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Census tracts are small, relatively permanent geographic entities within
counties (or the statistical equivalents of counties) delineated by a com-
mittee of local data users. Generally, census tracts have between 2,500 
and 8,000 residents and boundaries that follow visible features. When first
established, census tracts are to be as homogeneous as possible with res-
pect to population characteristics, economic status, and living conditions.

Block numbering areas (BNAs) are geographic entities similar to census 
tracts, and delineated in counties (or the statistical equivalents of counties)
without census tracts. For the 1990 census, the difference between census
tracts and BNAs generally was the type of organization doing the delinea-
tion. Local census statistical areas committees (CSACs), often working at 
the county level, delineated or reviewed census tracts. State agencies and
American Indian tribal authorities, sometimes with extensive assistance
from the Bureau of the Census, delineated BNAs. 

The Census Bureau uses census tracts and BNAs to collect, organize, tab-
ulate, and present the results of its decennial censuses. Both census tracts
and BNAs are an important part of the Census Bureau’s geographic hierar-
chy (see Figures 2-1 and 2-3 in Chapter 2). For the 1990 decennial census, 
the Census Bureau recognized 50,690 census tracts in the United States 
and Puerto Rico, and 11,586 BNAs in the United States, Puerto Rico, and 
the Outlying Areas under U.S. jurisdiction. Six States (California, Connect-
icut, Delaware, Hawaii, New Jersey, and Rhode Island) and the District of
Columbia are covered completely by census tracts.

The first recorded instances of the delineation of small geographic enti-
ties based on population, topography, and housing characteristics were 
the sanitary districts of a special vital statistics study associated with the 
1890 census. The Census Office, predecessor of the Census Bureau, 
worked with local officials in a number of cities to delineate a network 
of small geographic areas. 

Census Tracts and Block Numbering Areas   10-1

Census Tracts and Block Numbering Areas

Background

Chapter 10

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10-2   Census Tracts and Block Numbering Areas

These sanitary districts then were used to analyze and compare the effect
of population, topography, and housing on the mortality rate of the inhabi-
tants. The delineation of these sanitary districts was an important step in
the evolution of geographic statistical entities. This may have been the first
instance of Federal and local cooperation in designing a set of small geo-
graphic units based on population and housing characteristics.

Census Tracts

In 1906 Dr. Walter Laidlaw, Director of the Population Research Bureau of
the New York Federation of Churches, published an article putting forth
the idea of delineating and using small geographic areas as a method of
studying neighborhoods in New York city.

1

 Dr. Laidlaw had been studying

neighborhoods by using the 1900 census data for assembly districts (sub-
divisions of New York city’s boroughs) together with information from
other sources. In 1905, the State of New York changed the boundaries of
the assembly districts, thereby altering the geographic framework and
impairing the usefulness of all his information.

In search of a solution, Dr. Laidlaw proposed a scheme that did away with
both ward and assembly districts as data tabulation units. Instead of these,
he suggested the delineation of permanent small areas that would retain
their boundaries from census to census. His plan was to subdivide each
square mile of New York city into quarter sections of about 160 acres.
In 1909, he persuaded the Census Office to adopt the concept, and they
collected the 1910 census data in a manner that allowed for these tabula-
tions by small area. Interested data users then could purchase the data
summaries and arrange for their tabulation and publication. The Census
Office also delineated similar 

districts,

 later called 

census tracts,

 in seven

other cities: Baltimore, Boston, Cleveland, Chicago, Philadelphia, Pitts-
burgh, and St. Louis.

2

The Census Bureau collected data by census tract for these eight cities in
1910 and 1920; however, only New York city made immediate use of the
data. In the mid-1920s, Chicago and Cleveland purchased and published
their census tract data. By the end of the decade, 18 cities (the same 8,

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Census Tracts and Block Numbering Areas   10-3

and 10 new ones) were reviewing or delineating census tracts for the
1930 census.

