My geocoder hit the rooftop, but the Post Office says the address is invalid. How’s that?
This kind of outcome is quite possible. When working with spatial applications, it’s routine for me to “validate” and “geocode” addresses. People often use those terms interchangeably. Yet they are two distinct processes, which can sometimes yield contradictory results. Neither process can be said to be better than the other. It’s a matter of context.
When we “validate” an address, generally we mean that we are determining if the address exists. Within the US, the Postal Service (USPS) is the definitive source, through its Delivery Point Validation (DPV) process. Various vendors are licensed by the USPS to offer the process, including Pitney Bowes and Melissa Data.
When DPV processing flags an address as valid (i.e., the DPV value is Y), this means that the address exists. For example, DPV processing of the address of a newly constructed house at 705 Maddie Way, Marietta, GA, yields a Y. Already, it shows up clearly on Google Maps.
Since the address exits and is valid, you might expect it to “geocode” to the parcel or rooftop quality. Not necessarily. Passing DPV means the local jurisdiction has assigned an address and the Post Office has incorporated the address into its directory. On the other hand, geocoding an address is an entirely separate process, completely independent of the USPS.
Indeed, two of three geocoding vendors I checked return a parcel level geocode for 705 Maddie Way (i.e., spot on). The other vendor has some catching up to do, returning a geocode at the Postal Code level of resolution (i.e., ZIP Code centroid). So the address exists, per DPV, but the geocode quality can vary by vendor. Over time, though, we would expect all geocoding vendors to converge on the same or similar geocode.
At this point, you might conclude that DPV is always the way to go in evaluating your address data. Not so fast. The key phrase in DPV processing is that it determines if an address “exists” from the perspective of mail delivery. Consider 2930 Robinson RD, Marietta, GA, shown below in this image from Google Maps, and which is still representative to this day.
Until recently, an old farm house existed on this large lot. Plans were at one time to backfill in a cluster of new houses in place of it. This has yet to happen. The original address of 2930 Robinson RD still exists, is still known to the Postal Service, and therefore DPV processing still confirms the address with a status of “Y.”
From the geocoding perspective, three geocoding vendors resolve the address to the parcel center. So delivery point validation and geocoding can determine if a deliverable address exists, but not that a house or structure actually exists on the lot.
The address 296 Saxton ST in Rochester NY makes the point that DPV and geocoding sometimes can yield contradictory results. At present, a vacant lot occupies the address. Geocoding returns a parcel centroid, implying that the address exists. Yet DPV rejects the address (with a DPV value of N).
Further, neither process means a house on the lot is occupied. 3005 Canton Pines, Marietta, GA is still under construction. The house at 411 Jay St, Rochester, NY is boarded up. Yet both geocode to parcel level centroid as well as pass DPV with a score of Y.
One last point to make is that DPV processing considers the complete address, including an apartment number or suite. With geocoding, you are really mapping a parcel of land, and therefore the unit number is irrelevant.
Consider the large, multi- unit building at 4100 Massachusetts Ave in Washington DC. Omit the apartment number from the address (or get it wrong) and you’ll get a D (or S) from DPV processing. Likely, you’d have a problem delivering a letter to someone in the building unless you included the the apartment number in the address.
Finding it on a map, though is not a problem. It geocodes just fine, apartment number or not. Even geocoding it with a false apartment number yields a correct point. Generally, any address with a DPV code short of an N (or blank) means at least that the address of the lot exists, and therefore should geocode just fine – assuming the geocoder is as up to date as DPV.
￼Before selecting and paying for both geocoding and Delivery Point Validation (DPV) processing services, first evaluate your situation. If spatial analysis is your primary objective, and you feel your address data is decent, then likely a geocoding service is all you need. However, if you think your address data is substantially flawed or at best “dirty,” then Delivery Point Validation could be important in weeding out the flawed data. However, just because your addresses pass geocoding and DPV does not mean all of them correspond to warm bodies.