PostGIS WKT Raster - FAQ

1 - What is the meaning of "WKT" in WKT Raster?

WKT stands for "​Well Known Text". Is is an OGC standard to represent geographical information in a human readable textual representation. The WKT standard does not define a way to represent raster yet. PostGIS WKT Raster is a bit ahead on this

2 - How is WKT Raster different than the Oracle SDO_GEORASTER and SDO_RASTER types?

The major difference lies in the fact that each WKT Raster piece of raster (block or tile) is georeferenced (one-georeference-per-raster). In Oracle only the SDO_GEORASTER type supports georeference so that the location of every piece of raster stored as SDO_RASTER is aligned on a grid derived from the unique georeference (one-georeference-by-layer).

The major advantage of one-georeference-by-raster over one-georeference-by-layer is to allow:

• coverages to be not necessarily rectangular (which is often the case of raster coverage covering large extents. See the possible raster arrangments in the documentation)
• rasters to overlaps (which is necessary to implement lossless vector to raster conversion)

3 - Why do you say "WKT Raster goal is to implement the RASTER type as much as possible like the GEOMETRY type is implemented in PostGIS"? Vector and raster are very different and have always been threated differently in GIS packages.

WKT Raster see geospatial data in term of coverage. If the structure of vector and raster dataset are cery different, GIS analyst fundamentally always want to perform the same kind of operations on both types (overlay, map algebra, selection, etc…). Because those two formats are so different, traditional GIS packages have always implemented these operation in a way such that users have to learn and use different sets of tools to do basically the same thing. WKT Raster is a first attempt to build a single set of tool working transparently on both vector and raster formats. We expect that this simplification will allows application software developers to simplify the sets of tools and GUI they provide. This should, at the end, help users to spend more time on solving the problems they have instead of spending time on learning how to do the operation they have to do on a different data structure.

4 - Why does every tile is georeferenced?

5 - How come it is possible to store overlapping rasters in the same table? In a raster, tiles should not overlaps.

6 - How do a load my raster in the database using WKT Raster?

7 - How do I dump my raster from the database into the filesystem using WKT Raster?

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