This increased interest in census tracts was due largely to the promo-
tional efforts of Howard Whipple Green, a statistical consultant working
in Cleveland, Ohio. Having experienced data problems similar to those
faced by Dr. Laidlaw, he found that census tracts were a solution. In 1931,
the American Statistical Association appointed Mr. Green chairman of
its newly formed Committee on Census Enumeration Areas. Along with
this appointment came the unofficial assignment to promote the deline-
ation of census tracts in large cities throughout the country. Over the
next 25 years, he worked hard at this task, contacting interested people
in other cities, encouraging the formation of local committees, and pub-
In his dealing with the local committees, Mr. Green often found it conven-
ient to identify one individual in each city as a point of contact. He called
these individuals 

key persons

. The committees themselves became known

as 

census tract committees. 

These were the forerunners of the present-

day census statistical areas committees (see Chapter 3, “Local Census Statis-
tical Areas Committees and Other Local Assistance”). For the 1940 census,
the Census Bureau adopted the census tract as an official geographic entity
to be included in data tables of the standard publications of the decennial
census. This relieved the census tract committees of the need to purchase
the data tabulations and to fund their publication. In 1955, upon Mr. Green’s
retirement, the Census Bureau assumed the functions of promoting and
coordinating the delineation of census tracts.

Block Numbering Areas

Both census tracts and BNAs provide the geographic framework for delin-
eating block groups, assigning census block numbers, and tabulating and
presenting the resultant data. In 1940, the Census Bureau began publishing
census block data for all cities with 50,000 or more inhabitants. In cities
that had census tracts, it assigned the block numbers by census tract; in

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10-4   Census Tracts and Block Numbering Areas

cities without census tracts, it devised 

block areas

 to control the number-

ing. These block areas, renamed 

block numbering

 

areas

 

(BNAs)

 in 1960,

consisted of one or more enumeration districts, and sometimes city wards.
Their boundaries were major streets, railroads, and other physical features.
In 1970, the Census Bureau established its current procedure of number-
ing BNAs within a county (or statistically equivalent entity) beginning with
the number 9501. In 1970 and 1980, there was an increase in the number
of jurisdictions and areas receiving data by census block under the BNA
program (see Chapter 11, “Census Blocks and Block Groups”). Beginning
with the preparations for the 1980 census, the Census Bureau changed the
BNA delineation criteria to make BNAs more comparable in size and
shape to census tracts.

Census Tract and BNA Criteria

Over time, the Census Bureau and the census statistical areas committees
have developed a set of standards to guide the establishment and revision
of census tracts. Although not expressly mandated by any legislation, these
practices have evolved through custom and usage, and are now an integral
part of the principles, policies, guidelines, and criteria that the Census
Bureau uses to create and maintain census tracts. These rules promote
census tract consistency nationwide, and also serve to meet local needs
for small-area data.

Eligibility

The eligibility criteria for the census tract program has evolved over time
in response to user demand and the growth in metropolitan areas (MAs).
Initially, only metropolitan counties (or statistically equivalent entities) and
nonmetropolitan jurisdictions that met specified conditions could partici-
pate in the census tract program. Generally, local committees decided to
delineate census tracts in nonmetropolitan counties because (1) the coun-
ties were likely to gain metropolitan status as a result of an upcoming cen-
sus, (2) they had a population of at least 100,000, or (3) they contained a
city having at least 40,000 people. In addition, committees could establish
census tracts in nonmetropolitan counties adjacent to metropolitan areas

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Census Tracts and Block Numbering Areas   10-5

if the counties were part of the planning or jurisdictional region of an exist-
ing census statistical areas committee.

For the 1990 decennial census, the Census Bureau assigned block numbers
to all parts of the United States and the Outlying Areas. It opened the cen-
sus tract program to include all counties (and statistical equivalents) with
sufficient local interest to form a census statistical areas committee. All
other counties (or equivalents) were part of the BNA program. (For details,
see the section in this chapter entitled “Census Tracts and BNAs for the
1990 Census.”)

Basic Attributes of Census Tracts and BNAs

Even though local participation in the census tract program evolved over
time in response to user demand and the growth in MAs, the underlying
rationale for delineating census tracts has remained constant. They define
a set of small geographic areas for the enumeration, tabulation, and publi-
cation of census data.

