Changes between Version 102 and Version 103 of WKTRasterTutorial01


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Timestamp:
Jan 5, 2011, 1:48:01 PM (11 years ago)
Author:
pracine
Comment:

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  • WKTRasterTutorial01

    v102 v103  
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    183 ST_DumpAsPolygons returns polygons representing raster area sharing a single value and the associated value as a complex "geomval" object. The first item of the SELECT is the geometry and the second the associated value.
    184 
    185 Vectorizing all the table would have been way too long and the huge number of geometry would have been impossible to load in OpenJUMP (and most GIS software).
     183ST_DumpAsPolygons returns polygons representing raster area sharing a single value and the associated value as a complex "geomval" object. The first column returned by this query is the geometry and the second the associated value.
     184
     185Vectorizing all the table would have been way too long and the huge number of geometry would have been impossible to load in OpenJUMP (and most GIS software), this is why we limit the conversion to the tile number 3278.
    186186
    187187First notice: some pixels are grouped together into complex polygons. ST_DumpAsPolygons() not only blindly dump each pixels, it actually vectorize the raster grouping areas with shared values together. In a raster having a low range of values, this would result in a significantly lower number of polygons than the number of pixels. A higher number of different values (like continuous floating point value rasters) would result in many, many little polygons, most of them square corresponding to only one pixel. A future ST_Reclass() function should help reducing the number of values in a raster and give more analysis flexibility. For now you can always reclassify your raster BEFORE loading them into PostGIS WKT Raster.