28 September 2012

Benchmarking time: Mauna Kea, Hawaii

Posted by Jessica Ball

‘Benchmark’ is a general name for a fixed and marked point with a known elevation and location that is used for surveying. My first experience with benchmarks was through archaeological excavations, when we used them to provide a datum point for all the measurements (horizontal and vertical) that we made on the dig site. Many benchmarks are metal disks cemented into the ground, but they can also be engraved in stone (or even in buildings, as I found when I participated in a dig in England; our benchmark was on the back wall of an old church near the site). They’re often located on some high point, such as the summit of a mountain, but they can be found in any place where someone needed to do surveying (or other kinds of position-dependent work). Many of these markers are maintained by the NOAA’s National Geodetic Survey, although the USGS also places their own. The Geocaching website goes through all the technical terminology for the different types of geodetic markers, but most geologists I know just call them benchmarks.

In the course of my field work, I’ve gotten in the habit of ‘collecting’ benchmarks with photos, but I didn’t know hunting for benchmarks was an actual named activity! The great Wikipedia assures me that ‘benchmarking‘ or ‘benchmark hunting’ is an actual thing, with its own fansites and everything. The Geocaching.com website even has a section devoted to benchmarking. It’s a big thing. At any rate, I thought it would be a fun activity to post photos of my benchmarks each week – and, like Callan’s Friday Fold, to ask for photos of your favorite benchmarks! I’ll start with my favorite, the USGS benchmark on top of Mauna Kea’s highest point.

I've got this under control...

13,796 feet above sea level (but 33,476 feet from base to summit!)

As you can see, most of the USGS benchmarks show their elevation (in feet) and the date the benchmark was put in place. Because I was able to wrap myself around this one, I’m going to assume that some erosion went on in the vicinity of the disk, which was presumably flush to the ground surface at some point. (Another little tidbit – you can buy a copy of this benchmark at the Mauna Kea Visitor Information Station, if you’re utterly geeky like me.)

I have a limited number of benchmark photos, so I’d like to intersperse my own with any you’d like to share! Feel free to email me at magmacumlaude AT gmail DOT com with benchmarks you’d like to show off. I’ll post a new one every week, with a little backstory each time.