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Mapping Shorelines Using Airborne Lidar
Abstract
Introduction

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

Mapping Historical Shorelines
Abstract
Rectification
Ground Surveys
 
Digital Image Rectification

Rectification of aerial photographs involves the establishment of ground control points that link each image to its corresponding aerial coverage on a digital orthophoto quarter quad (DOQQ) that serves as the base map. Points are chosen on the image that can be matched to points on the DOQQ. Road intersections and other cultural features are preferred as reference points rather than natural features. However, in many cases cultural features are lacking and features such as trees, shrubs, and the edges of water bodies are used. Where possible, points are evenly spaced across the image, with special emphasis on the edges of the image, and on areas near to the shoreline.

 

Once all the ground control points have been established, the image is rectified. Once the rectification is complete, the image is made semitransparent and overlain on the DOQQ. In some areas a "double image" occurs, indicating that either more ground control points are needed, or one or more existing ground control points have been placed incorrectly. In these cases, images are reprocessed to correct any remaining distortion.

Ground control point selection in ER Mapper's geocoding wizard. The program calculates the approximate RMS error for each point.

Click on images to enlarge.

 
A poorly rectified image overlain on a DOQQ. A double image indicates that more ground control is needed. A well-rectified image overlain on a DOQQ as shown by a sharp image.
 

Mesh of Delaunay triangles.

Rectification by Triangulation

In a Delaunay triangulation, each triangle is shaped as equilaterally as possible by connecting each ground control point (GCP) to its two nearest neighbors. During rectification each GCP snaps to its corresponding location on the base map and each triangle is stretched and squeezed to fit by calculating the polynomial coefficient of each triangle. Because multiple polynomials are used, local distortion can be corrected. A large number of well-placed GCP's could result in an almost complete correction for any geometrical

errors. If the GCP's could be placed with 100 percent accuracy, accuracy would be a function of image resolution and photograph scale.
 
Sources of Error
Errors in the data are difficult to measure and have several possible causes:
•Lack of suitable ground control point locations on some images •Artifacts caused by the triangulation process
•Poor choice of ground control points because of interpretation error •Incorrect position of mapped shoreline due to poor quality of image or interpretation error
•Severe image distortion •Positional inaccuracies in the base map DOQQ
 

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