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Home > Explore! > Coastal Marine Geology > LIDAR - Casco > Background and Purpose


Assessment of LIDAR for Simulating Existing and Potential Future Marsh Conditions in Casco Bay

Background

Light Detection and Ranging (LIDAR) topographic data has revolutionized the rapid collection and subsequent analysis of highly accurate coastal topography. LIDAR data can be collected over relatively large geographic areas, and has been typically used to support mapping of a variety of different shoreline features, along with post-storm impacts.

Previous LIDAR flights have been conducted along southern coastal Maine. These include flights for the National Oceanic and Atmospheric Administration (NOAA) Coastal Services Center in 2000 and 2004. The Maine Geological Survey (MGS) has made extensive use of available 2004 LIDAR data in conjunction with aerial orthophotographs to support a variety of its shoreline mapping efforts within the State of Maine, mostly in inspecting coastal features along the shoreline such as dunes and beaches (Dickson, 2007, Slovinsky, 2009, Slovinsky and Dickson, 2006). The Root Mean Square Error (RMSE) vertical accuracy of the NOAA 2004 LIDAR was 6.7 cm, with an overall vertical accuracy of 13.4 cm, with a 2 meter point spacing (NOAA CSC, 2004).

MGS has also used the 2004 NOAA LIDAR and aerial imagery as part of a demonstration project to inspect the impacts of sea level rise on coastal floodplains and inundation levels (Slovinsky and Dickson, 2006). As part of this project, MGS simulated the impacts of sea level rise on tidal inundation levels, and also used tidal elevations as proxies for existing coastal wetland boundaries. For example, high marsh species, which dominate most of southern Maine marshes, typically exist between mean high water and highest annual tide tidal elevations (HAT), while low marsh species typically are found below mean high water and above mean sea level (Gehrels, 1994; Frey and Basan, 1985; Jacobson and Jacobson, 1989). These tidal elevations were calculated by MGS using NOAA National Ocean Service (NOS) Tide Charts (NOS, 2009a), and benchmark data sheets for Portland, Maine (NOS, 2009b). LIDAR raster data was used to create simulations of both existing marsh conditions, and potential future marsh conditions after sea level rise.

2006 FEMA LIDAR Data

2006 LIDAR coverage
Figure 1		 In 2006, the Federal Emergency Management Agency (FEMA) conducted aerial Light Detection and Ranging (LIDAR) flights in southern Maine to support Flood Insurance Rate Map remapping efforts for the FEMA Flood Plain Map Modernization Program. The data collection efforts were to provide FEMA compliant elevation, topography and contour maps of Cumberland and York County in Maine, in addition to bare earth (vegetation removed) topographic data of the coastal region of Cumberland and York County, Maine. The area of LIDAR coverage included communities that surround Casco Bay (Figure 1).

The LIDAR data had to meet certain standards to be determined adequate for FEMA use (FEMA, 2002), such as data needing to have a vertical Root Mean Square Error (RMSE) that does not exceed 15 cm (this is considered to be 30 cm accuracy at the 95% confidence interval, or about 1 vertical foot). Interestingly, metadata supporting the FEMA LIDAR listed an 18.5 cm RMSE for the bare earth data, or an overall 37 cm, which is outside listed standards.

The LIDAR topographic data, including TINs, bare earth grids (.xyz format files), and first and last return raw .xyz format files, were provided to the Maine Office of Global Information Systems (MEGIS) by the FEMA contractor; MEGIS provided the LIDAR data to MGS at our request.

Project Purpose

Because of potential inaccuracies in the 2006 LIDAR data as evidenced by the higher than standard RMSE and overall vertical accuracies, MGS was requested to conduct field investigations to compare LIDAR elevation data with field recorded Real Time Kinematic Global Positioning (RTKGPS) data collected at specific field sites. Subsequent comparisons between the LIDAR and groundtruthing RTK data could help determine whether or not the 2006 FEMA data was adequate for using tidal elevations to simulate existing marsh surfaces within select locations, and conducting subsequent mapping simulations of the potential impacts of sea level rise on those marsh surfaces.

Last updated on March 3, 2010