CGS Home CSG Staff
Mapping Shorelines Using Airborne Lidar
Abstract
Introduction
Calibration

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

Mapping Historical Shorelines
Abstract
Ground Surveys
 
Calibration
 
At the beginning or end of every survey flight a calibration survey is conducted. The calibration survey consists of flying orthogonal passes across an unambiguous surface such as a road or runway. A ground-based Global Positioning System (GPS) survey is conducted along the calibration surface. If no ground survey has been conducted, two to four passes are flown parallel to the calibration target to serve as reference data. The flight data from the calibration area are compared with the ground GPS or airborne reference line to determine estimates of scanner roll corrections, scanner scale corrections, and vertical bias adjustments.
Intensity image of calibration target with ground GPS survey overlain. Before Calibration Adjustment: Mean Error = 0.015 m, RMS = 0.138 m. After Calibration Adjustment: Mean Error = -0.028 m, RMS = 0.061 m.                              Click on image to enlarge.
 

Vehicle used for GPS ground survey with antenna mounted on roof. A total station is used to determine the height of the antenna above the ground surface. Also in the center of the photo is a GPS base station set-up.

Click on images to enlarge.

GPS trajectory over calibration target. The ground GPS survey used for calibration is overlain on the trajectory. During this flight two reference passes were flown parallel to the calibration road. Also note in this figure the wide, flat turns at the end of passes. The aircraft cannot bank more than 15 degrees, or the GPS receiver will lose lock on satellites, causing potential problems for the computation of the aircraft trajectory.

 

Calibration and bias errors can be seen when carefully examining cross sections of overlapping flightlines. A scanner scale error will cause the flightline to appear either concave or convex as in strip 1. A bias error will appear as a vertical offset as in strip 2. The tilts in strips 3 and 4 represent scanner roll errors.
 
 
Comparison between ground GPS and lidar points before and after calibration and bias adjustment. After the calibration is adjusted and bias corrected, the lidar matches the ground GPS.                                                                          Click on images to enlarge.

Top of page