Measuring in Google Earth

[empip]

Castle Combe race circuit.
Official lap 1.85 miles.

Measured at inside of track (yellow line) 9,710 ft = 1.84 miles
Measured on centre line 1.86 miles
0.5% error not bad ! .. It does undulate a little

 

[securespark]

I wish I had your patience!

 

[tim west]

Looks like you clipped a corner there pip

 

[empip]

Method in the madness !
Looking at some local fields, required a datum measure check... 67 points using 'path', measure at a 'known distance' - the track - Leads me to reasonably assume that measurement in the local area to the track will be pretty accurate using GE.
A useful piece of free kit if there was such a thing.