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Geographic Information Systems - GIS Mapping Project Applications
Technical Papers (Authored by Dodson's Engineering Staff)
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Federal Emergency Management Agency (1999) Dodson & Associates, Inc., as a part of a joint venture, is preparing a re-study of the Brays Bayou watershed in Harris County, Texas. The study serves as a pilot project for additional re-studies to be completed in the future. The results of the study include updated flood hydrologic and hydraulic modeling data for the watershed and updated flood plain maps that are based on the same up-to-date topographic data on which the updated hydraulic models are based. In addition to providing the final mapping results, the study is establishing methods, procedures, criteria, and requirements for future study efforts in Harris County and other areas in the U.S. that are subject to ground subsidence. The project also includes an analysis of the effects of subsidence on flooding depths in the Brays Bayou watershed. In addition to these issues, the project includes traditional hydrology and hydraulic analyses such as unit hydrograph preparation, definition of design storms and floods, flood and reservoir modeling, rating of hydraulic structures, backwater profile computations, and flood plain mapping.
The scope of the study is extensive.
It includes the main channel of Brays Bayou as well as several
major tributaries. In This completely digital FIS approach is consistent with FEMA’s move toward digital technology for processing requests for map changes. More accurate study results, faster study completion, and easier study revision all result from these new procedures.
Flood Protection
Planning Study Orange County Drainage District
Orange County, Texas
LIDAR Topographic Data Post Processing Harris County, Texas (2002) Dodson & Associates, Inc. provided post processing services for all LIDAR data sets for the area of Harris County. Dodson has also developed proprietary software for the visualization and analysis of LIDAR data. This software has proven to be extremely useful for the thorough evaluation of the completeness and internal consistency of the LIDAR data set.
What is LIDAR? LIDAR systems are airborne laser systems, flown aboard rotary or fixed-wing aircraft, that are used to acquire x, y, and z coordinates of terrain and terrain features that are both manmade and naturally occurring. This is a new technology and is intended to benefit the preparation of Flood Insurance Studies (FISs), Flood Insurance Restudies (RFISs), Limited Map Maintenance Program revisions (LMMPs), and other NFIP products. LIDAR systems consist of an airborne Global Positioning System (GPS) with attendant GPS base station(s), an Inertial Measuring Unit (IMU), and a light-emitting scanning laser. The system measures ranges from the scanning laser to terrain surfaces within a scan width beneath the aircraft. Scan widths will vary, depending on mission purpose, weather conditions, desired point density and spacing, geometry of the system's oscillating or rotating mirrors, and other factors. The time it takes for the emitted light (LIDAR return) to reach the earth's surface and reflect back to the onboard LIDAR detector is measured to determine the range to ground. The other two components of LIDAR systems are the airborne GPS, which ascertains the in-flight three-dimensional position of the sensor, and the IMU, which delivers precise information about the altitude of the sensor. Why is Post Processing Required? For hydraulic modeling, the contractor must provide high-resolution, high-accuracy, "bare-earth" ground elevation data. To restrict data to ground elevations only, the contractor must remove elevation points on bridges, buildings, and other structures and on vegetation from the LIDAR-derived data. In addition to randomly spaced LIDAR points, before and after removal of data associated with structures and vegetation, the contractor must produce a bare-earth DEM, with regular 5-meter point spacing in eastings and northings. In accordance with NSSDA, the contractor must use Triangular Irregular Network (TIN) linear interpolation procedures when validating the vertical accuracy of the DEM. In addition to DEMs, the contractor is required to produce breaklines for stream centerlines, drainage ditches, tops and bottoms of stream banks, ridge lines, road crowns, levees, bulkheads, road/highway embankments, and selected manmade features that constrict or control the flow of water (e.g., curb lines). Harris County Flood Control District Harris County, Texas (2000)
Memorial Village Water Distribution System Model Harris County, Texas (1995)
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As part of its Map Modernization
Plan, FEMA is performing an assessment of advanced technologies for
possible use in the preparation of National Flood Insurance Program
(NFIP) maps and related products. In April 1999, FEMA completed
Appendix 4B of the Flood
Insurance Study Guidelines and Specifications for Study Contractors.
Appendix 4B presents the guidelines and specifications that must
be used for the application of Light Imaging Detection And Ranging (LIDAR)
systems for gathering the data necessary to create digital elevation
models (DEMs), digital terrain maps, and other NFIP products.
Basic sediment information for
Harris County was prepared using GIS technology to characterize
erosion potential in each of the 22 major watersheds within Harris
County.