CGS Home CSG Staff
Mapping Shorelines Using Airborne Lidar
Abstract
Introduction
Datum Adjustment

Measuring Shoreline Change Along Bays and Oceans Using Historical Aerial Photography and Airborne Topographic Lidar Surveys

Mapping Historical Shorelines
Abstract
Ground Surveys
 
Vertical Datum Adjustment

Postprocessed lidar data are provided in the UTM coordinate system using the NAD83 datum. Vertical measurements are expressed as heights above the reference ellipsoid (HAE). Using the G99 model, HAE heights are converted to orthometric heights relative to NAVD 88, which approximates mean sea level (MSL). A local MSL correction is then applied to the orthometric height based upon vertical information from the bay-side tide gauge at Port Bolivar. Water levels at the time of the survey from the Galveston Entrance Channel North Jetty tide gauge are compared with heights from the Digital Elevation Model (DEM) as a check upon the conversion from HAE to height above MSL. Below is the formula for the HAE to MSL correction:

Hmsl = HAElidar + NG99SSS + MSL correction

 
Ellipsoid and Geoid Heights
Schematic of relationships between ellipsoid and geoid heights.

 

Click on images to enlarge.

 
GEOID99 Model
GEOID99 model along a portion of the Texas Gulf of Mexico shoreline. The geoid model is used to convert HAE to NAVD 88 orthometric heights.
 
Water level at Galveston Entrance Channel North Jetty tide gauge, July 16-18, 2001. The water level is indicated by a dashed line showing the time the aircraft passes over the north jetty. A sampling of water levels from the DEM matches the water level at the tide gauge.

Shoreline levels and horizontal shifts on beach profile BEG02 if a local mean sea level or alongshore geoid anomaly gradient were not applied.

Click on images to enlarge.