OpenStreetMap
Yes, these house numbers in Portland really have leading zeros. As the numbered avenues increase from east to west, they increment by 100 for every block. For example, buildings between SW 19th Ave and SW 20th Ave are between 1900 and 2000. In the city center, SW 1st Ave is very close to the Willamette River. However, as you head south the river bends leaving a bunch of streets that are east of SW 1st Ave, but still west of the Willamette river. Mathematically speaking, it might make more sense for them to have negative house numbers, but I guess just adding a leading zero has the same effect.
Looking at various maps providers, these are often not really handled correctly and the leading zero is often not preserved. It will be interesting to find out if there are any addresses where the only differentiating factor is the leading zero. The hilly terrain prevents building houses on both side of the SW 1st Ave axis in most cases.
Discussion
Comment from raykendo on 6 October 2014 at 13:57
Sounds a little like Chicagoland suburbs. Areas not incorporated by a city would receive an address prefix based on a mile-wide grid centered on Chicago. So, a north-south street a little more than 3 miles north of the centerline would be 3N###.
The problem is, the numbering system is zero-based. I have an address that is 0N###. This doesn’t work well with most address searches. I often have to drop the “0” when I want directions from most major direction search engines.
Comment from Ryan_Peterson on 9 October 2014 at 17:41
There is a case where the only difference is a leading 0. There is 841 SW Gaines St up by OHSU on top of the hill, then there is 0841 SW Gaines St down in south waterfront. This is the only example I have found so far, but I wouldn’t be surprised if there were more.
Comment from FTA on 31 October 2014 at 02:54
I am glad to see that OSM DOES retain the leading 0 :)
Comment from Michael Kale on 30 June 2015 at 03:54
Thank you Peter for this explanation! I’ve always been confused about those addresses.
Comment from Peter Dobratz on 30 June 2015 at 04:18
I think the whole situation is unusually confusing. I’m not sure about the exact methodology for determining house numbers, but it seems like in many cases “repeats” are avoided by cleverly selecting the numbers. However, there are a number of addresses that only differ by the presence or absence of a leading zero. Here is the list of such addresses that I’ve found so far:
I would imagine it would be quite confusing for people unfamiliar with the area to navigate to these places.