If you go to Lownsdale Square in downtown Portland and look closely around the base of the old Spanish-American War monument, you'll eventually come across the tiny metal disk pictured above. You might have seen disks like it around town before; like the others, this disk is a surveying benchmark. Benchmarks are altitude references, basically adding a vertical dimension to go along with the latitude & longitude info provided by milestones and such.
The benchmark in Lownsdale Square isn't just a random one I bumped into. It reads "Initial Class A Benchmark No. 00", which seems to indicate that it's the fundamental datum that the others in town are measured relative to. Which is actually a bit confusing, since the city's history page about benchmarks mentions a different location, part of a building at the SW corner of Front & Washington. So I'm not sure how to explain the discrepancy here. I'll update this post if I happen to learn the answer.
In any case, the City of Portland has one system of benchmarks, and the city also has benchmarks for various nationwide systems as well. For example, contrast this benchmark with the one at the top of Elk Rock, which was emplaced by the US Coast & Geodetic Survey (now the National Geodetic Survey) rather than the City. For info on what all the markings on the latter disk represent, this forum thread seems to discuss them rather exhaustively. Coincidentally, it seems that Elk Rock tops out at almost exactly 200 feet above the height of benchmark 0 if I'm understanding things correctly. Benchmark 0 is 56.667 feet above sea level, while the one at Elk Rock is 257 feet.
PortlandMaps has a layer that shows both extant and missing benchmarks. This map shows the vicinity of benchmark no. 00, with five more extant ones and two missing ones within a 3 block radius. The Transportation Bureau offers a separate benchmark locator that can search by various criteria, including street name, benchmark number, and section / quarter section identifiers. The City of Lake Oswego's benchmark catalog is online too (with ~350 benchmarks), and info about Clark County's system is here, for readers in the 'burbs who don't want to miss out on all the geeky fun. The City of Portland benchmarks alone surely have to number in the thousands, so making a project out of finding them all seems a bit tedious and unrewarding.
Not everyone is as easily dissuaded as I am. It turns out there's an entire hobby around "benchmarking", which is sort of like geocaching without the caching. Geocaching.com has a page about Benchmark Zero including a comment thread with several exciting photos similar to the one you see here.
This point is curiously close to the putative zero point for the Stark Street Milestones, which is said to be at the Multnomah County Courthouse next door. The benchmark obviously came later though. It's affixed to a memorial for a war that happened decades after the milestones went in. The metal disc that replaced the original Willamette Stone also serves as a benchmark, as it turns out.
As you might imagine (if you've thought about it at all, which I hadn't until now), figuring altitude above sea level is more complex on water than on land, especially when you have both tides and varying river levels to worry about. For some info on how this works in our part of the world, see a paper from NOAA's National Ocean Service: "Vertical Control in a Tidally Influenced Complex River System". FWIW.
