[gdal-dev] SetSpatialFilter not working?

[Spencer Gardner]

I'm writing a script with 2 primary layers, a road layer and an intersection layer.  I have designed the script to cycle through the intersections and remove any that have less than 3 road segments connected to it.  I have set a buffer of 1 foot to account for any misalignment.  The script runs just fine (using almost 24,000 intersection nodes) but doesn't identify any intersections for removal.

After troubleshooting, I have found that counting the road features after setting the spatial filter returns an unusually high number of road segments (usually between 9 and 12) for each intersection so that no intersection returns less than 3 segments.  The same phenomenon occurs if I remove the 1-foot buffer.  If I test the layers in ArcView by selecting roads that intersect a specific node, the result is correct.  Is there something in my code that could be causing this problem?  The documentation says the SetSpatialFilter method "may be inaccurately implemented" - would this be a symptom of this problem?  Is there another way of doing this type of analysis?

Thanks for the help.  Please excuse my lack of knowledge as I'm fairly new to this.

Spencer Gardner
University of Wisconsin-Madison
Department of Urban and Regional Planning





# import modules

import os, sys
try:
  from osgeo import ogr

except:
  import ogr

startDir = os.getcwd()

tempDir = '/tmp'

# set the working directory

os.chdir(startDir + '/Roads')



# get the shapefile driver

driver = ogr.GetDriverByName('ESRI Shapefile')



# open nodes.shp and get the layer

nodesDS = driver.Open('nodesdelete.shp', 0)

if nodesDS is None:

  print 'Could not open nodesdelete.shp'

  sys.exit(1)

nodesLayer = nodesDS.GetLayer()



# open 2009roads.shp and get the layer

roadsDS = driver.Open('2009roads.shp', 0)

if roadsDS is None:

  print 'Could not open 2009roads.shp'

  sys.exit(1)

roadsLayer = roadsDS.GetLayer()
print 'found' + str(roadsLayer.GetFeatureCount()) + 'total road features'

# set up the array to store id's to be deleted
deleteNodes = []


# set up the progress bar
count = 1
totalNodes = nodesLayer.GetFeatureCount()
print 'found ' + str(totalNodes) + 'total nodes'

# loop through each node to count the number of connected roads
nodesLayer.ResetReading()
nodesFeature = nodesLayer.GetNextFeature()
while nodesFeature:

  # buffer the nodes by 1 foot (to get only the adjacent roads)
  nodesGeom = nodesFeature.GetGeometryRef()
  bufferGeom = nodesGeom.Buffer(1)


  # use bufferGeom as a spatial filter on the node to get all roads

  # within 1 foot of the node

  roadsLayer.SetSpatialFilter(bufferGeom)

  roadCount = roadsLayer.GetFeatureCount()
  print str(roadsLayer.GetFeatureCount())
  if roadCount < 3:
    deleteNodes.append(nodesFeature.GetFID())

  # show progress
  count += 1
  sys.stdout.write('.')
  progress = divmod(1000*count, 24000)
  if progress[1] == 0:
    print str(count) + '/' + str(24000)

  # move to the next node
  nodesFeature.Destroy()
  nodesFeature = nodesLayer.GetNextFeature()

# delete the features
print 'deleting features\n'
for node in deleteNodes:
  nodesLayer.DeleteFeature(node)
  sys.stdout.write('.')


# close the shapefiles

nodesDS.Destroy()

roadsDS.Destroy()