已知数列\(\{a_{n}\}\)是首项为\(1\)，公差为\(2\)的等差数列，\(S_{n}\)是其前\(n\)项和，则\( \overset\lim{n\rightarrow \infty } \dfrac {S_{n}}{a_{n}^{2}}=\) ______ ．