src/Census.py

   1 import pgdb
   2
   3 import Configuration.Defaults
   4 import GPS
   5 import SummaryFile1
   6
   7
   8 class Database:
   9     """
  10     This class wraps all of the operations that we'd like to perform
  11     on the census database. Most of the utility scripts will just call
  12     one or two methods from within this class.
  13     """
  14
  15     def __init__(self,
  16                  initial_host=Configuration.Defaults.DATABASE_HOST,
  17                  initial_database=Configuration.Defaults.DATABASE_NAME,
  18                  initial_username=Configuration.Defaults.DATABASE_USERNAME,
  19                  initial_srid=Configuration.Defaults.SRID):
  20
  21         self.connection = pgdb.connect(host=initial_host,
  22                                        database=initial_database,
  23                                        user=initial_username)
  24         self.srid = initial_srid
  25
  26
  27     def __del__(self):
  28         self.connection.close()
  29
  30
  31     def find_average_population_density(self, coords):
  32         """
  33         Find the average population density at a set of GPS coordinates.
  34         """
  35         cursor = self.connection.cursor()
  36
  37         query = """
  38         SELECT population_density
  39         FROM blocks
  40         WHERE ST_Contains(blocks.the_geom,
  41                           ST_SetSRID(ST_Point(%.6f, %.6f), %d));
  42         """
  43
  44         sql_params = (coords.longitude, coords.latitude, self.srid)
  45         cursor.execute(query, sql_params)
  46         rows = cursor.fetchall()
  47         cursor.close()
  48
  49         # SQL queries that return no results get returned as [[None]].
  50         if rows[0][0] != None:
  51             return rows[0][0]
  52         else:
  53             return 0
  54
  55
  56     def find_contained_population(self, well_known_text):
  57         """
  58         Find the population contained within a geometric object,
  59         given in OGC Well-Known Text format.
  60         """
  61         cursor = self.connection.cursor()
  62
  63         # We're ready to build our query, one step at a time. First, we store
  64         # the Text->Geom conversion in a variable; this just makes the query a
  65         # little easier to read.
  66         geometric_object = "ST_GeomFromText(%s, %d)"
  67
  68         # We want to compute the population "under" the geometric object. We
  69         # can compute the percentage of a block that is covered by taking the
  70         # area of (the intersection of the object and the block) divided by
  71         # the total area of the block.
  72         #
  73         # Once we know the percentage covered, we just multiply that value by
  74         # the total population in the block to find the population that is
  75         # covered. The sum of these values over all blocks is our final
  76         # result.
  77         #
  78         query = """
  79         SELECT SUM(pop100 *
  80                    ( ST_Area(ST_Intersection(%s, blocks.the_geom))
  81                    / ST_Area(blocks.the_geom) )
  82                ) AS covered_population
  83                """ % geometric_object
  84         sql_params = (well_known_text, self.srid)
  85
  86
  87         # Join our two block tables, so that we have both the demographic
  88         # and geometric data.
  89         query += """
  90         FROM blocks
  91         """
  92
  93
  94         # We only need to calculate the covered population for the blocks
  95         # that actually intersect our object.
  96         query += """
  97         WHERE (ST_Intersects(%s, blocks.the_geom))
  98         """ % geometric_object
  99         # geometric_object hasn't been substituted yet, so we need
 100         # to add the sql_params twice.
 101         sql_params += sql_params
 102
 103
 104         # And we only take the first result, since they're all going to be the
 105         # same (our query returns the sum once for each block).
 106         query += """
 107         LIMIT 1
 108         """
 109
 110         cursor.execute(query, sql_params)
 111         rows = cursor.fetchall()
 112         cursor.close()
 113
 114         # SQL queries that return no results get returned as [[None]].
 115         if rows[0][0] != None:
 116             return rows[0][0]
 117         else:
 118             return 0
 119
 120
 121
 122     def get_block_geometry_as_wkt(self, blkidfp00):
 123         """
 124         Find the geometry of a (uniquely-identified) block, in
 125         Well-Known Text format.
 126         """
 127         cursor = self.connection.cursor()
 128
 129         query = """
 130         SELECT ST_AsText(the_geom)
 131         FROM blocks
 132         WHERE blkidfp00 = %s;
 133         """
 134         sql_params = (blkidfp00,)
 135
 136         cursor.execute(query, sql_params)
 137         rows = cursor.fetchall()
 138         cursor.close()
 139
 140         # Just pass on the None if that's what we got from the
 141         # database.
 142         return rows[0][0]
 143