For the 1990 census, the Census Bureau changed the concept of the BNA
dramatically. By redefining the BNA from a geographic area delineated
solely as the framework for assigning census block numbers (1940 through
1980), to an entity sharing the same basic attributes as the census tract
(1990), the Census Bureau has established a nationwide set of comparable
small geographic areas.

The census tract and BNA criteria recognized by the Census Bureau identify
boundary, size, and demographic requirements, and establish conventions
for numeric identification and stability.

Boundary requirements

  The need for appropriate boundaries is a long-

standing concern of census geography. Census tract and BNA boundaries
generally follow permanent, visible features, such as streets, roads, high-
ways, rivers, canals, railroads, and high-tension power lines. Pipelines and
ridge lines may be acceptable when no other choice is available.

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10-6   Census Tracts and Block Numbering Areas

The most important attribute of census tract/BNA boundaries is that they
be visible, that is, readily identifiable in the field.

The Census Bureau often is urged to accept the use of governmental unit
boundaries as census tract and BNA boundaries. Census tract/BNA bound-
aries always follow the boundaries of States and counties (or their statistical
equivalents). Census tract and BNA boundaries follow other governmental
unit boundaries only in selected instances. Many early census tract plans
covered only large cities; as a result, the city limits were census tract bound-
aries. This posed no problem at a time when governmental unit boundaries
remained unchanged for long periods of time, and thus, their location was
well known. Later, as annexations became more frequent and incorporated
places expanded into surrounding areas, the governmental unit boundaries
in many States became more irregular and subject to change, and fewer
people knew their precise location. This lack of stability meant that the gov-
ernmental unit boundaries became less suitable as census tract boundaries.

For the 1970 census, the Census Bureau began providing data for that por-
tion of each census tract inside a governmental unit, and for the census tract
as a whole. Until then, the only way to obtain census tract data within a gov-
ernmental unit was by recognizing the governmental unit boundaries as
census tract boundaries. Currently, the Census Bureau makes the full range
of census data available for all governmental units and for census tracts/
BNAs, thereby negating the need for census tract/BNA boundaries to
follow governmental unit boundaries.

Demographic requirements

  When first delineating census tracts, the Census

Bureau requests that the average population of all census tracts in a county
(or statistically equivalent entity) be about 4,000 people (approximately
1,500 housing units), with individual census tracts ranging from 2,500 to
8,000 inhabitants (1,000 to 3,000 housing units). For the 1990 census, the
Census Bureau requested that the average number of housing units in each
BNA be around 1,500 (approximately 4,000 people), ranging from 600 to
3,000 housing units (1,500 to 8,000 inhabitants).

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Census Tracts and Block Numbering Areas   10-7

The Census Bureau also requests that at the time each census tract/BNA is
established, it contain (if possible) a population whose housing and socio-
economic characteristics are similar. Because the characteristics of neigh-
borhoods and other small areas change with time, census tracts/BNAs
may become less homogeneous in succeeding censuses.

Numeric identification

  To facilitate data processing and publication, the

Census Bureau identifies census tracts and BNAs by number rather than
name. Each census tract has a basic census tract number composed of no
more than four digits, and may have an optional two-digit decimal suffix.
(Leading zeroes appear in electronic media products, but do not appear
on the Census Bureau’s maps or in the printed reports.) All BNAs have
a four-digit basic number and may have an optional two-digit decimal
suffix. The Census Bureau uses the numbers 1 to 9499.99 to identify cen-
sus tracts, and 9501 to 9989.99 to identify BNAs.

Sometimes the Census Bureau recommends a range or series of census
tract/BNA numbers to the census statistical areas committee or agency
participating in the BNA program to avoid duplication with adjoining
counties. For example, if two counties in the same MA both contain
census tracts numbered 101 through 110, the Census Bureau might rec-
ommend that one county renumber their census tracts 1101 through
1110, and the other renumber theirs 2101 through 2110.

A permanent numbering system is desirable since it helps data users
make intercensal comparisons of information by census tract. Census
tract updates often involve the subdivision of an existing census tract (or
census tracts) into two or more new units. When new census tracts (splits)
occur within an established set of census tracts, the Census Bureau recom-
mends retaining the original four-digit census tract number and adding a
two-digit decimal suffix. As a result, Census Tract 101 may be split into
Census Tracts 101.01, 101.02, and so forth, depending upon how many
new census tracts are created. If a census tract identified by a suffixed
number is subsequently split, the census statistical areas committee usu-
ally drops the existing suffix and utilizes the next available suffixes.

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10-8   Census Tracts and Block Numbering Areas

Figure 10-1 depicts the most common scheme for numbering split cen-
sus tracts (represented by Census Tract 6) and an alternative numbering
scheme that some census statistical areas committees have chosen to use
(represented by Census Tract 12). If two census tracts merge, the Census
Bureau recommends that the census statistical areas committee retain the
number of the more populous census tract.

The Census Bureau provides a unique census tract/BNA identifier (a
numeric suffix of .99) to report statistics about people aboard civilian
or military ships. These 

crews of vessels

 census tracts/BNAs refer to

the water near the piers, docks, or onshore facilities associated with the
ships; they do not represent any land area or any specific area of water.

Figure 10-1.  

Recommended Renumbering of Split Census Tracts

Original

First Split

Second Split

6.01

6.02

6

6.03

6.04

6.05

6.06

Original

First Split

Second Split

12.01

12.02

12

12.11

12.12

12.21

12.23

12.22

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Census Tracts and Block Numbering Areas   10-9

Census Tracts and BNAs for the 1990 Census

The 1990 BNA Delineations

In preparation for the 1990 decennial census, the Census Bureau expanded
the delineation of BNAs so that all counties (or statistically equivalent enti-
ties) not in the 1990 census tract program would have BNAs. To do this,
it developed a program for the governments of States, American Indian
tribes, Puerto Rico, and the Outlying Areas to participate in the delineation
of BNAs and block groups. This effort paralleled the delineation, or review
and update, of census tracts and block groups being undertaken by the
census statistical areas committees.

The Census Bureau contacted State/territorial governors and requested
that they designate an agency to coordinate the delineation of BNAs for
the 1990 census. It offered them two options for participation in the 1990
BNA program. Under the first option, the State/territorial agency deline-
ated the BNAs (in some instances, with assistance from interested county
or local agencies). Under the second option, the Census Bureau deline-
ated the BNAs and sent the delineations to the designated State/territorial
agency for review and concurrence. Although many States chose one
option or the other, several combined both approaches. In Florida and
Illinois, the State governor declined to participate in the BNA program
and the Census Bureau delineated the 1990 BNAs.

For the 1990 census, the Census Bureau recognized some census tracts
and BNAs that did not conform completely to established criteria. This
was due to a number of factors, including Census Bureau enumeration
and tabulation requirements, TIGER System constraints, and special
arrangements reflecting the unique needs of data users.

Census Tract/BNA Boundary Discrepancies

Data users first saw geographic products showing the 1990 census tracts
and BNAs on the Precensus Local Review Maps. In some instances, they
discovered a discrepancy between the location of the census tract or

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10-10   Census Tracts and Block Numbering Areas

BNA boundary that the Census Bureau had previously agreed to recognize,
and the location shown on the precensus maps. If the census statistical
areas committee or the BNA participant notified the Census Bureau of a
census tract or BNA boundary discrepancy and requested a correction,
the Census Bureau corrected the discrepancy.

Resolving census tract/BNA boundary discrepancies was complicated
further by an additional commitment made to data users. Following the
1980 census, many data users complained about two types of geographic
inconsistencies that made using the 1980 census data difficult for a major-
ity of data users—duplicate block numbers in a census tract/BNA and
block groups consisting of more than one contiguous cluster of blocks
(discontiguous block groups). The Census Bureau agreed to correct this
for the 1990 census.

When resolving census tract/BNA boundary discrepancies, the Census
Bureau expanded the area of a census tract/BNA wherever possible.
After expanding the census tract/BNA, Census Bureau staff flagged the
census tract/BNA gaining area and the census tract/BNA losing area.
This was accomplished by retaining the basic census tract/BNA number
of the changed census tracts/BNAs and adding a special two-digit suffix.
When assigning the special suffixes, Census Bureau staff began with .98
and assigned subsequent numbers in descending sequence, .97, .96, .95,
and so forth (see Figure 10-2).

As a result of the promise not to create discontiguous block groups or
duplicate 1990 census block numbers, the Census Bureau did not expand
the area of a census tract/BNA if such a revision caused the expanding
census tract/BNA to include a census block that was discontiguous with
other blocks sharing the same block group identifier, or if resolving the
census tract/BNA boundary discrepancy created duplicate 1990 census
block numbers. Under these circumstances, the Census Bureau created a
separate census tract/BNA composed of the census block(s) in question.
The Census Bureau assigned a new census tract/BNA number to the
newly created census tract/BNA by retaining the basic number of the

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Census Tracts and Block Numbering Areas   10-11

Figure 10-2.  

Expanding a Census Tract or BNA for Boundary Resolution

Example 1: 1990 Precensus Map

Example 2: Corrected 1990 Census Tract

Census Tract Boundary

Block Boundary

6.01

Census Tract Number

201

Block Number

101

102

103

201

202

203

301

302

204

206

207

401

402

403

404

405

506

505

504

503

502

501

205

6.01

6.02

101

102

103

201

202

203

301

302

204

206

207

401

402

403

404

405

506

505

504

503

502

501

205

6.98

6.97

When resolving census tract/BNA boundary discrepancies, the Census Bureau
expanded the area of a census tract/BNA wherever possible. In Example 1 above,
Block 205 was included in Census Tract 6.01 even though the approved census
tract plan had included it in Census Tract 6.02. Because Census Tract 6.02 does
not contain a Block Group 2, expanding Census Tract 6.02 does not create a
discontiguous block group or duplicate any 1990 census block number in that
census tract; thus, Block 205 simply becomes part of Census Tract 6.02.

After expanding a census tract/BNA, the Census Bureau flagged the affected
census tracts/BNAs by adding special two-digit suffixes (beginning with .98 and
then descending) to the basic census tract/BNA numbers. In Example 2, Cen-
sus Tract 6.01 has been renumbered as 6.98 and Census Tract 6.02 as 6.97.

In Examples 1 and 2, it was possible to expand Census Tract 6.02 to include the
affected census block (205). If expanding the census tract would have created
discontiguous block groups or duplicate block numbers, the Census Bureau
would have created a new, separate census tract/BNA as shown in Figure 10-3.

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10-12   Census Tracts and Block Numbering Areas

census tract/BNA losing area and appending the special suffix. The census
tract/BNA losing area also was renumbered by assigning the special suffix
(see Figure 10-3).

Figure 10-3.  

Creating a New Census Tract or BNA for Boundary Resolution

Example 1: 1990 Precensus Map

Example 2: Corrected 1990 Census Map

Census Tract Boundary

Block Boundary

6.01

Census Tract Number

201

Block Number

601

602

603

701

702

703

801

704

601

602

603

604

605

706

705

704

703

702

701

10.98

10.02

802

10.97

601

602

603

701

702

703

801

704

601

602

603

604

605

706

705

704

703

702

701

10.01

10.02

802

The Census Bureau would not expand the area of a census tract/BNA when
resolving a census tract/BNA boundary discrepancy if such a revision caused
the census tract/BNA to include a census block that was discontiguous with
other blocks in the same block group or created duplicate 1990 census block
numbers. In Example 1, Block 704 was included in Census Tract 10.01, even
though the approved census tract plan had included it in Census Tract 10.02.
Because Census Tract 10.02 already contains a Block 704, the Census Bureau
could not expand Census Tract 10.02 to include a second Block 704.

