2017 Countywide Contours

Click here to access the data directly from the Illinois State Geospatial Data Clearinghouse.The contours are created from a bare earth DEM derived from the source lidar and 3D breaklines. The contours are created using controlled and tested methods to limit the amount of error introduced during contour production but smoothing applied to the contours does introduce some deviation from the original source lidar and bare earth DEM. The contours were produced from the raster bare earth DEM and inherit the accuracy of the DEM.The project specifications require the accuracy (ACCz) of the derived DEM be calculated and reported in two ways: 1. The required NVA is: 19.6 cm (0.64 ft) at a 95% confidence level, derived according to NSSDA, i.e., based on RMSE of 10 cm (0.33 ft) in the “bare earth” and "urban" land cover classes. This is a required accuracy. The NVA was tested with 116 checkpoints located in bare earth and urban (non-vegetated) areas. 2. Vegetated Vertical Accuracy (VVA): VVA shall be reported for "brushlands/low trees" and "tall weeds/crops" land cover classes. The target VVA is: 29.4 cm (0.96 ft) at the 95th percentile, derived according to ASPRS Guidelines, Vertical Accuracy Reporting for Lidar Data, i.e., based on the 95th percentile error in all vegetated land cover classes combined. This is a target accuracy. The VVA was tested with 79 checkpoints located in tall weeds/crops and brushlands/low trees (vegetated) areas. The checkpoints were distributed throughout the project area and were surveyed using GPS techniques. See survey report for additional survey methodologies. AccuracyZ has been tested to meet 19.6 cm (0.64 ft) or better Non-Vegetated Vertical Accuracy at 95% confidence level using RMSE(z) x 1.9600 as defined by the National Standards for Spatial Data Accuracy (NSSDA); assessed and reported using National Digital Elevation Program (NDEP)/ASRPS Guidelines.