Notes on using sdelayer to mosaic data into ArcSDE

For those who aren’t familiar, ArcSDE is server software that sits atop a database to spatially enable it. The resulting geodatabase is able to store geographic features (e.g. roads, buildings, endangered habitats) along with more common SQL data types. ArcSDE can also store georeferenced rasters such as scanned plans/maps or satellite/aerial imagery. To load raster data as a continuous layer one typically mosaics many images, using ArcGIS Desktop or the sderaster command. ArcGIS Desktop is more flexible, it accepts many image formats and can resample images that don’t perfectly align, but it’s slow and struggles with large jobs. The sderaster command is faster and scriptable, but it accepts only tiffs and it’s very fussy about them. What follows are some notes that will remind me the next time and possibly help others.

1. If the real world pixel size (i.e. units m/px or ft/px) on lines 1 & 4 of the world file are not exactly equalt to the value you calculate then correct the world files. If present also correct the geotiff headers. Ordnance Survey 10K tiles may need this correction.
2. Pyramidding is only visually effective if it can create pixels with averaged colour values. Monochrome or colour mapped images do not satisfy this requirement.
3. The documentation states that ArcSDE cannot mosaic images with a colour map. This also applies in some cases where no colourmap is present, such as 4 bit greyscale. Use the -N switch to be safe and then reapply the colour map with sderaster -o colormap, once all images are mosaiced.
4. Geotiff headers are tricky to remove in place, en masse. When preprocessing images be sure that either the world file and/or geotiff tags are correct, or generate your tiffs without getiff tags.
5. GDAL is much faster than ImageMagick for altering bit depth. The libtiff utilities are faster yet.
6. Until ArcSDE 8.3 SP, sderaster required that the top left image be loaded into a raster first, or it wouldn’t mosaic anything.

Those wishing to save the effort of loading images into ArcSDE, might wish to look into ArcGIS ImageServer or a WMS server. To update world files in a batch try the following python script:

import os
import sys
import fileinput
import re
import glob

def replace_in_files(filenames, pattern, rterm):
    '''Replace all occurances of regex pattern with rterm in sequence of filenames'''
    regex = re.compile(pattern)
    for line in fileinput.input(filenames, inplace=True):
        sys.stdout.write(re.sub(rterm, line))

if __name__ == '__main__':
    pattern = sys.argv[1]
    rterm = sys.argv[2]
    filenames = glob.glob(sys.argv[3])

    replace_in_files(filenames, pattern, rterm)