Saturday, September 26, 2020

Bench marks

Okay, we all know what a benchmark is, don't we? It's a standard, fixed point, maybe for pay, maybe for achievement, or cost, against which other more variable points are measured. If I do a slightly different job than, say, a nurse, my pay will go up or down pro-rata with that of the nurse. The nurse's pay is the benchmark by which my pay is calculated.
But, and I'm not being a smart arse here because I only learned about it a few weeks ago, did you know that we pass benchmarks all the time in our daily lives, yet, very few of us have actually seen one? (Yes, you probably knew that and I'm way behind the times.)

Basically, it's a surveying thing, dating from the mid-nineteenth century. Apparently, it was important to the Victorian builder that they knew the height above sea level of every new building. What they did, they marked a point on a building - one that would likely survive for a while - where the precise height above sea-level had been calculated. This was occasionally marked by a plate screwed into the stone, or, more usually, by a cut horizontal line with an arrow beneath it. In rural areas, the frequency was five benchmarks per one square kilometer; in urban areas, between thirty and forty. By use of theodolites and measuring angles and distances between benchmarks and new builds, any point in Ireland could be calculated for height above sea-level.

So, how do you know where they are? Well, you have to check old Ordnance Survey maps. These can be found online here. Simply select, say, Historic 6" Black and White and then zoom in. Sometimes, the maps say B.M. and a number, sometimes merely the arrows and a number. The number refers to the feet above the national benchmark. In the map below, of Dublin city centre, there are benchmarks on Queen Maeve's Bridge (middle, east side), on the river wall, half way along Usher's Quay, on the corner of Arran Quay and Fr. Matthew Bridge etc.

19th Century OS map of Arran Quay and Usher Quay, Dublin

So, armed with the map on my phone, I set out walking to find me some benchmarks. I had been warned that many might no longer exist, due to weathering or buildings being knocked down or resurfaced but I was surprised that the first five I came across, I couldn't find the benchmark. The problem seemed to be that these were tall buildings and had a huge area to check for what might be a very faint mark. It was only on the sixth and subsequent sites that I realised that all the marks were on the block above ground level. Not sure if this is true for every benchmark but certainly, all the ones I found were less than a foot above ground.

Benburb Street, B.M. 23.6

Arran Quay / Fr. Matthew Bridge B.M. 24.7

Usher's Quay (directly opposite St. Paul's Church B.M. 24.4

Queen Street Bridge (adorned by stone) B.M. 29.1


South-west corner of Rory O'More Bridge B.M. 21.4

The Law Library (formerly King's Hospital) where the front facade turns inwards, right hand side B.M. 39.7

All very interesting but this is a lighthouse blog. What is the relevance to lighthouses?
Well, the National Benchmark - the mark from which all other benchmarks were measured -  was the mark recorded on the 8th April 1837 at a Spring Tide at Poolbeg Lighthouse. This benchmark remained for well over 100 years, until they discovered that the spring tide at Poolbeg was nearly seven feet lower than that at Malin Head. Since 1970, Malin Head was the new benchmark, although all this theodolite stuff has now been superceded by GPS. But its an interesting part of our history nonetheless.
More can be found on benchmarks here


  1. Hi Pete, I really like the blog. Can I ask, where did you source the maps showing the Dublin benchmarks?

    1. Yes, the maps are from the Irish Townland and Historical Map Viewer. Takes a bit of navigating but once you get the hang of it, its easier. You need to go into Layer and select either 1st OS edition (1820-40) or Last OS edition (c.1900) The link is

    2. Thank you for sharing this, I really appreciate it. Keep up the good work.