I am using an API (To get crimes in city ) where I have found a field entitled "Census Tract Number" and am getting data formatted with (longitude, latitude, census tract number). What does this last field represent? Is there any way to find these "census tract numbers"? For example, for the value "census tract= 8100.2010", where can I find data about that census tract number?

The results I receive are:

Nb_census_tract = census_tract_of_address("200 N State Chicago IL 60601")

I found this definition:

Census Tract: The census is conducted by the US Census Bureau every 10 years. It identifies certain geographic areas by census tract and census block. The census track reflects median numbers when it comes to age and income. It is a generalization about a specific area, not by any means an exact science or number.

I tried to load the shape file in Postgis, and I found census tract data on the official website, but it's not specific to every block/building in the city (I found something like: Poverty rate in city, ...) and most of data are older then 2010. Then, I tried another approach--I found a service where it shows census information (even in 2014, although I couldn't find the dataset in the official website). But this service gives you information from an address. I don't want to do geocoding as it takes time and my data is huge.

As far as I know Decennial Census are the data where you can only get fundamental demographic and housing data. I tried to explore this API, but it works by getting lat/long, but in my case I have polygons.

I thought about getting the center of the polygon, but it's not effective.

My questions are:

  • There are many things that are confusing about your post. For one "4700.4003" or "8100.2010" are not valid forms of Census Tracts. I tried entering in my own information into the two links you provided and got results that I expected, which includes proper and recognizable GEOID formats. As far as recent Census data, you could try here. I haven't tested this, but you could see if this provides an open source way of getting at the data.
    – Kotebiya
    Commented Apr 2, 2014 at 10:57
  • @Kotebiya : Thanks ! I am using City-Violent API. You can check : data.seattle.gov/Public-Safety/2014-City-Violent-Crimes-to-date/… . You can find "8100.2010" in one of the raws (USE CRTL+F to find it ) as I can't mention the raw's number, it's updating every-time. Commented Apr 2, 2014 at 17:10

2 Answers 2


I think I know what is going on with the weird Census Tract numbers you are getting. Census tract numbers come in many forms. However, the number you are shown in the Seattle city violent crimes appears to be both the tract number and the block number (e.g. "4700.4003"). I am guessing that the first number prior to the period is the tract number, which is expressible in its GEOID form as "004700", which also shows up as "Census Tract 47", or tract number 47.00. It is inherently 6 digits long, although the last 2 digits can be thought of as decimals and the leftmost numbers may disappear because they are leading zeroes. The numbers after the period are 4 digits long, because block numbers are always 4 digits long no matter what.

Given this information, I would forget about worrying what the Census tract number is. If you have the latitude and longitude I would suggest taking a different approach to matching the locations to areas with demographic data.

Make an account with NHGIS.org (They are a free publicly-provided service) and download the latest data from the 2008-2012 ACS along with corresponding shapefiles to go with it. With these shapefiles in your hands, and the lat/long coordinates of the crime incidents, you may be able to perform some type of intersect analysis to find which Census tract each crime was located in. If you are so inclined, you can do this instead by Census block group which is a smaller geographic division than Census tract, but may suffer from a larger degree of statistical error than Census tracts.

I hope this was helpful.

  • Thanks! This is very helpful. From what I've read, Census data contains many fields from Housing to Population data. In order to do my study I am focusing on every block. Can you please suggest other data sets where I can find data about their housing units, Taxes, householders, .. Do you recommend that I use Census tracking with this API using the classification you suggested: developer.usatoday.com/docs/read/Census Commented Apr 3, 2014 at 1:15
  • 1
    The availability of data is a tradeoff between geographic specificity and availability of reliable data. Basically, the narrower your scope of geography, the less data you'll have and the data that is available suffers from larger margins of error. As far as the link in your comment, since they don't provide instructions on where to get an API key, I can't make any recommendations one way or another. I don't think you need information on every housing unit or householder to perform analysis. The process to even try to obtain that information would take years.
    – Kotebiya
    Commented Apr 3, 2014 at 13:29
  • The Seattle crime data file indicates that the census tracts are "2000 census tracts." Census tracts are adjusted for every decennial census, so you cannot reliably use 2000 Census tract IDs to look up data in the 2010 decennial census or the 2012 ACS. If you are comfortable with GIS (as your question suggests), you can get 2010 census shapefiles for Washington here: www2.census.gov/geo/tiger/TIGER2013/TRACT/tl_2013_53_tract.zip Commented Apr 9, 2014 at 19:50
  • Actually, this person would have to use 2012 shapefiles if they want to use 2012 Data. The Census publishes demographic data for different geographic areas based on the borders coinciding with the shapefile release of that year. But I did not mention that because going to NHGIS.org would have avoided complications.
    – Kotebiya
    Commented Apr 9, 2014 at 21:32
  • I don't believe that census tracts change except as part of preparation for each new decennial census, when adjustments are made to come back to the optimal ~4000 residents. Commented Apr 29, 2014 at 13:18

The US Census now provides KML versions of geographic boundaries. Being XML based, these are much easier to use for non-GIS developers. You can find the KML version of census tracts at:


From the Census site:

The cartographic boundary files are simplified representations of selected geographic areas from the Census Bureau’s MAF/TIGER geographic database. These boundary files are specifically designed for small scale thematic mapping.

  • I would mention to a new project as well, launched two days ago. check it out: plenar.io Commented Sep 25, 2014 at 5:55

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.