Copt Howe Cup and Ring markings: circles and ellipses

 This boulder in Langdale has rock art on it from perhaps 5000 years ago called "cup and ring" markings.

On inspection, the circles looked perfect. I'm impressed and was wondering how they were able to do this. Chalk on a string method perhaps??

Today I tried measuring the circles on the photographs and found that they have a bigger diameter horizontally than vertically. Now is this because of the angle of the photograph? Next time I'll take a ruler with me but I think it will be easy enough to test if viewing a circle from an angle distorts the perception of circle. I was interested if a circle like this is actually an ellipse and wondered if the ratio of perpendicular diameters gave the eccentricity. Sadly not: the formula for the eccentricity of an ellipse requires knowing the position of the foci. See this