Surveying Benson Beach with an ABMS

The North Head ABMS is comprised of eight video cameras pointing south towards the Mouth of the Columbia River (MCR) with a merged view that spans the area shown by the black arrows in Figure 2. The regions of interest for ABMS monitoring are: Benson Beach, the North Jetty, Site E, and Peacock Spit. On Benson Beach, quantitative maps of the shoreline and offshore sandbar locations can be made daily. Directly offshore on Peacock Spit, the ocean wave direction and period can be measured as can surface currents. Further away, at Site E, the presence or absence of wave breaking is monitored from this web site with ABMS snapshot images taken and delivered every twenty minutes (communication links permitting).

To be able to use oblique camera images for surveying the nearshore, the cameras and objects in the camera views are surveyed into a local coordinate system. The information obtained in the images can then be rectified into a Cartesian map of image pixel intensity (I) values ranging from 0 to 255.

An image consists of a 640 (u) x 480 (v) matrix of pixel intensity (I) values (0-255). We convert from image (u,v) coordinates to world (x,y,z) coordinate using photogrammetry methods. The problem consists of transforming from 2 knowns (u,v) to 3 unknowns (x,y,z). To do this, we can solve for (x,y) by constraining z to a known elevation, for instance the tidal stanza. To obtain these "geometry" solutions, we survey the local real-world coordinates of the camera focal plane and a few ground control points (GCPs) in the camera's field-of-view.


An image consists of a 640 (u) x 480 (v) matrix of pixel intensity (I) values (0-255). We convert from image (u,v) coordinates to world (x,y,z) coordinate using photogrammetry methods. The problem consists of transforming from 2 knowns (u,v) to 3 unknowns (x,y,z). To do this, we can solve for (x,y) by constraining z to a known elevation, for instance the tidal stanza. To obtain these "geometry" solutions, we survey the local real-world coordinates of the camera focal plane and a few ground control points (GCPs) in the camera's field-of-view.

Example image view from a ABMS site at Duck, North Carolina. The camera is sitting on top of a 110 foot tower. The white lines mark the oblique image view.


Example image view from a ABMS site at Duck, North Carolina. The camera is sitting on top of a 110 foot tower. The white lines mark the oblique image view.

Photogrametric coordinate system for obtaining the geometry solutions.


Photogrametric coordinate system for obtaining the geometry solutions.

The height above sea level of the North Head Lighthouse is ideal for ABMS mapping of coastal features. The lighthouse focal plane sits 65 feet above the ground and approximate 250 feet above sea level. Maximizing height of the ABMS cameras provides the best alongshore spatial resolution of features. A lighthouse mounting provides nominally 1m cross-shore and 3m alongshore resolution at a distance of 1km from the lighthouse. Resolution at the distant jetty over 3km away is nominally 2m cross-shore and 20m alongshore.