Seafarers navigate using charts. Today those charts are mostly electronic and accurate to within a few inches. In earlier times, a mariners charts were much less precise. As discovery proceeded, information about coasts, islands, depths, currents and winds increased. Gradually, charts evolved becoming more exact and therefore more useful. Charts were also of great strategic value and were often kept very secret as empires expanded and vied for new territories. Charts were invaluable early ocean maps.