Changes between Version 40 and Version 41 of WKTRasterTutorial01

Timestamp:

Jun 11, 2010, 12:14:47 PM (14 years ago)

Author:

pracine

Comment:

—

WKTRasterTutorial01

@@ -134 +134 @@
 As you can see, unlike most raster coverage loaded into a GIS, the area covered by this single table raster coverage is not strictly rectangular and the two big missing square areas are not filled with nodata values. Here you have only one table for the whole coverage which is build from many raster files loaded in a single step. You don't need 13 tables to store 13 rasters. We did this with one band SRTM files in TIF format, but it could have been 3 bands BMP files or JPEG and the number of files and the total size of the coverage does not really matter (PostgreSQL has a limit of 32 terabytes)...
 
-If you want to go to the pixel level and verify the integrity of the values associated to a sample of the raster, (thanks to Jorge Arevalo and GDAL) you can view a vectorization of one of the tile (vectorizing all the table would be way too long) in OpenJUMP. Just type the following SQL query in the same query dialog:
+If you want to go to the pixel level and verify the integrity of the values associated to a sample of the raster,  you can (thanks to Jorge Arevalo and GDAL) view a vectorization of some tiles in OpenJUMP. Just type the following SQL query in the same query dialog:
 
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-You will notice that a small area in the upper left corner of the tile is missing. This is the nodata value area. Any analysis function in PostGIS WKT Raster takes the nodata value into account. That means those area will be omitted from the operation. For example in the upcoming intersection operation, if one buffer falls in a nodata value area, st_intersection() and st_intersects() will ignore any part of the raster with nodata values possibly resulting in no intersection at all.
+Vectorizing all the table would be way too long and probably impossible to load in OpenJUMP. 
+
+You will notice that a small area in the upper left corner of the vectorized tile is missing. This is an area with nodata value pixels. Any analysis function in PostGIS WKT Raster takes the nodata value into account. This means that those areas will be omitted from the operation. For example in the upcoming intersection operation, if a buffer completely overlaps a nodata value area of one tile, st_intersects(rast, geom) will return false for this tile/buffer couple and st_intersection(rast, geom) will return an "EMPTY GEOMETRY". If the buffer partially overlap a nodata value area and intersect other parts of the tile, st_intersects(rast, geom) will return true for this tile/buffer couple and st_intersection(rast, geom) will only returns parts of the tile with values, omiting parts with nodata values.
 
 You can verify that your raster has a nodatavalues set and the effective value of the nodata value by doing this query: