American Community Survey vs. Decennial Census: What’s the Difference

Photo by timothy muza on Unsplash

This post explains the differences between the American Community Survey (ACS) and the Decennial Census and how those differences will impact your work.

Background: Long Form and Short Form

Remember back to the Census 2000 when some people were complaining about the Census form taking forever to fill out? Well, those people received the Census 2000 Long Form which was sent to about 18 million households and collected detailed demographic, economic and housing data. The rest of the US households received the simple Census 2000 Short Form, which was used to count the population and collect basic demographic data. So for the Census 2000, 2 forms were used: the Long Form and the Short Form.

Fast-forward to the Census 2010. The Census 2010 did not use the Long Form, only the Short Form. Enter the ACS. The American Community Survey (ACS) is an annual demographic survey of the United States. It provides the detailed demographic, economic and housing data that was once supplied by the Decennal Census Long Form.

2000 2010
Count + Basic Data Short Form Census 2010
Detailed Data Long Form ACS 5 year estimates

However, the ACS is only sent to 3 million households each year, a statistically small sample especially when compared to the Census 2000 Long Form which was sent to 18 million households in 1 year. So to produce the detailed demographic data, the ACS combines data from several years to produce multi-year estimates. For example, the ACS combines data collected from 2006 + 2007 + 2008 to get 3-year ACS estimates for geographies with at least 20,000 persons. ACS 5 year estimates, which will be out in December 2010, will provide data for geographies with fewer than 20,000 persons.

Similar Data; Different Methodologies & Format

The ACS and the Decennial Census provide the same types of data data–that is, demographic, economic and housing data for the US. However, the methodology of these 2 tools differs greatly.

ACS Decennial Census
What is

It?

Continuous

demographic

survey

Count of the

population

Data

Average

characteristics

OVER TIME

Counts characteristics for a SPECIFIC DATE
How Often

Every 1 year

(more timely)

Every 10 years (less

timely)

House-

holds

11 million households over 5 years

18 million households

in 1 year

Sampling Error

Higher Error (1.75

percent

higher than Census

2000*)

Lower Error
Accuracy Less Accurate More Accurate

*[Revised: this information (and much of the blog post) is based on the Census Bureau’s Brian McKenzie’s awesome presentation here]

To sum up the table above, the ACS more timely but less accurate than the Decennial Census, because the data is collected over a series of years (instead of just 1 year), and fewer households are surveyed.

Because the ACS is less accurate, ACS data includes margins or error and is released at 90% confidence limits. Here’s an example of ACS data.

Austin city
Estimate Error Percent Error
Hispanic/Latino 261,672 +/-3,404 35.0 +/-0.4

This means that we can say with 90% confidence that the number of Hispanic persons in Austin is between 258,268 and 265,076 (or 261,672 plus or minus 3,404). We can also say with 90% confidence that the percentage of Hispanic persons in Austin is between 34.6% and 35.4%.

How Does All of this Impact You?

  1. ACS demographic data is less reliable than Decennial Census data. It’s going to be more important than ever use ACS data as a starting point only and to support the data fieldwork, interviews, and data from other sources.
  2. Doing basic calculations with ACS data is going to require an elementary understanding of statistics. For example, calculating percentage change between Census 2000 data and ACS estimates will require basic statistics, because the numbers are not comparable (they are apples to oranges).

If you’re interested, let me know in the comments if a blog post on how to accurately calculate comparisons using Decennial Census data and ACS estimates would be helpful. Or you can check out the Census’ guidance on the topic.  Any questions about the differences between ACS and Decennial Census data? Are there still points of confusion?

Right now, you can get ACS data for your projects in seconds with the Free Poverty Report beta. When the ACS 5 year estimates come out in December, this data will be part of the Cubit system.

National Land Cover Data Set: when there’s no Local Land Use GIS data

Co-written by Aaron Herman

Finding land use land cover data is a pain. Unless your project is within an MPO/COG that has historic and current land use shapefiles that are easy for your GIS guru to access, you can be up a creek without a paddle when it comes to finding good land use data.

Despite the data challenges, many planners HAVE to have land use data for their projects. For example, over 90% percent of transportation environmental impact statements address land use impacts in their indirect impacts analyses according to this unscientific survey. So what should you do if your project is in an area that doesn’t have local land use data? Or what if you have a giant project area and the land use data set for one area doesn’t jive with the land use data set for another area?

USGS Land Use Data

If you can’t find good land use data from the typical local venues (I’ll blog about “typical local venues for land use data” next week), check out the US Geological Survey’s 2001 National Land Cover Data Set (or USGS NLCD 2001). The USGS 2001 National Land Cover Data Set is the latest, most accurate, nearest-to-complete and standardized land use coverage for the entire United States. Yup, you can get land cover data for the whole USA enchilada.

NCLD2001
2001 National Land Cover Data Set

Positives of the USGS 2001 National Land Cover Data Set

  • Nationwide land use data for the entire United States
  • Standardized land use types makes for easy apples-to-apples comparisons
  • Have historic data & are working on more current data set (2006) for visualizing land use trends over time

There is a USGS NLCD 1992 data set. And the land use experts over at the Multi-Resolution Land Characteristics Consortium are working on a USGS NLCD 2006 data set. You can use these USGS National Land Cover Data Sets to visualize land use changes over time.

Visualizing Land Use Change Over Time with National Land Cover Data Sets
Visualizing Land Use Change Over Time with National Land Cover Data Sets

Negatives of the USGS 2001 National Land Cover Data Set

  • 2001 is old
  • National land use data won’t be as detailed or current as local land use data
  • The data is provided as a image. While it’s possible to convert the image to acreage estimates, it’s technically difficult to do and would involve custom scripting.

Head to Head Battle: Local Data vs. USGS 2001 National Land Cover Data

Generalized Land Use 2005, Twin Cities Metropolitan Council
Generalized Land Use 2005, Twin Cities Metropolitan Council
2001 NLCD Data
2001 NLCD Data

Let’s compare the land use data provided by Twin Cities’ Metropolitan Council and the USGS NLCD 2001. I was able to download Generalized Land Use 2005 data from the Metropolitan Council for the seven-county area. For comparison, displayed below is the USGS 2001 National Land Cover data for roughly the same area. These images are pretty small–if you need bigger images or want more information about land use land cover data sources, scroll down to the end of the blog post where you can get more information in a pdf.

You can get much more specified data with the local Metropolitan Council data set. For example, I can determine Single Family Attached versus Single Family Detached versus Mobile Homes using the Metropolitan Council’s data. The USGS 2001 National Land Cover data provides generalized data on Developed High Density, Developed Low Density and Developed Medium Density. But not every project area has good, accessible land use data like the Metropolitan Council data. And that’s where USGS 2001 National Land Cover Data comes in handy.

Bottom Line

If you have a small project in an area that has good local land use data, local data is your best bet. If you have a large project area (think multi-county, multi-MPO/COG, especially multi-state) or you can’t find good local data, then check out the USGS 2001 National Land Cover Data Set.

If you need more information about where to get land use land cover data, Aaron Herman researched and wrote an excellent whitepaper on the subject. This blog post is based on his research. If you want a pdf of his whitepaper, contact me here and I’ll send it to you.

Do you have another favorite back-up land use data set that you use? Leave a comment so we can check it out.

Road Miles by State

Photo by Dan Gold on Unsplash

This blog post was updated on March 25, 2019. I find myself googling for this data once a month or so. I just can’t seem to remember where to find it on the Federal Highway Administration’s website. Below are states by total 2017 lane miles–which is the most recent dataset available today and is dated August 2018.

Road Miles by State: Sorted from Most to Least

STATE TOTAL LANE MILES
Texas 679,917
California 394,383
Illinois 306,614
Kansas 289,948
Minnesota 286,708
Missouri 276,619
Florida 274,149
Georgia 272,017
Ohio 262,377
Michigan 256,207
Pennsylvania 251,271
New York 239,763
Wisconsin 239,027
Iowa 235,048
Oklahoma 234,729
North Carolina 227,544
Alabama 211,339
Arkansas 210,532
Tennessee 203,474
Indiana 202,417
Nebraska 193,712
Colorado 184,913
North Dakota 177,882
South Dakota 167,838
Washington 167,112
Kentucky 167,092
Virginia 163,648
South Carolina 162,694
Oregon 162,575
Mississippi 161,909
New Mexico 161,015
Montana 150,257
Arizona 144,959
Louisiana 130,020
Idaho 107,376
Utah 103,208
Nevada 101,666
New Jersey 85,000
West Virginia 80,114
Massachusetts 77,557
Maryland 70,792
Wyoming 63,319
Maine 46,851
Connecticut 45,855
New Hampshire 33,328
Alaska 31,597
Vermont 29,276
Delaware 13,954
Rhode Island 12,741
Hawaii 9,781
District of Columbia 3,454
U.S. Total 8,765,578
Source: U.S. Department of Transportation Federal Highway Administration. Office of Highway Policy Information. Highway Statistics Series. Functional System Lane-Length – 2017. Released August 23, 2018. You can download an Excel or PDF version of this data here.

Pie Graph Showing 2017 Lane Miles by State
And just for fun, here’s a pie graph of this table.

Helpful Links with Road Miles by State data
If you need historic data like lane miles for 1980 to 2007, check out Functional System Data; Estimated Lane – Length page. If you need Function System data for 2008 (i.e. interstates, freeways, arterial, collectors and local), check out FHWA’s Functional System Lane-Length – 2008 Lane-Miles page.

And below are links to FHWA’s Functional System Lane-Length by year. You can download PDF and Excel versions of the data on these pages.

Got questions? Email us.