US Census Geocoding - Frequently Asked Questions - 17.1

Question

How come when I look at the data returned, the FIPS state does not match to the actual state? For example,

2600 ARMY PENTAGON

WASHINGTON DC 20310

The FIPS State code for DC is 11, but I am getting a 51 returned from Census Geocoding which is VA.

Answer

The data source provider tries to place ZIP codes where they are actually located. This may differ from the location (city or county) that handles the mail. Improved placement is part of the value-add.

DC is host to a good number of these examples. The main post office in DC handles mail for many government agencies located outside of the district. That's why some of them are returned with a state of VA or MD rather than DC. The most obvious example is the Pentagon. The mail comes from the main DC Post Office, but the facility is located across the river in VA.

 

Question

Is it possible for a Zip+4 to cross state or county lines?

Answer

ZIP codes can and do cross county and state lines. A ZIP+4 range may span a county line, but its centroid will fall in one county or the other as will a geocoded address from that range. The centroid latitude/longitude and the address latitude/longitude could very well be different. That's why address level geocoding (Interpolated Rooftop+) is considered more accurate. A ZIP+4 range may include both even and odd address and may also run along a county or state line. In these cases, the ZIP+4 centroid will land right on the street and the FIPS assignment is basically a coin toss. An individual address in that range, however, should geocode to the left or right of the street depending on how the data source provider assigned the address.

There are areas where no city style street addressing exists. There are also areas where it does, but the data source provider has not added it in their database yet. Geocoding addresses with these ZIP codes will yield a latitude/longitude based on the ZIP centroid. Depending on the ZIP, this could be the location of the post office or the geographic center of their approximated delivery area.

The following examples illustrate several of these points:

130 ROBERTS DR IUKA MS 38852

State Code = 28 (MS)

FIPS State Code = 01 (AL)

This is a MS ZIP that delivers in both MS and AL. This address falls in AL.

299 CINDY AVE PROTEM MO 65733

State Code = 29 (MO)

FIPS State Code = 05 (AR)

This is a MO ZIP that also delivers into AR. This address falls in AR.

The above two examples illustrate why some ZIPs cross state lines - it's the best way to deal with a formidable natural barrier. In these cases, the state associated with post office will not match the state of the physical address. The data source provider uses the ZIP code to "geocode" an address, but the associated Census geography is derived from where that address actually falls.

 

Question

I am passing PO BOX addresses to the Census Geocoder and not getting back a latitude/longitude that represents a point better than the centroid for the Zip5, why is this?

Answer

Rural Routes, Highway Contracts, PO Boxes, and General Delivery addresses will only get a Zip5 centroid hit. There is street geography in the US that has E911 addressing, but does not have a Zip+4 assignment (usually in rural areas, where the USPS has Rural Routes and POBs). These street address ranges would not be represented in the Zip+4 product, they would return a match during a postal geocoding process.

 

Question

My record failed US Postal Matching but I am getting Latitude and Longitude values. Why?

Answer

The geocoding function of the US Postal Matcher is based on various address components. Even if the US Postal Matcher failed the record, it may be able to return a centroid match based on zip code or city data.

 

Question

In the US Interpolate data the hm_acc_ind (accuracy indicator) has the following description for values:

A: An average distance of less than 25 yds/house

B: An average distance of 25 to 100 yds/house

C: An average distance of 100 to 250 yds/house

D: An average distance of 250 to 750 yds/house

E: An average distance of more than 750 yds/house

Could you explain what "An average distance of less than 25 yds/house" means?

Answer

The accuracy field is used as a general gauge of how accurate the Latitude and Longitude values are that are returned. Because information is provided only on ranges of house numbers as a whole instead of specific house numbers, there will be a certain margin of error. This is due to houses not being exactly the same distance apart from each other or that all the houses in a range truly exist. In calculating the Latitude and Longitude, the average distance per house number and the number of houses on the street segment are used as guides as to how far down a street segment the house is assumed to be positioned. The accuracy field is simply an alphabetic score that is associated with a distance-per-house range.

 

Question

Why don't my FPO/APO military address records get a Latitude and Longitude value?

Answer

FPO and APO designations are for military addresses outside of the United States and do not have a physical address associated with them. FPO addresses are typically ships at sea and APO addresses are Army Postal Servicing centers. These will not have a Latitude and Longitude value associated with them.

 

Question

What if the service member is stationed stateside?

Answer

Stateside military facilities have standard US Mailing addresses and would be candidates to have Latitude and Longitude assigned.