Changes between Version 102 and Version 103 of WKTRasterTutorial01

Timestamp:

01/05/11 13:48:01 (14 years ago)

Author:

pracine

Comment:

—

WKTRasterTutorial01

@@ -181 +181 @@
 }}}
 
-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. 
-
-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).
+ST_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. 
+
+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), this is why we limit the conversion to the tile number 3278.
 
 First 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.