Background
I first came across benchmarks doing this cache in Birmingham - http://www.geocaching.com/geocache/GC1KCZZ_how-high. Benchmarks are fun to search for (like geocaches!). Look out for them on the island – there may be more than you think! Read on to find out what benchmarks are.
What are benchmarks?
Benchmarks were made by Ordnance Survey to record height above mean sea level. Once the height of one is known, the height of the next can be found using spirit levelling.
There are many different types of benchmarks and a list can be found here - http://maps.jochta.com/types.htm. All the benchmarks below are examples of pivot benchmarks. They look like an upwards arrow and are carved into stone.
Benchmarks were used for the Ordnance Datum Newlyn (ODN) which is the national height system for mainland Great Britain. The ODN has around 190 fundamental benchmarks which all stem from the single “mother” benchmark at Newlyn in Cornwall. From the fundamental benchmarks, tens of thousands of lower order benchmarks were established.
The Isle of Man has no link to the ODN and therefore has its own local mean sea level datum at Douglas Jetty. All the benchmarks here were levelled in 1866 except the datum at Douglas jetty which was levelled in 1865.
Lower order benchmarks are no longer used so the number of benchmarks in existence is being reduced. When I was looking for benchmarks for this cache, many of the buildings that had benchmarks on have been replaced with newer buildings or the benchmarks have been rendered over.
However, the 190 fundamental benchmarks in the UK are still maintained and used by Ordnance Survey. Unlike the lower order benchmarks, they sit on bedrock so are immune to any slip and movement.
Finding the cache
To find this cache, you will need to visit the site of 4 pivot benchmarks in Douglas and the site of the Isle of Man datum. You can visit these in any order but they are listed below in the suggested order.
The cache can be found at N54.H(A+1).(F+F)(G+F)(D-F-F) W004.(F+F)C.DBE. The values of A-H can be found by visiting the 5 stages.
I recommend driving to the cache location (it is a 5 minute drive or 30 minute walk from the final stage) and there is parking very nearby.
This cache is placed with the kind permission of the Department of Infrastructure.
Victoria Road 1 (N54.09.656 W004.28.618)
This benchmark is 36.546 metres above mean sea level and can be found on the step into one of the properties. This benchmark is clearly visible from the road – please do not enter any private property. Answer the following question: when facing the benchmark, how many water stop taps are there to the left of the benchmark on the pavement below the white wall? A.
Victoria Road 2 (N54.09.602 W004.28.656)
This benchmark is 32.644 metres above mean sea level and is again on the step into one of the properties. It is visible from the road – please do not enter any properties. Answer the following question: What number house does the step with the benchmark on lead to? BC.
Villa Marina Gardens (N54.09.411 W004.28.753)
This benchmark is 24.445 metres above mean sea level. It can be found carved on a building to the right of its entrance. Answer the following question: How many sides does the building with the benchmark on it have? D.
Crellin’s Hill (N54.09.273 W004.28.867)
This benchmark is 24.171 metres above mean sea level and is carved into the wall on the south side of the hill. Answer the following question: How many stones are there between the stone with the benchmark on and the pavement? Do not count the stone with the benchmark on. E.
Douglas Jetty (N54.08.946 W004.28.438)
This benchmark was the Isle of Man datum – this was a known marker to which all other benchmarks in the Isle of Man were related. It was on the end of the jetty on the north side – 1.737 metres below mean sea level. Unfortunately, I do not believe this benchmark is here anymore but I would be happy if anyone could prove me wrong – you need to time your search around low tide though.
Answer the following question: What is the number on the rescue boat on the plaque? FGH.