But the sea isn't level

Our house is a shade over 600 metres above sea level. If you say that in feet it's just shy of 2,000 feet which, in the UK, would be hilly. The Yorkshire Three Peaks Challenge with Pen-y-Ghent at 694 metres, Whernside at 736 metres and Ingleborough at 723 metres are all a bit lower than the humble, but 800 metre high hill, Xirivell, at the back of our house. Just a little further away the Sierra del Carche range, which you can see from Pinoso and which you drive alongside on the way to nearby Jumilla, rises to 1371 metres which is just a few metres up on Ben Nevis at 1345 metres. That said the Grampians, the Lake District or the Machynlleth Hills call for high tech footwear, cuben fibre gear and trekking poles while Xirivell is much more a flip flops and shorts hill. The difference is the height of the surrounding flatland.

The Spanish Ordnance Survey, the National Geographic Institute (IGN from it's initials in Spanish), began work on the first topographic maps in 1857. One of the essentials for making maps which show heights, the topography, is to have reference points. The most basic of these is a base level and it's sea level that is used. The trouble is that the sea level, for all sorts of reasons, varies from place to place on the various seas and oceans. The IGN decided that it would use Alicante as the place to measure sea level to give the basis for heights in Spain. They chose Alicante because it was quite easy to measure sea level there using basic instrumentation. The Mediterranean is relatively calm, the tidal variation is limited and it's a comfortable place to be based. Cádiz or Santander might have been just as comfortable but the Atlantic has much bigger tidal variations and more frequent and violent storms. 

So, between 1870 and 1872 the IGN measured the sea level just by where, nowadays, there is a statue which is called El regreso de Ícaro con su ala de surf - it's the one that I always thought was a surfer until I wrote this piece. Apparently it's a homage to those who try to dream and soar. The IGN used a metal ruler fastened to the wall with divisions every 5 mm and they measured the sea level every three hours between 9 in the morning and six in the evening for a couple of years. The mean variation was 0.43 metres. That gave them their sea level, the mean sea level, Point Zero or Cota cero in Spanish. I'm told there is a metal plaque on the seawall to mark the spot though I've never noticed it myself.

Now the IGN had its baseline they needed a spot on dry land to measure from. They chose the first step of the main staircase in Alicante Town Hall. Heaven knows why. The Town Hall is just 230 metres in a straight line from Point Zero and that first staircase step is 3.4095 metres above mean sea level. This measuring point was given the name NP1. It's still the point from which the height of anywhere in Peninsular Spain is measured. If you're ever in Alicante it's easy to have a look. Anyone can walk in the front door of the Town Hall. The staircase is obvious enough, though you may notice that funny golden Dalí statue before it. You probably won't notice the rather unimpressive bronze plaque, which is NP1, without looking for it. Then again the standard inch, foot, yard etc in Trafalgar Square aren't exactly overwhelming either. 

Once the IGN had something to go on it developed the precision levelling network (Red Española de Nivelación de Alta Precisión) which wasn't completed till 1924. By drawing 92 lines between various places running along more than 16,500 kms of  railway lines and roads they were able to place over 18,000 signs which marked fixed heights. Nearly 2,200 of those marks were principal signs which were placed in public spaces like town halls and railways stations. You will see them from time to time as you wander around Spain. The one in the header photo is from Villena railway station.

It has to be said that in today's world a metal ruler on a seawall isn't as accurate as it gets. The real variations in sea level depend on lots of factors, like the curvature of the earth, the amount of ice held in the polar ice caps, the phases of the moon and the like. Nowadays there are various tide meters dotted around the Spanish coast and satellites also measure sea level - I presume though I don't know - that the two can communicate with each other so that there is accurate information about actual and mean sea levels at any time. Nonetheless, as we look out from our garden to the Salinas de Sierra mountain chain opposite we can say that the Pico de la Capilla is at 1238 metres above sea level at Alicante and that's close enough for us.