Yellowlegs (Sandpiper) - Is it a Greater or Lesser Yellowlegs?

In Albums: nature birds Philadelphia

A yellowlegs sandpiper standing in water with vegetation.

Aug 19th, 2012, by Alex Zorach

This photo shows a yellowlegs, a type of sandpiper with yellow legs. Is it a greater or lesser yellowlegs? These species are notoriously difficult to tell apart. I think this is a greater yellowlegs but I'm not 100% sure.

I took this photo at the John Heinz National Wildlife Refuge.