Visualizing Hierarchies - Spatial Mathematics: Theory and Practice through Mapping - page 153

First-order subdivision
(typically a county)

Second-order subdivision
(typically an MCD, CCD, or UT)

Place
(incorporated or
census designated)

MCD

MCD

MCD

104
201

MCD 005

101A

9501
9502

205

Place
1740

202
204

203

MCD

MCD 005

Elm street

102

302 9502 101B

304

103

201
202

203

301 205

105

104
201

303
305

101A

203

206

205

306

203

202

206

204

204

Census tract or
block numbering area (BNA)

Block group (BG) Block

Figure 7.2 Census Small-Area Geography. MCD stands for Minor Civil Division; CCD
stands for Census County Division; UT stands for Unorganized Territory. Source:
US Census Bureau, http://www.census.gov/geo/www/GARM/Ch2GARM.pdf

Tract 3 into block groups with numbers in the 300s. The block group with
numbers in the 200s is divided into blocks numbered 201, 202, 203, 204, 205,
and 206. Block number 203 is focused on in the lower right. Another route to
the same location, Block 203, may be found by following Minor Civil Division
(MCD) data, across the top of Figure 7.2.

It is important to note, as shown in the added circle in Figure 7.1 , that sta-
tistical areas may cross political area boundaries. Furthermore, while statisti-
cal areas are often bounded by physical features in the landscape; political
area boundaries are frequently along imaginary lines. Census statistical area
boundaries do not necessarily nest within political area boundaries, nor are
they very often coincident with other data collection areas, such as precincts,
election districts, recreational or tax districts, or ZIP codes.

The most important thing in working with census data is to ensure that your
statistical data cover the same level of geography (such as block group) as the
statistical area features you are working with. The statistical areas form the
physical boundaries of your area of investigation (the G part of GIS ), while the
statistical data provide the demographic information for that area (the I part of
GIS ). If you are working with block groups, for example, make sure that the
demographic data you use are at the block group level.