UKH

Dotted black line but with 'directional arrows' 

Not seen these arrows before...

I think its the symbol for a pipeline, but its an odd looking route for one.

On Os maps that's a pipeline with the arrows pointing the direction of the flow.

Yup, pipeline.

I'm guessing it's to fill man made Lochs, used for hydro and or drinking water. Minimum height loss being more critical than using the least amount of pipe, just angled enough to flow. There are lots about most are surface channels, not buried. 

Thanks everyone.

I found a pic here: https://www.geograph.org.uk/photo/4520130

There are some on the slopes of Ben Lawers above Loch Tay as well

Hugging the contours, to give a gentle fall. An adit would follow this path, too.

That seems to be bringing water from Loch Eilde Mor to the Blackwater Reservoir, presumably to produce power for the aluminium smelter at Kinlochleven. Am I right in thinking the smelter no longer exists? If so do they still generate hydro power?

The Kinlochleven smelter closed a couple of decades ago; I believe the hydro power is now sold to the grid.

I thought it was a path for the haggis being as they can only walk around the mountain in one direction

It's a one way path. 

Slightly off topic, though interesting to note this particular aqueduct (pipeline) was built by German POWs during WWI: https://her.highland.gov.uk/monument/MHG27947

That really is interesting, thanks.

This reference may interest you too...

"The First World War brought about a massive rise in the demand for aluminium and the Blackwater Reservoir had to be expanded to cope with the extra electricity requirement. Five hundred British troops and 1,200 German prisoners of war were brought in to build a five-mile aqueduct to lead water from Loch Eilde Mhor into the Blackwater."

James Miller The Dam Builders - Power from the Glens (Birlinn, Edinburgh, 2002) p.16

Very interesting. I think this may have featured in a TV programme a few years back (Countryfile possibly?). There's a sentence in there that seems odd

'It held some 1000 German and Belgian prisoners of war, conscientious objectors and 500 British troops of a low medical category.'

Had those Belgians been fighting alongside the Germans or, possibly, were conscientious objectors?

There's water carrying channels in mining areas of the Lakes called leats. Used to carry water to various mine processing buildings downhill while losing as little height as possible. It looks like this is something similar whether enclosed pipes or open channel,  probably a similar feature and use. 

Leats are seen all of the country where there was historical mining, not just lakes. Typical they'd carry water for water wheels or to processing mills and as such they were open not enclosed/pressurised pipe for turbines, a bit like ditches or artificial streams. Often if you examine the map carefully you'll see the top of the leats is a small dam/pool to regulate flow.

This one is a pipe so not the same as a leat, but it's all interesting history to see for those of us who walk about with eyes actually open  

The Romans were fantastic at this kind of thing. I suspect they brought the technique to Britain as I can't imagine the natives had the necessary skills or incentive to do it. A few years ago I went to hunt down the Roman 'aqueduct' shown on the map north of Hadrian's Wall which supplied their Aesica fort. It wriggles around all over the place round the undulations and keeping a very slight gradient. I don't think it would ever have been much more than a dug cutting or wooden troughs, there's certainly nothing much to see now. It starts at a pool in the Caw Burn called Fond Tom's Pool and there is limestone outcropping so it's almost like a little bit of the Dales.

The Scottish equivalent is called a lade.

Have you (or anyone else) read the book?

A quick check of the world's favorite tax dodging online retailer shows it for a penny less than £6 for the Kindle version and I'm tempted.

they are called "Bisses" in French.  often make lovely walking.  A lot were made in the period after the "little ice age" when alpine pastures were being populated and when agricultural practices were changing

They seem to have different names depending on where in France, here in the southern Alps they seem to be 'béal', although I think that only refers to canals/ditches for irrigation rather than lades which, from memory, are mostly for getting water to mills & power stations.

No that's a myth or at least a gross exaggeration, they don't actually fall over. Here's an example from the Glasgow Kelvingrove museum, the leg difference is relatively minor

https://en.m.wikipedia.org/wiki/File:Haggis_scoticus.jpg

Not just mining, but anywhere there was a mill - a Leat is an artificial stream following almost but not quite across the contours, often diverting a stream flow into a reservoir or mill-pond.  Whereas a wooden trough used to take the water again at a gentle gradient across steeper or uneven ground or to supply a mill wheel is called a "Launder" - so called because it would be a place in a village with clean fresh flowing water often elevated above ground level so the best place for washing linen. 

In North Wales there is a system of Leats around the Eastern and Southern edges of the Carneddau redirecting water flow from most rivers/streams towards the Dulyn and Cowlyd systems from where it is used as drinking water supply and for hydroelectric power at Dolgarrog.