The Geoid

Getting back to normality today.

We took Caley to the beach at the west end of Loch Laggan but as we suspected it was gone.
As we drove past Laggan Dam the water level was just about lipping over the spillways and the operators had one of the syphons open. The dam in full overflow is quite spectacular but the syphons alone project a massive foamy jet which thunders down to the gorge below and generates huge clouds of mist. (It’s a good blip to keep in mind for a rainy day).

Loch Laggan is ultimately controlled by the dam which spills at approximately 250m AOD (above ordnance datum). Given that Loch Laggan proper is about 6km upstream of the dam I would guess that the Loch was a couple of feet higher again. When the dam is low in summer the water course between the Loch outlet at Moy and the dam is more of a meandering river with a couple of sections breaking in to rapids. In this situation the difference in level between dam and loch can be over 4 metres.

Anyway, when we got to the loch there was a lightly trodden track on sodden grass at the top of the submerged beach. It was full of interest for Caley, for all of us in fact.
AOD; there is an interesting concept. I have listened to some very odd interpretations of the heights of mountains (of anything in fact) as noted on OS maps. In respect of Ben Nevis I recall one young man thinking it was the height measured from the gate passed at the start of the tourist path. Another more mature gentleman suggested it was related to the sea level at Loch Linnhe.
The heights or summit levels of all our mainland mountains are indeed related to the sea level but specifically the average sea level as observed between 1915 and 1921 at a small harbour at Newlyn in Cornwall. An earlier (now redundant) datum related to docks in Liverpool.
The advent of the GPS surveying techniques adds a layer of complication to this as the OS datum which we perceive as level (how water behaves in a lake) actually undulates dramatically with respect to the datum used by GPS which is a smooth mathematical model spanning oceans and continental plates. Despite its apparent magical powers we can not accurately measure height by GPS without some prior knowledge and measurement of this undulation. The OS are continuing to refine this “geoid model” which may result in some mountains changing height marginally.