Elevation GRID (10 Meter) for Webster County, Mississippi - 2007

Feature locations were were validated via cross-checks with exisiting data resources including photographs, maps, imagery, and scientific reports.

Primary Source of data

Digital source to verify elevation values in feet msl from the topographic maps

Hydrography dataset was used in generation of the digital elevation model as input of the stream linework and polygonal water bodies

Used in clipping the digital elevation model

Used to compare the elevation values of the new 10 meter DEMs

The 2006 NAIP data was used in some situations where the NHD dataset did not match the contour linework. The aerial photography was used to help edit the contour linework and NHD linework to more closely match the actual hydrography on the ground

Used in original coding of elevations.

The 1:24,000 contour mylar separates were scanned at 1000 dpi. They were brought into Hitachi's Cad/Core software to vectorize. The .tic and .lin files generated from Cad/Core were brought into a blank UTM coverage for transformation. The file was then projected into the MSTM projection. The lines were then cleaned and dangles corrected. The lines were then coded for elevation, contour type, and depression/hilltop using the separates and the paper 1:24,000 topographic maps. The quads that make up a county were then appended and clipped using a one mile buffer of the county border. The Arc/Info TOPOGRID command was used to generate the 10 meter GRID based on the Elevation variable (in feet). The GRID was brought into ERDAS Imagine to check for anomolies and data coding errors. Once several checks were made and verification of a smoothed GRID, the GRID was clipped on the county border.

The exisiting 1:24,000 contour quadrangle vector coverages were converted into vector shapefiles. The contours for a county were appended. A 400 meter county border buffer was created. The contours for a county were clipped on this 400 meter buffer to ensure a smoothe model at the border. Using the 3-D Anayst Topo-to-Raster command, a 10 meter digital elevation model (DEM) was generated. The resulting DEM was inspected for anomolies using a visual inspection in ArcMap of the grey-scaled image. Breaks in the contour lines between the quadrangles were edgematched. Any elevation value discrepencies were also fixed and/or addressed at these areas. A new DEM was generated and clipped on the county border (with a 10 meter buffer to ensure a smooth mosaic with surrounding counties). The resulting DEM was compared with the established USGS National Elevation Dataset (NHD) 30 Meter DEM. Using the Spatial Analyst MINUS command, a difference GRID was generated. This was used to find extreme elevation differences between the 30 meter DEM and the new 10 meter DEM. Errors in elevation were verified using the DRGs, changes made, and a new DEM generated. Using the 3-D Analyst HillShade command, a 3-D surface was generated. Overlaying the  NHD linework and polygons, visual inspection was made by MARIS staff to check for spikes, dips, and other discrepencies in the model. Potential errors in the elevations were then verified or discounted using the DRGs as a background layer. There were instances where the NHD was editied to more correctly match the 2006 NAIP aerial photography. This was in situations where the NHD dataset conflicted with both the contour linework and the imagery in a severe manner. Once this process was complete, a new 10 meter DEM was generated using the contour file, NHD stream and polygonal water bodies, and 1:24,000 county border file as input into the 3-D Analyst Topo-to-Raster command. Both the DEM and the contour vector file were sent to USGS/NGTOC III for inspection. Their staff performed a QA review of the data for completeness and accuracy in topographic characterization. In addition, a statistical sampling of vertical accuracy was performed by USGS/NGTOC III to ensure the vertical accuracy was met. Any deficiences/errors noted in a given county data set was returned to MARIS via x,y coordinates and a description of each error. These changes to the contour shapefile were made by MARIS staff and a new DEM generated. This procedure continued until inspection of the DEM resulted in few errors in elevation values in a county. At that point, those changes were made and the final 10 meter DEM was generated. The data was then readied for publication on the MARIS web site as well as distributed to the USGS for possible inclusion in the NED system