In resolving this type of census tract/BNA boundary discrepancy, the Census
Bureau created a separate new census tract/BNA comprised of the block(s) in
question. In Example 2, it created the new Census Tract 10.97, comprised of
Block 704 from Census Tract 10.01. Additionally, the Census Bureau renum-
bered Census Tract 10.01 as 10.98 (because 10.01 is the census tract that lost
area). Census Tract 10.02 was not renumbered because nothing in it changed.

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Census Tracts and Block Numbering Areas   10-13

Census Tracts/BNAs and Governmental Unit Boundaries

The Census Bureau discourages the use of governmental unit boundaries as
census tract/BNA boundaries because of the need to freeze the census tract/
BNA boundaries at the time of census block numbering, which occurs sev-
eral years before a decennial census. Once the census tract/BNA boundaries
are frozen, any changes to the governmental unit boundaries, whether as a
result of annexations, detachments, or mapping corrections, result in the
census tract/BNA boundaries continuing to follow the former (incorrect)
location of the governmental unit boundary. The result can be the loss of
the intended 

nesting

 relationship between census tracts/BNAs and the

governmental unit.

County Boundary Updates

Holding the boundaries of counties (or statistically equivalent entities) as
census tract/BNA boundaries is a fundamental requirement of the census
tract/BNA programs. Census tracts and BNAs are subdivisions of counties,
and they nest within counties. Because the Census Bureau needed to have
the census tracts/BNAs delineated before numbering the 1990 census
blocks, it had to approve the census tract/BNA plans several years before-
hand. Knowing that it would be necessary to update some county or State
boundaries after the establishment of census tracts/BNAs and the assign-
ment of 1990 census block numbers (but before data tabulation), the Cen-
sus Bureau designed a method to accommodate the latest (January 1, 1990)
State and county boundary changes. Changes in the 1990 census tract/BNA
boundaries ordinarily would require renumbering some census blocks,
yet the Census Bureau had to design a method of updating State or county
boundaries without changing any census block numbers. As a result, the
Census Bureau recognized as census tract/BNA boundaries both the super-
seded and the corrected State or county boundaries. The result was the
formation of (usually) small census tracts/BNAs, often containing little or
no population or housing units, that represented the territory affected by
the State or county boundary update (see Figure 10-4).

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10-14   Census Tracts and Block Numbering Areas

Figure 10-4.  

Effect of County Boundary Changes on Census Tracts and BNAs

Example 1: County Boundaries at the 

Example 2: Updated County Boundaries

Time of Block Numbering

3

BIBB COUNTY

PIKE   COUNTY

1

5602

5601

2

3

BIBB COUNTY

PIKE   COUNTY

1

5602

5601

2

2

3

Example 3: Updated County Boundaries and

Renumbered Census Tracts/BNAs

2

Census Tract/BNA Number

County Boundary

Census Tract/BNA Boundary

2.98

5603.98

5604.98

3.98

BIBB COUNTY

PIKE   COUNTY

1

5602

5601

Any changes to a county or State boundary that occur after census block numbering
result in one county losing part of a census tract/BNA and another county gaining part
of a census tract/BNA. Example 1 above shows the boundaries of Bibb County and
Pike County, and the boundaries of their respective census tracts/BNAs at the time the
Census Bureau assigned the census block numbers. When the Census Bureau updated
the county boundaries, Bibb County lost area to Pike County; Pike County 

gained

 parts

of Census Tracts 2 and 3 as shown in Example 2.

After updating the county boundary, the Census Bureau renumbered the affected
census tracts/BNAs by adding the special suffix to the census tracts that 

lost

 area.

As shown in Example 3, the Census Bureau renumbered Census Tracts 2 and 3 in
Bibb County, creating Census Tracts 2.98 and 3.98. The Census Bureau then assigned
new census tract/BNA numbers in the county 

gaining 

territory, using numbers that

fit within the numbering scheme of that county. In Example 3, Pike County gained
Census Tracts 5603.98 and 5604.98.

