Question

What is the meaning of 2 r and 3 r

Answer 1

The 2 parallel red lines indicate metalled roads. 
2r shows that the relative height of the metalled road is 2 metres and 3r shows that the relative height of the road is 3 metres. it can also indicate that the height of the land along the road is 2 metres and 3 metres respectively which indicates a slope in the road

Answer 2

The r repeesents the relative height or depth of a thing above or below sea level.

If it is a mountain, hill or any raised body , r represents relative height. 

If it is a well, tank or depression or any suppresed body r represents relative depth.

Here I think r is for relative height.