]>
gitweb.michael.orlitzky.com - dead/census-tools.git/blob - src/Census.py
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
28 self
.connection
.close()
31 def find_average_population_density(self
, coords
):
33 Find the average population density at a set of GPS coordinates.
35 cursor
= self
.connection
.cursor()
38 SELECT population_density
40 WHERE ST_Contains(blocks.the_geom,
41 ST_SetSRID(ST_Point(%.6f, %.6f), %d));
44 sql_params
= (coords
.longitude
, coords
.latitude
, self
.srid
)
45 cursor
.execute(query
, sql_params
)
46 rows
= cursor
.fetchall()
49 # SQL queries that return no results get returned as [[None]].
50 if rows
[0][0] != None:
56 def find_contained_population(self
, well_known_text
):
58 Find the population contained within a geometric object,
59 given in OGC Well-Known Text format.
61 cursor
= self
.connection
.cursor()
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)"
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.
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
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
)
87 # Join our two block tables, so that we have both the demographic
94 # We only need to calculate the covered population for the blocks
95 # that actually intersect our object.
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
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).
110 cursor
.execute(query
, sql_params
)
111 rows
= cursor
.fetchall()
114 # SQL queries that return no results get returned as [[None]].
115 if rows
[0][0] != None:
122 def get_block_geometry_as_wkt(self
, blkidfp00
):
124 Find the geometry of a (uniquely-identified) block, in
125 Well-Known Text format.
127 cursor
= self
.connection
.cursor()
130 SELECT ST_AsText(the_geom)
132 WHERE blkidfp00 = %s;
134 sql_params
= (blkidfp00
,)
136 cursor
.execute(query
, sql_params
)
137 rows
= cursor
.fetchall()
140 # Just pass on the None if that's what we got from the