- SPECIAL REPORTS
- THE MAGAZINE
The coastal communities of North Carolina live under constant threat from the effects of tidal and storm surge. This eastern seaboard state was ranked as the third most vulnerable to flooding in the U.S. in the recent Surging Seas report by Climate Central,1 and the state’s own research has shown that up to 4,300 miles of state-maintained highways within the coastal plain are at risk.
Under flood conditions, local governments and residents look to the state for advice on the likely extent of inundation and which evacuation routes they should take if they are forced to abandon homes. This essential service is provided by the Geospatial and Technology Management Office (GTMO), located in the North Carolina Department of Public Safety (NCDPS), which conducts regular simulations using a sophisticated computer model to test its systems and to carry out detailed scenario assessments.
Almost half of the state’s total area is within the region referred to as the coastal plain. Flood events are therefore likely to have a significant impact and will affect a large population--which is why North Carolina has invested in carefully building a complex GIS database to support the model’s output. The database is the responsibility of the state’s GTMO, which makes it available to other agencies involved in North Carolina’s State Emergency Response Team (SERT). One major weakness was identified in the model: The limited resolution of highway elevation measurements was compromising the reliability of flood penetration modeling along highways which, in turn, was preventing safe evacuation route planning.
“We needed to know the extent of inundation across the entire highway network, and the impact on infrastructure assets such as bridges and sidewalks,” says John Dorman, director of GTMO. “Local governments and residents rely on evacuation route advice, yet we were getting a wide spread of results from the model simply because the resolution of our elevation dataset was too coarse.”
North Carolina had pioneered the use of LiDAR to measure elevation above sea level when it established the first state-wide dataset in 2000. The data had been captured using an airborne platform, a technique that requires ground reference posts to be inserted for calibration. That initial dataset was based on 4 to 5 meter nominal spacing of posts, which resulted in a 15-centimeter resolution. The state had also used LiDAR to capture a “first floor” map of building elevations so that it had a complete ensemble of information.
GTMO calculated that it really needed highway elevation measurements to a resolution of 7 centimeters or better to make the model output more robust. “We looked at how we could best do this, and our conclusion was that ground-based mobile LiDAR was the only viable option,” says Dorman.
His team had identified that around 4,300 miles of low-lying highways across 20 counties were particularly vulnerable to tidal and storm surges up to 14 feet (4.3 meters).
GTMO used one of its contractors, ESP Associates P.A. (ESP), to conduct a survey of the vulnerable highways and create a new elevation dataset. While the primary requirements were driven by the need for high-resolution data of the highways themselves, GTMO also wanted to capture information to show which of its infrastructure assets--including bridges, ramps, walkways and signs--were also at short- and long-term risk from flooding. The program deliverable was a new dataset of mapping-grade elevation measurements along the 4,300-mile network that could be incorporated into the state’s GIS database.
The project was organized in three phases: planning, LiDAR data capture and post processing. During the planning phase, the team reviewed the available data of the vulnerable highways and intersected it with inundation layers provided by GTMO. From this, the team developed a contiguous data capture plan.
The Trimble MX8 is a purpose-designed, vehicle-mounted system that combines high-resolution LiDAR, an array of high-performance CCD cameras and dual GPS positioning. LiDAR captures the precise properties of a target scene by illuminating it with pulses of light and measuring the time delay and signal characteristics of the reflection. This is relatively simple when the LiDAR is in a fixed position and static, but when moving at highway speeds, as in the MX8, accuracy and resolution depend on complex proprietary algorithms–a skill for which Trimble is well known.
The MX8 captures dense point clouds that can be processed to give mapping- or survey-grade accuracy. It collects data on visible or unobstructed features at highway speeds at densities ranging from approximately 300 to 1,000 points per square meter. Soft topography that is unobscured is captured at the same accuracy as hard topography. The MX8 system achieves mapping-grade accuracies from a fully integrated position and orientation system that combines GPS and inertial sensors, and survey-grade accuracies when supplemented with project control and planning.
ESP performed a number of quality control functions during data capture, including intermittent ground-truth surveys to validate the vertical accuracy of the collected data using the existing North Carolina Real Time Kinematic Network. The total data capture time for all 4,300 miles was around 65 days at a productivity of 66 miles per day, including quality control.
Following the data capture, Trimble Trident-3D Analyst software was used to analyze and interpret the point cloud data. This software tool offers a user interface with robust object positioning, measurement and data layer creation capabilities. It uses automated features to increase productivity and reduce turnaround time.
Following the automated point cloud classification, ground points were exported to LAS files for digital elevation model (DEM) development and additional processing. The bare earth ground data was tiled into dimensions that matched the North Carolina state-wide LiDAR elevation grid (10,000 feet x 10,000 feet). Approximately 1,020 tiles were delivered with updated roadway elevation data.
The project was successfully completed, and the state’s GIS database now contains accurate, high-resolution measurements of 4,300 miles of highways that are most vulnerable to tidal surge. ESP Associates’ Director of Water Resources and GIS David Key believes technology was a key factor in delivering excellent outcomes. “The MX8 platform and Trident Analyst software allowed our team to work quickly and efficiently in what was a very complex project,” he says. “The state’s updated GIS database can now be used with confidence to accurately carry out scenario modeling and emergency planning.”
This updated database will serve the state of North Carolina during future hurricane seasons by helping the authorities carry out more robust predictions of inundation and better manage emergency response. The enhanced elevation dataset is already allowing more reliable scenario modeling.
John Dorman is delighted with the overall achievements of the project. “It is critical for emergency managers and first responders to have highly accurate, detailed road data to efficiently prepare for, respond to and mitigate against coastal flood events,” he says. “There is no doubt that our new capabilities will help to protect property and save lives, as well as provide improved emergency services to North Carolina residents.”
1. Climate Central, “Surging Seas,” a report on sea level rise, storms, and global warming’s threat to the U.S. coast, March 2012 ( www.sealevel.climatecentral.org).
For more information about ESP Associates, visit www.espassociates.com. Additional information about the Trimble MX8 Mobile Spatial Imaging System and Trimble Trident Analyst software can be found at www.trimble.com/drive.