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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
Introduction
Ground Surveys
 

Introduction

Digitally rectified aerial photographs have become an important tool in historical shoreline mapping. They are replacing the need for traditional methods such as using a zoom transfer scope to project shorelines onto a base map. Digitally rectified aerial photographs have all the elements of a photograph, but the image distortion caused by tilt of aircraft, camera lens, and relief displacement has been corrected. Also, the image is georeferenced and therefore may be combined with other forms of geographic data in a geographic information system (GIS).

Historical shoreline maps of Texas Gulf Coast bays and shorelines are being created through digital rectification techniques and GIS. Aerial photographs are digitally scanned at high resolution and imported into Earth Resource Mapping's ER Mapper software, where they are manually registered to a base image/map and then rectified. The rectified images are imported into ArcView GIS software, where all shore and vegetation lines are digitized into a GIS database.

 

Aerial Photographs

Historical aerial photographs since the 1930's are available. Much of the mapping is based on 9 x 9 inch vertical aerial photographs with scales ranging from 1:20,000 to 1:24,000. Some mapping uses photomosaics from the 1930's and 1950's. These mosaics are composed of photographs that were pieced together over a 7.5-minute USGS topographic quadrangle. The result is a composite image that covers roughly the same area as a topographic sheet. All photomosaics and aerial photographs are digitally scanned at 600 to 1,000 dots per inch in a flatbed scanner.

 

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Portion of a 1930 photomosaic. Digitally scanned photomosaics are rectified to produce georeferenced images.
 

Distortion

Distortion increases the farther a point is from the center of an image because all points on an image are represented as if they are being viewed from the center of the image. Distortion is also caused by the unavoidable tilting of the aircraft. Because aerial photographs are distorted and lack a spatial reference, they cannot be brought directly into a GIS environment.

1982 9" x 9" contact print of color-infrared aerial photograph before rectification. The image is distorted and lacks spatial reference.

Click on image to enlarge.

 

USGS Orthophotos

1995/96 U.S. Geological Survey digital orthophoto quarter quads (DOQQ's) serve as base maps to which other photographs are registered. They are created by using camera and flight information, digital elevation models, and ground control points. The color-infrared film is scanned with 24-bit color and 1-meter spatial resolution. They meet National Map Accuracy Standards for 1:12,000-scale maps (i.e., 90 percent of well-defined test points must fall within 10 m of their real location).

1995 USGS digital orthophoto quad. These photogrammetrically corrected images serve as base maps to which other aerial photographs are warped.

Click on image to enlarge.