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In this paper, we propose a novel relay selection scheme based on geographical information, aiming to minimize the symbol error probability (SEP) for wireless ad hoc relay networks. The proposed scheme utilizes only the source-relay and relay-destination distances as selection criteria to choose the best relay. In particular, such geographical information is often available during network initialization. Thus, the proposed scheme does not need extra procedures to examine which relay has the best channel quality and, hence, reduces significantly the computational complexity. By contrast, most existing relay selection schemes rely on strict channel state information (CSI) and coherent time intervals, drastically increasing the difficulty in practical realizations, especially for large relay networks in which the fading channels are sufficiently fast-varying. In our scheme, the accuracy of the distance information is the main factor resulting in non-best relay selection. An ideal scheme (in which the distances between nodes are exact) and a practical scheme (in which there exists distance estimation errors) will be discussed respectively. Simulation results demonstrate the proposed relay selection method can efficiently improve the SEP performance and outperform the random relay selection scheme.