Coastal maps and charts represent one of the few longer-term datasets which can potentially be used to analyse coastal change over the last 200 years. Mapping of coastal areas has often been unreliable, inconsistent and a low priority. One feature which has been mapped periodically is the line of the high and low water lines of the tides. Tidal line mapping has periodically been carried out by the Ordnance Survey throughout the late nineteenth and first half of the twentieth centuries; data were largely recorded using land survey techniques. The Ordnance Survey was legally obliged to map and print the tidal lines of coastal areas as they represented the seaward edge of the parish boundary. Tidal lines are transient features and were often difficult, if not dangerous to record in the field. As a result, from 1947 onwards, the Ordnance Survey experimented with photogrammetric techniques in an attempt to improve the consistency and reduce the costs of tidal surveys. This paper reviews the mapping of tidal lines and in particular discusses the evolution and development of photogrammetric mapping techniques by the Ordnance Survey as applied to tidal line surveying.