How high? Multi-cache
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Difficulty:
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Terrain:
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Size: 
(micro)
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Background
An educational multi for my 30th offering, this cache is based on some of the geodetic "bench marks" to be found in central Birmingham. The cache should not take more than 30 minutes. I didn't know what a bench mark was until I had to look them up online having DNFd a cache in Devon at a bench mark. Now I know what they look like, I see them all over the place! If you have no idea what I'm going on about, read on...
About benchmarks
I do not pretend to be an expert on bench marks so the quotes below are lifted straight from the Ordnance Survey website:
"Ordnance Survey bench marks (BMs) are survey marks made by Ordnance Survey to record height above Ordnance Datum. If the exact height of one BM is known then the exact height of the next can be found by measuring the difference in heights, through a process of spirit levelling.
Most commonly, the BMs are found on buildings or other semi-permanent features. Although the main network is no longer being updated, the record is still in existence and the markers will remain until they are eventually destroyed by redevelopment or erosion.
Bench marks are the visible manifestation of Ordnance Datum Newlyn (ODN), which is the national height system for mainland Great Britain and forms the reference frame for heights above mean sea level. ODN is realised on the ground by a network of approximately 190 fundamental bench marks (FBMs). From these FBMs tens of thousands of lower-order BMs were established. The network has had little maintenance for 30 years, and in some areas (mining areas for example), subsidence has affected the levelling values. In these regions the BMs cannot be relied upon to accurately define ODN. There are approximately 500 000 'lower order' BMs still remaining. This number is reducing due to property development, road widening and so on."
There are numerous types of benchmark in existence. The best summary of these I have found is on this site.
There are three orders or levels of precision in the height measurement of bench marks with one or first being the most accurate (FBMs) and three or third being the least accurate. On the Ordnance Survey site, bench marks are quoted to four decimal places of height in metres. Third-order BMs are precise to two decimal places and second-order BMs are precise to three decimal places - so still pretty accurate!
About the cache
To find the cache you will need to visit three of the 47 bench marks recorded in the square km referenced SP0687 (type or paste this number into the search box on this OS linked page to see them all). Specifically, you will see one of the ubiquitous "cut bench marks", one of the reasonably common "flush brackets" and a "cut bench mark with bolt" (described on the site linked above as "old and rare" and "highly prized"). You can visit them in any order but the suggested start point is at the listing co-ordinates. The trail is wheelchair accessible but a companion may need to recover the cache itself.
The cache is located at N52 2A.BCD W001 5E.FGH. The values of A-H are derived as follows.
The cut bench mark [N 52 28.871 W 001 53.961].
This is a third order bench mark given at 140.1016 metres above mean sea level cut into the West angle of St Philip's Cathedral. Directly behind you is a gravestone remembering Mary and Richard Morton. Richard Morton was A7 years old when he died on H May 1838. Mary was G8 years old when she died.
The flush bracket [N 52 28.813 W 001 54.235]
This is a second order bench mark given at 140.2238 metres above mean sea level fixed onto the Chamberlain Square wall of the Council House, immediately below the clock tower. Just around the corner is a plaque giving details about the clock tower and its bell. Measured from pavement level, the clock tower is 152ft Ein high. There are 15B steps to clock face level.
There is another second order flush bracket on the corner of the Council House nearest the Town Hall.
The cut bench mark with bolt [N 52 29.109 W 001 54.378 - this is an average of two visits where the co-ords (both averaged for several minutes) were wildly inaccurate on googlemaps!]
This is a third order bench mark given at 128.4671 metres above mean sea level cut into the North West corner of St Paul's Church. About 10 metres East of the mark is a plaque on the church wall in memory of Henry H Aston who died in 1D6C. Subtracting the second digit from the first digit of his age at death gives F.
There is also a very rare "pivot" bench mark on a step in the North west corner of the square. Thanks to baggie boy for finding it!
The cache itself
... has been upgraded (5 June 2009) from a 35mm film pot to a small clip lock box with room for small TBs, coins and trade items. If you trade, please trade up or even. Bring something to write with. The final location has not been chosen for its beauty or charm but for what I hope proves to be its longer term security. Nevertheless, please watch out for muggles.
Waymarking
For those of you who are into waymarking, I have set these three and a few other city centre bench marks up as waymarks on waymarking.com. So you can get two finds for the price of one! The waymarking.com links for the three locations you have to visit (including pictures of the bench marks) are:
St Philip's - WM5GXA .
Council House - WM5GXD.
St Paul's - WM5GXF .
You have to take and upload a photo of the bench mark to log a find of these waymarks. You can look the rest up for yourselves!
Additional Hints
(Decrypt)
Qverpgyl haqre gur gbc fgrc. Vtaber gur boivbhf. Gehfg lbhe TCF.