Why do contour lines extend into open water on some US Topo maps?
Traditional topographic maps (maps published before about 2000) derived hydrography and hypsography from a single data source, usually the same aerial photographs. In addition, the maps were drafted by hand, which allowed precise visual integration of different feature classes. On traditional topographic maps, non-bathymetric contour lines in open water were considered errors.
US Topo maps, the new digital product produced since 2000, are mass-produced from national GIS databases. Different data layers come from different sources collected at different times. The ortho image, the elevation data, and the water data from these databases will usually not represent the same point in time at any particular spot. Water levels rise and fall, meandering stream channels shift, so water extents and elevation data collected at different times will often not match exactly. The problem is exacerbated if one or both data layers are relatively old or from low-resolution sources, or if extreme seasonal or climate variations have occurred between data collection dates.
In Alaska, where contours are generated from recent lidar (light detection and ranging) or ifsar (radar) elevation data, the older hydrography data is updated to reflect very major stream course or water body changes. However, minor changes and differences due to fluctuating seasonal or annual climate variations are not reflected in the hydrography data. These source differences can result in contours intersecting water features. Such occurrences are common where lidar or ifsar sensors detect stream banks and shoals during low-water conditions and, therefore, are allowed to remain in the US Topo to allow the user to visually see the ground surface based on the newly collected elevation source.