bin/find_avg_population_density

   1 #!/usr/bin/python
   2
   3 """
   4 Find the average population density of a set of GPS coordinates.
   5 """
   6
   7 import sys
   8 import os
   9 import site
  10 import pgdb
  11 from optparse import OptionParser
  12
  13 # Basically, add '../src' to our path.
  14 # Needed for the imports that follow.
  15 site.addsitedir(os.path.dirname(os.path.abspath(sys.argv[0])) + '/../src')
  16
  17 import Configuration.Defaults
  18 import ExitCodes
  19 import GPS
  20 import StringUtils
  21 import SummaryFile1
  22
  23
  24 """
  25 Parse the command line options. There's some magic involved here;
  26 otherwise, optparse woule like nothing more than to interpret GPS
  27 coordinates such as '-78.000000' as options.
  28 """
  29 for idx, value in enumerate(sys.argv):
  30     # Please oh please don't rename this script to something which
  31     # validates as a negative float.
  32     if (StringUtils.is_negative_float(value)):
  33         sys.argv.insert(idx, '--')
  34         # We have to break here, otherwise the negative argument
  35         # will get shifted down the list, and we'll hit it again
  36         # in the next iteration of enumerate(). And then it'll
  37         # get shifted down the list...
  38         break
  39
  40 usage = '%prog [options] <longitude> <latitude>'
  41
  42 # -h (help) Conflicts with -h HOSTNAME
  43 parser = OptionParser(usage=usage, add_help_option = False)
  44
  45 # Use this module's docstring as the description.
  46 parser.description = __doc__.strip()
  47
  48 parser.add_option('-h',
  49                   '--host',
  50                   help='The hostname/address where the database is located.',
  51                   default=Configuration.Defaults.DATABASE_HOST)
  52
  53 parser.add_option('-d',
  54                   '--database',
  55                   help='The database in which the population data are stored.',
  56                   default=Configuration.Defaults.DATABASE_NAME)
  57
  58 parser.add_option('-U',
  59                   '--username',
  60                   help='The username who has access to the database.',
  61                   default=Configuration.Defaults.DATABASE_USERNAME)
  62
  63 (options, args) = parser.parse_args()
  64
  65 if len(args) < 2:
  66     print "\nERROR: You must supply a longitude and latitude.\n"
  67     parser.print_help()
  68     print '' # Print a newline
  69     raise SystemExit(ExitCodes.NOT_ENOUGH_ARGS)
  70
  71
  72 coords = GPS.Coordinates()
  73 coords.longitude = float(args[0])
  74 coords.latitude = float(args[1])
  75
  76 conn = pgdb.connect(host=options.host,
  77                     database=options.database,
  78                     user=options.username)
  79
  80 cursor = conn.cursor()
  81
  82 ###########
  83 # WARNING #
  84 ###########
  85 #
  86 # Most GIS software, including PostGIS and the associated libraries,
  87 # store and manipulate GPS coordinates in (longitude, latitude) format
  88 # rather than (latitude, longitude) format.
  89 #
  90
  91 query = """
  92 SELECT population_density
  93 FROM (sf1_blocks INNER JOIN tiger
  94       ON sf1_blocks.tiger_blkidfp00=tiger.blkidfp00)
  95 WHERE ST_Contains(tiger.the_geom,
  96                   ST_SetSRID(ST_Point(%.6f, %.6f), 4269));
  97 """ % (coords.longitude, coords.latitude)
  98
  99 cursor.execute(query)
 100 rows = cursor.fetchall()
 101
 102 if len(rows) > 0:
 103     avg_density = rows[0][0]
 104     print str(avg_density)
 105 else:
 106     print 'Error: No rows returned.'
 107     print 'Did you pass (longitude, latitude) in the correct order?'
 108     raise SystemExit(ExitCodes.NO_RESULTS)
 109
 110 conn.close()