I don't quite agree that it's a generalised tangent. The cairn at Rhyd Uchaf is where it is because of the terrain and the streams on either side of it. You couldn't move it nearer to Maen Llia without it being in the water and off its hillock. Of course, the cairn could have been the first feature there, then Maen Llia conceivably could have been placed x number of metres from it. But what if the hengiform was there first? And how does all this relate to the broadly contemporary cairns on Fan Nedd and Fan Llia (all of which are intervisible with this group).

I totally appreciate that Cerrig has done more fieldwork there than any of us. But the surveying points still need to be factored in if the equidistance aspect is going to hang together convincingly. You'd be surprised (well, maybe you wouldn't) how many people think that a line they've measured using their fabric tape measure, on a slope, without any correction to take the slope into account, will give the same measurement as if they measure the same line shown on a 2D map that has been accurately surveyed.

It looks from the Coflein notes that the sites may have been measured using modern surveying techniques. As I said, without "modern" (i.e 18th century!) surveying equipment, using only technology available to the builders, it would be very difficult to achieve proper equidistance on site over such uneven terrain, without even so much as a steel chain to use. Far more likely that there would be some degree of error. It may sound perverse, but in some ways I'd be more convinced if the sites appeared equidistant when paced out, but actually aren't when checked using modern equipment.