AbstractA map should treat the critical problems in a way understandable to the intended reader. Some maps appropriately show only one or two pertinent hydrogeologic parameters. Point‐data maps make little or no interpretation of the data. Four‐dimensional maps‐those that include an elapsed span of time or projection of hydrogeologic variables into the future‐represent a high degree of interpretation of data; they are most useful to the map reader. The hydro‐geologist has an obligation to his reader to design maps that are as useful as