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Census Tracts and Block Numbering Areas   10-15

To identify all census tracts/BNAs affected by a county boundary update,
the Census Bureau added a special suffix in the range of .70 to .98 (starting
with .98 and assigned in descending sequence) to the basic census tract/
BNA number of each census tract/BNA that lost territory. The Census
Bureau also assigned a new census tract/BNA number to the portion of
the census tract/BNA in the county that gained territory. These new cen-
sus tract/BNA numbers fit within the numbering scheme of each county,
but were identifiable by the special suffix. The addition of these special
suffixes fulfilled data user requests for a flag to identify any areas changed
after the Census Bureau produced the products used in the early 1990
census operations. Because many census tracts/BNAs with this special
suffix have very small areas with little or no population or housing, some
users have chosen to aggregate one or more such census tracts/BNAs
with an adjacent census tract/BNA for data analysis.

Default Census Tract/BNA Numbers

One of the changes brought about by the TIGER System was the need to
include all area (land and water) within a census tract/BNA. Rather than
extending the census tract/BNA boundaries into the Great Lakes or out
to the three-mile limit in coastal waters, the Census Bureau decided to
close off the census tract/BNA boundaries along the shoreline or just
offshore. The Census Bureau then assigned a default census tract/BNA
number 0000 to the coastal and Great Lakes waters not assigned to any
other census tract/BNA.

Relationships to Other Geographic Entities

In the decennial census geographic hierarchy, census tracts/BNAs are
subdivisions of, and nest within, counties (and their statistical equivalents).
The block groups, the next lower level in the decennial census geographic
hierarchy, are subdivisions of census tracts/BNAs and always nest within a
specific census tract/BNA. The Census Bureau assigns census block num-
bers within block groups to identify the smallest geographic areas for
which it collects and tabulates census data. It does this by using the block

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10-16   Census Tracts and Block Numbering Areas

group number as the first digit of the block number. Thus census
blocks are subdivisions of, and nest within, a specific block group.

The relationship of census tract/BNAs to county subdivisions (census
county divisions and minor civil divisions) and places (incorporated
places and census designated places) varies. Many States have incorpo-
rated places such as cities, boroughs, and villages, and minor civil divi-
sions (MCDs) such as towns and townships. The boundaries of some of
these governmental units are not well known locally or shift frequently
as a result of annexations. In these States, the Census Bureau discourages
the use of these governmental unit boundaries as census tract/BNA
boundaries; data users will find that the layout of the governmental
units seldom corresponds to the census tract/BNA framework. In the
New England States, where governmental unit boundaries change infre-
quently and are well known locally, data users generally will find a nest-
ing relationship between census tracts/BNAs and governmental units.
Wherever possible, the Census Bureau has continued the practice of
encouraging congruency between census county divisions (CCDs) and
census tracts/BNAs, and does so by revising the CCD boundaries
when a census tract/BNA needs to change.

The areas and boundaries of other census geographic entities bear no
geographic relationship to census tracts/BNAs because there are differ-
ent reasons for their establishment. Their boundaries, therefore, may
or may not conform to those of the census tracts/BNAs. Such entities
include census designated places (CDPs), voting districts, school dis-
tricts, American Indian reservation and subreservation areas, Alaska
Native villages, and congressional districts. Many data users inquire
about the geographic relationship between census tracts/BNAs and
ZIP Code areas (geographic entities that approximate the assignment
of ZIP Codes by the U.S. Postal Service)—census tracts/BNAs rarely
correspond to ZIP Code areas.

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Census Tracts and Block Numbering Areas   10-17

Notes and References

1

Laidlaw, Walter, “Federation Districts and a Suggestion for a Convenient and Scien-

tific City Map System,” The Federation of Churches and Christian Organizations in

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Federation

, Vol. IV, No. 4, 1906.

2

Swift, Arthur L., Jr., “Doctor Laidlaw’s Vision, the Early Years: 1906-1926, ” American

Statistical Association, 

Golden Anniversary of Census Tracts. 1956, 

Washington, DC:

n.p., 1956.

3

Green, Howard Whipple, “A Period of Great Growth and Development: 1926-1946,”

American Statistical Association, 

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ington, DC: n.p., 1956 [reprinted in the Census Bureau’s 

Proceedings of the National

Geographic Areas Conference, Putting It Together for 1990, 

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Government Printing Office, 1984].


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