Changes between Initial Version and Version 1 of CatalogueForQIS

Timestamp:

Feb 28, 2012, 12:38:44 PM (12 years ago)

Author:

aperi2007

Comment:

--

CatalogueForQIS

@@ +1 @@
+When you have a set of raster that are a mosaic. If really useful a catalogue to use all ads un unique raster.
+Qgis is capable to use the VirtualCatalog of gdal.[[BR]]
+To create a catalogue (on windows) usable from qgis 1.9, you can create a file ascii with a list of all the raster with their paths. Like this:[[BR]]
+
+{{{
+D:\dati\folder1\folder2\folder_11\raster_1.tif
+D:\dati\folder1\folder2\folder_12\raster_2.tif
+D:\dati\folder1\folder2\folder_13\raster_3.tif
+D:\dati\folder1\folder2\folder_13\raster_4.tif
+D:\dati\folder1\folder2\folder_13\raster_5.tif
+...
+}}}
+(save the file as "my_list.txt for example)
+[[BR]]
+
+Please, notice that every raster could be on distinct path.[[BR]]
+To create the catalog use this call.[[BR]]
+
+gdalbuildvrt.exe -resolution average -allow_projection_difference -addalpha -hidenodata -input_file_list my_list.txt -overwrite my_catalog.vrt [[BR]]
+
+After the catalog is create it could be used from qgs. But the gdalbuildvrt don't compute the statistics of the rasters in the catalogue and unfortunately qgis,
+if detect that the stats are not available in the catalog, try to compute they for each raster (ouch!). If the rasters are huge and on a shared remote folder. This step could be more time expensive.[[BR]]
+So is really a good idea to add the stats to the catalogue before use it on qgis :)[[BR]]
+Qgis really need only of min/max nothing else. Also to speed up this step you could use without any problem an approximated stats computing.[[BR]]
+To add this approx-stats to the gdal you can use this simple and smart python code:[[BR]]
+
+[Thanks to Giuseppe Sucameli & Faunalia for this smart code]
+
+{{{
+#!/usr/bin/env python
+
+from osgeo import gdal
+from gdalconst import *
+
+def main( path, approx ):
+        ds = gdal.Open( path, GA_ReadOnly )
+        if ds is None:
+                return "Unable to open", path
+
+        for i in range(ds.RasterCount):
+                band = ds.GetRasterBand(i+1)
+                band.ComputeStatistics( approx )
+
+        ds = None
+
+if __name__ == "__main__":
+        import sys
+
+        approx = False
+        path = ""
+
+        for i in range( len(sys.argv) ):
+                if i == 0:
+                        continue
+
+                if sys.argv[i].lower() == "-approx":
+                        approx = True
+                else:
+                        path = sys.argv[i]
+                        break
+
+        if path == "":
+                print "Usage:", sys.argv[0], "[-approx] <datasource>"
+                sys.exit(0)
+
+        main( path, approx )
+}}}
+
+Save it as "computestat.py" and use it with this sintax:[[BR]]
+computestats.py -approx my_catalog.vrt
+
+et voila!
+Now QGis start fast and smart using the raster catalogue
+[[BR]]
+
+Happy catalog-ing
+[[BR]]
+Andrea Peri
+
+------
+