1.1. Vertical Accuracy Vertical accuracy is the principal criterion in specifying the quality of elevation data, and vertical accuracy requirements depend upon the intended user applications.1 There are five principal applications where high vertical accuracy is normally required of digital elevation datasets: (1) For marine navigation and safety. (2) For storm water and floodplain management in flat terrain. (3) For management of wetlands and other ecologically sensitive flat areas. (4) For infrastructure management of dense urban areas where planimetric maps are typically required at scales of 1 inch = 100 feet and larger scales. (5) For special engineering applications where elevation data of the highest accuracy are required. Whereas there is a tendency to specify the highest accuracy achievable for many other applications, users of elevation data must recognize that lesser standards may suffice, especially when faced with the increased costs for higher accuracy elevation data. When contracting for lidar-derived elevation data, it is important to specify the vertical accuracy expected for all final products being delivered. For example, when contours or gridded digital elevation models (DEMs) are specified as deliverables from lidar-generated mass points, a TIN may first be produced from which a DEM or contours are derived. If done properly, error introduced during the TIN to contour/DEM process should be minimal; however, some degree of error will be introduced. Accuracy should not be specified and tested for the TIN with the expectation that derivatives will meet the same accuracy. Derivatives may exhibit greater error, especially when generalization or surface smoothing has been applied to the final product. Specifying accuracy of the final product(s) requires the data producer to ensure that error is kept within necessary limits during all production steps