sure if the DGGS has the concept of vector elements, points, lines and polygons; in our understanding, it is rasterized on the 2D-plane and voxelized in 3D-space. For instance, the relationship ‘cross’ is only capable when lines exist, but for DGGS, which does not have the concept of lines, how should we interpret this ‘cross’ relationship? And, another problem is, how should we apply the DE-9IM model on a 3D system?
Based on above considerations, we are wondering if we should substitute the DE-9IM model with other topological query method; in other words, if we should abandon the concepts of point, line, or polygon and only consider the concept of discrete grid zones. In the grid system, the zones are hierarchical, encoded, and the difference between points, lines and polygons are pretty blurred. For example, a GeoSOT 1°×1° 2D zone has a length and width of about 100 kilometers on the equator. It is counted as only one zone in quantity, but it is a polygon in terms of geographical coverage. We suggest computing the topological relationship between zones directly. For example, there might be 4 types of topological relationships between zone sets (a single zone(cell) is a zone set with only one zone): Touch (adjacent, common edges in 2D, and common surfaces in 3D), Intersects (common surfaces in 2D, and common bodies in 3D), Disjoint (no common edges or common bodies), Equal (two zone sets can be completely overlapped), Contains or Within (One zone set is the subset of another zone set).