]>
gitweb.michael.orlitzky.com - dead/census-tools.git/blob - Census.py
b72da0cbd7133e66f9fb9f5c63b7c641fc58180a
3 import Configuration
.Defaults
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.
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
):
21 self
.connection
= pgdb
.connect(host
=initial_host
,
22 database
=initial_database
,
23 user
=initial_username
)
24 self
.srid
= initial_srid
29 self
.connection
.close()
34 def find_average_population_density(self
, coords
):
36 Find the average population density at a set of GPS coordinates.
38 cursor
= self
.connection
.cursor()
41 SELECT population_density
43 WHERE ST_Contains(blocks.the_geom,
44 ST_SetSRID(ST_Point(%.6f, %.6f), %d));
47 sql_params
= (coords
.longitude
, coords
.latitude
, self
.srid
)
48 cursor
.execute(query
, sql_params
)
49 rows
= cursor
.fetchall()
52 # SQL queries that return no results get returned as [[None]].
53 if rows
[0][0] != None:
59 def find_contained_population(self
, well_known_text
):
61 Find the population contained within a geometric object,
62 given in OGC Well-Known Text format.
64 cursor
= self
.connection
.cursor()
66 # We're ready to build our query, one step at a time. First, we store
67 # the Text->Geom conversion in a variable; this just makes the query a
68 # little easier to read.
69 geometric_object
= "ST_GeomFromText(%s, %d)"
71 # We want to compute the population "under" the geometric object. We
72 # can compute the percentage of a block that is covered by taking the
73 # area of (the intersection of the object and the block) divided by
74 # the total area of the block.
76 # Once we know the percentage covered, we just multiply that value by
77 # the total population in the block to find the population that is
78 # covered. The sum of these values over all blocks is our final
83 ( ST_Area(ST_Intersection(%s, blocks.the_geom))
84 / ST_Area(blocks.the_geom) )
85 ) AS covered_population
86 """ % geometric_object
87 sql_params
= (well_known_text
, self
.srid
)
90 # Join our two block tables, so that we have both the demographic
97 # We only need to calculate the covered population for the blocks
98 # that actually intersect our object.
100 WHERE (ST_Intersects(%s, blocks.the_geom))
101 """ % geometric_object
102 # geometric_object hasn't been substituted yet, so we need
103 # to add the sql_params twice.
104 sql_params
+= sql_params
107 # And we only take the first result, since they're all going to be the
108 # same (our query returns the sum once for each block).
113 cursor
.execute(query
, sql_params
)
114 rows
= cursor
.fetchall()
117 # SQL queries that return no results get returned as [[None]].
118 if rows
[0][0] != None:
125 def get_block_geometry_as_wkt(self
, blkidfp00
):
127 Find the geometry of a (uniquely-identified) block, in
128 Well-Known Text format.
130 cursor
= self
.connection
.cursor()
133 SELECT ST_AsText(the_geom)
135 WHERE blkidfp00 = %s;
137 sql_params
= (blkidfp00
,)
139 cursor
.execute(query
, sql_params
)
140 rows
= cursor
.fetchall()
143 # Just pass on the None if that's what we got from the