How accurate are the geographic locations?
The spatial accuracy of the geologic source information varies. The source geologic map scales range from 1:100,000 to 1:500,000. In addition, analysis near state boundaries may be limited by the geologic map interpretation differences across state boundaries. This New England Lithology coverage is limited by variances related to these differences in source materials used and interpretation of geologic information between the CONN and NECB lithochemical mapping. The spatial component of this data layer was compiled from five different digital sources and, therefore, incorporates any limitations associated with those individual state geologic data layers as well as their source paper maps. The digital sources were provided with minimal documentation, so it is unknown how or with what accuracy they were digitized. The scales of the original state bedrock maps range from 1:100,000 to 1:500,000.
The DFIRM Database consists of countywide vector files and associated attributes produced in conjunction with the hardcopy FEMA FIRM. The published effective FIRM and DFIRM are issued as the official designation of the SFHAs. As such they are adopted by local communities and form the basis for administration of the NFIP. For these purposes they are authoritative. Provisions exist in the regulations for public review, appeals and corrections of the flood risk information shown to better match real world conditions. As with any engineering analysis of this type, variation from the estimated flood heights and floodplain boundaries is possible. Details of FEMA’s requirements for the FISs and flood mapping process that produces these data are available in the Guidelines and Specifications for Flood Hazard Mapping Partners. Horizontal accuracy was tested by manual comparison of source graphics with hardcopy plots and a symbolized display on an interactive computer graphic system. Independent
• How accurate are the heights or depths? • Where are the gaps in the data? What is missing? Topo_Catchment replaces DRNBASIN completely. The area that was updated is King County’s Lidar extent. This includes all of King County and Basins in Snohomish that are part of the Lake Washington – Cedar Watershed. • How consistent are the relationships among the observations, including topology? Not all Catchment or Sub-Basin polygons have a Identification attribute value. Some catchments outside of King County have no ID value. Not all Basins have Sub-Basins.
The horizontal positional accuracy of these data is dependent on that of the source data sets. Positional accuracy reports from those data sets are reproduced here. For QUAT79: This coverage was generated by Environmental Systems Research Institute as one component of a multi-layer Integrated Terrain Unit (ITU) database. Automation was limited to polygons with area of one square mile or greater and linear features greater than one-half mile in width. Boundaries which were generally coincident with boundaries of other ITU layers (e.g., soil associations or surface water bodies) may have been adjusted to be exactly coincident. Overlay with the published map shows significant differences in some areas. This data can only be used to show the generalized Quaternary boundaries within Illinois. For MORAINES: Horizontal positional accuracy was verified in October, 1991 by manual overlay of source and hard-copy plots. Lines within one line-width of the source were deemed acceptable. Accuracy of
