Tuesday, 19 August 2014

Normaal Amsterdams Peil - Amsterdam Ordnance Datum

During a recent visit to Amsterdam I was able to discover more about the Dutch vertical datum; from its humble beginnings in the 17th Century through to being the origin point of the European levelling network.

NAP, Normaal Amsterdams Peil, approximates to MSL, Mean Sea Level.  However its origins date to the Amsterdam Peil (Amsterdam Level) which was based on the mean level of the IJ, the main waterway of Amsterdam.

There is an informative visitor's centre in the City Hall which you can visit for the grand sum of 1 Euro.  As NAP is lower than the ground floor, the exhibition is in the basement. The top of the brass mound sitting on the white pillar is zero.
14 08 14 NAP (7)
And the water levels indicate the current height of the canals.
14 08 14 NAP (2)
The exhibition covers the history of vertical datums in the Netherlands.  It included a display of tide poles, which I forgot to photograph, showing how each region initially had its own vertical datum.  This was a very common phenomena 350 years ago and  improved over the centuries as geodetic networks increased their geographic reach.

The display of levels below took me back.  I remember many hours of levelling the campus at Newcastle University...  I left the exhibition knowing that my Euro had been well spent.
14 08 14 NAP (5)
I then set off to find the only surviving marker stone of the level of the sea dyke in the 17th Century.  But on my way I discovered my first ever Dutch benchmark in Dam Square. The NAP exhibition had a display of these so I knew what to look for.
14 08 14 NAP (8)
There are apparently 50,000 of these in the Netherlands.  So that's only 49,999 to go.

In the pouring rain I then walked to Eenhoornsluis, Unicorn Sluice, Korte Prinsengracht. This level marked the mean high tide during the summer - the height of the sea dyke.  For more details there is a good paper here.
14 08 14 NAP (10)
The inscription reads "Sea dyke's height, being nine feet and five inches above City Datum". What an absolute treat to find.
14 08 14 NAP (9)

Wednesday, 6 August 2014

Arran - 6 Trigs and a Lighthouse

What a great week on Arran!  Not only did I manage to bag six trigs in five days, but I managed to do this in a mix of weather states.  The weather forecasts were sublime.  I'm pretty sure they predicted the same thing each day "sunny intervals with scattered showers" so some days we had glorious sunshine and got burnt, and other days we shivered in our waterproofs.  It wasn't even height related - our highest trig, 874m, was admittedly the wettest bag but on subsequent days (forecast "sunny intervals with scattered showers") it was perfectly clear at that height with the cloud base somewhere near the International Space Station.

Our cottage was in Lamlash and, joy of extreme joys, I could actually see the trig on Holy Island opposite without having to leave the lounge.

So the first trig was TP3987, Holy Island.  A wonderfully hot day which rendered this a shorts and tshirt trig.  Not to mention Budhist prayer flags to add a bit of flair.
14 07 28 Holy Island Arran (28)
It was clear enough to see the Ailsa Craig.
 14 07 28 Holy Island Arran (34)
On the way back down we detoured around the south east of Holy Island to view one of Stevenson's latest lighthouses, and the first square one constructed.
14 07 28 Holy Island Arran (37)
14 07 28 Holy Island Arran (39)
A well maintained lighthouse.
14 07 28 Holy Island Arran (41)
The next day we tackled the highest point on Arran, Goat Fell.  TP3393, Goat Fell, stands at 874m and it's an easy climb up from sea level.  It was pretty cold and blustery at the top as we shivered in the clouds.  We didn't hang around for long.
14 07 29 Goat Fell (3)
On Wednesday I decided we could bag a trig on the way to see the Machrie standing stones.   We parked on the delightfully named String Road, and literally hoicked straight up the ridge. It amazes me that one encounters bogs even at the tops of hills, as we did with this one.  Living near Dartmoor, this shouldn't surprise me at all.

We had noticed the day before that the Goat Fell trig had had a recent coat of paint and wondered if there was a trig pillar maintenance army on Arran.  On arriving at TP0848 An Tunna we were convinced.  Not only had this trig been recently painted, but the grass was still white.
14 07 30 2 TP0848 An Tunna (2)
There is so much white gloss being used that it has filled in the hollows underneath the metal hoops used for lashing one's theodolite to the tribrach.
14 07 30 2 TP0848 An Tunna (3)
And I am certainly not impressed with the black gloss paint over what would have normally been a beautifully weathered brass flush bracket.  Whah :-(
14 07 30 2 TP0848 An Tunna (4)
Anyway, not enough to upset me for long.
14 07 30 2 TP0848 An Tunna (6)
After some standing stone bagging - I wonder if these were prehistoric prototypes of a trig pillar - I realised that our route home took us right next to a drive-by trig.

TP1322, Bennan, is such a trig.  Minimal exertion needed.  It is a drive up a small track and it stands next to a small reservoir.  As expected, this had also met the same fate as the previous white trig.
14 07 30 4 TP 1322 (6)
Now admit it: a trig in the sun with a view is a truly beautiful thing eh?
14 07 30 4 TP 1322 (2)
Thursday's trig was found on the way to see the King's Caves on the west coast of Arran.  This took a couple of goes to reach as our initial approach was via a fern covered slope.  Now I can do ferns...but not when they are taller than me.  So we approached via a forest (and a bit of fence hopping), to find TP6539, Torr Righ Mor.
14 07 31 TP6539 (6)
View across to the Mull of Kintyre.
14 07 31 TP6539 (4)
My last trig was on the Friday after a visit to the Arran Distillery in the north of the island.  Again, an easy find as you simply park up at the village of Catacol and climb up.  When you run out of hill to climb you know you're there.  And there is even something to lean on whilst you await the arrival of your photographer...
14 08 31 TP2046 - Catacol (1)
TP2046, Catacol.
14 08 31 TP2046 - Catacol (5)
14 08 31 TP2046 - Catacol (6)
I never did make it to my planned 7th trig.  On Saturday the forecast weather (sunny intervals with showers) turned out to be pretty wet, as in constant heavy rain.  And although I'm waterproof and don't mind a bit of precipitation, lack of waterproof trousers put me off and the tea shop called instead.  And, anyway, best leave some for my next visit eh?