What do the symbols of triangle and circle averaged over the Nashua River Rail Trail?

We biked from Ayer, MA, at Pepperell, MA, on the Nashua River Rail Trail. Along the way they circle and triangle symbols painted on the pavement, but I could not figure out the sequence of symbols of meaning or if it is placed at some distance from the role of specific itinerary. I have not found the answer in some online searches. Does anyone know? Thank you!

which means there is a campsite near where it was a triangle inside a circle.

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