GIS.contours

Airborne Laser Terrain Mapping Survey utilizing dual-frequency airborne GPS control and conventional control were conducted in Waupaca County.  The horizontal coordinates are referenced to the  Waupaca County Coordinate System NAD83,  (Feet), and the vertical datum is NAVD88 (Feet).

LiDAR data collection and quality control has been conducted in accordance wih the Federal Emergency Management Agency (FEMA) guidelines and specifications for gathering data necessary to create digital elevation models using the guidelines and specifications in place at the beginning of the task order.  The survey includes select areas and over and within Waupaca County.  Waupaca County LiDAR Survey was performed using government furnished maps  outlining specific areas for the purpose of gathering data necessary to create digital elevation models in accordance with the guidelines and specifications directed by owner, and contract Scope of Work.  The raw LiDAR data density (posting) will be approximately 3-meters.  The total area of the survey is 70,339.72 acres.

The data design specifications for this product includes several directed to insure its quality, consistency, and useability.  These specifications address some of the technical issues associated with orthoimage production: 1) the tops of buildings and other structures elevated above the surface of the ground 'tilt' away from the center point of the source photography and from their own bases as a result of perspective and their height which is not compensated for in digital elevation models of the ground surface and 2) individual orthoimages may be assembled from several source images (strips).

All ground control processing and adjustment is performed using published coordinate horizontal and vertical datums (e.g. NGS CORS).  For deliverables, Corpscon for Windows Version 5.11.08 (geoid 99) was used for horizontal and vertical datum conversion as well as for coordinate system conversion purposes (e.g. UTM to Stateplane).

The accuracy assessment was performed using a standard method to compute the root mean square error (RMSE) based on a comparison of ground control points (GCP) and filtered LiDAR data points. Filtered LiDAR data has had vegetation and cultural features removed. Testing was performed prior to gridding of the filtered LiDAR data points and construction of the Intergraph .dtm file format. The RMSE figure was used to compute the vertical National Standard for Spatial Data Accuracy (NSSDA). Ground control was established by 3001, Inc. A spatial proximity analysis was used to select edited LiDAR data points contiguous to the relevant GCPs.  A search radius decision rule is applied with consideration of terrain complexity, cumulative error and adequate sample size.  Cumulative error results from the errors inherent in the various sources of horizontal easurement.  These sources include the airborne GPS, GCPs and the uncertainty of the accuracy of the LiDAR data points. This accuracy is achieved prior to the subsampling that occurs through integration with the inertial measurement unit (IMU) positions that are recorded. It is unclear at this time whether the initial accuracy is maintained. The horizontal accuracy of the GCPs is estimated to be in the range of approximately 1 to 1.6 inches.  Finally, sample size was considered. The specification for the National Standard for Spatial Data Accuracy is a  inimum of 20 points to conduct a statistically significant accuracy evaluation (Minnesota Planning, 1999, Positional Accuracy Handbook, Minnesota Planning Land Management Information Center, St. Paul, Minnesota., p.3). Most statistical texts indicate that a minimum of 30 sample points provide a reasonable approximation of a normal distribution.  The intent of the NSSDA is to reflect the geographic area of interest and the distribution of error in the data set  (Federal Geographic Data Committee, 1998, Geospatial National Standard for Spatial Data Accuracy, Federal Geographic Data Committee Secretariat, Reston, Virginia, p.3-4).