Ley lines

Characterizing alignments
One precise definition that expresses the generally accepted meaning of Watkins's ley lines defines an alignment as:

a set of points, chosen from a given set of landmark points, all of which lie within at least an arc of 1/4 degree.
Watkins remarked that if this is accepted as the degree of error, then:

"if only three accidentally placed points are on the sheet, the chance of a three point alignment is 1 in 720."
"But this chance by accidental coincidence increases so rapidly in geometric progression with each point added that if ten mark-points are distributed haphazard on a sheet of paper, there is an average probability that there will be one three-point alignment, while if only two more points are added to make twelve points, there is a probability of two three-point alignments."
"It is clear that a three-point alignment must not be accepted as proof of a ley by itself, as a fair number of other eligible points are usually present."
"A ley should not be taken as proved with less than four good mark-points. Three good points with several others of less value like cross roads and coinciding tracks may be sufficient."
The Leyhunter's Manual (page 88), 1927
Lines and points on a map cover wide areas on the ground. With 1:63360 (1-inch-to-the-mile) maps a 1/100-inch (1/4 mm) wide line represents a path over 50 feet (15 m) across. And in travelling across a sheet, an angle of 1/4 degree encompasses something like an additional 600 feet (200 m).