tommy's scratchpad

I recently had the opportunity to prototype WMS visualization of meteorological data.  MapServer, GDAL and Python to the rescue!  Here are the steps I took to make it happen.

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The data, (GRIB), is a GDAL supported format, so MapServer can handle processing as a result.  The goal here was to create a LAYER object.  First thing was to figure out the projection, then figure out the band pixel values/ranges and correlate to MapServer classes (in this case I just used a simple greyscale approach).

Here’s the hack:

import sys
import osgeo.gdal as gdal
import osgeo.osr as osr

if len(sys.argv) < 3:
 print 'Usage: %s <file> <numclasses>' % sys.argv[0]
 sys.exit(1)

cvr = 256  # range of RGB values
numclasses = int(sys.argv[2])  # number of classifiers

ds = gdal.Open(sys.argv[1])

# get proj4 def and write out PROJECTION object
p = osr.SpatialReference()
s = p.ImportFromWkt(ds.GetProjection())
p2 = p.ExportToProj4().split()

print '  PROJECTION'
for i in p2:
 print '   "%s"' % i.replace('+','')
print '  END'

# get band pixel data ranges and classify
band = ds.GetRasterBand(1)
min = band.GetMinimum()
max = band.GetMaximum()

if min is None or max is None:  # compute automagically
 (min, max) = band.ComputeRasterMinMax(1)

# calculate range of pixel values
pixel_value_range = float(max - min)
# calculate the intervals of values based on classes specified
pixel_interval = pixel_value_range / numclasses
# calculate the intervals of color values
color_interval = (pixel_interval * cvr) / pixel_value_range

for i in range(numclasses):
 print '''  CLASS
  NAME "%.2f to %.2f"
  EXPRESSION ([pixel] >= %.2f AND [pixel] < %.2f)
  STYLE
   COLOR %s %s %s
  END
 END''' % (min, min+pixel_interval, min, min+pixel_interval, cvr, cvr, cvr)
 min += pixel_interval
 cvr -= int(color_interval)

Running this script outputs various bits for MapServer mapfile configuration.  Passing more classes to the script creates more CLASS objects, resulting in a smoother looking image.

Here’s an example GetMap request:

Users can query to obtain pixel values (water temperature in this case) via GetFeatureInfo.  Given that these are produced frequently, we can use the WMS GetMap TIME parameter to create time series maps of the models.