to determine if bathymetric conditions had stabilized as a result of breakwater 

 construction. 



Broken armor unit surveys 



A survey of broken/cracked armor stone above the waterline on the 320-m-long 

 (1,050-ft-long) St. Paul Harbor outer main breakwater was conducted four times 

 during the monitoring period. Surveys were conducted in July 1993, June 1994, 

 June 1995, and June 1996. During the inspections, each broken armor stone was 

 identified and photographed, and its approximate location relative to breakwater 

 station and distance from a baseline was recorded. The baseline was the approxi- 

 mate centerline of the structure. Armor stones with hairline cracks were not 

 counted; only those that were cracked all the way through. A geological assessment 

 of the broken stone was conducted during the June 1995 survey. 



Photog ram metric surveys 



Photogrammetric surveys, as well as ground surveys for control, were conducted 

 during May 1994 and May 1996. To establish control for the photogrammetric 

 work, monuments were established on the breakwater. Groxmd surveys were initi- 

 ated from existing known monuments, which included National Geodetic Survey 

 stations and a Corps of Engineers station. They were established by GPS control 

 and electronic land surveying techniques. In addition, targets were established at 

 intervals of about 55 m (180 ft) along the sea side, harbor side, and approximate 

 center of the breakwater. Each target was marked with a drill hole 0.64 cm (1/4 in.) 

 in diameter, and 0.64 cm (1/4 in.) deep, and painted with a circular target to ensure 

 visibility in aerial photography. Atypical target is shown in Figure 23. Targets 

 were electronically surveyed to form control by which the accuracy of the photo- 

 grammetric survey work could be validated. Horizontal positions were based on the 

 Alaska State Plane Coordinate System and elevations were referenced to mean lower 

 low water datum. 



Aerial photography is a very effective means of capturing images of large areas 

 for later analysis, study, visual comparison to previous or subsequent photography, 

 or measurement and mapping. Its chief attribute is the ability to freeze a moment in 

 time, while capturing extensive detail. Low-altitude aerial photography was 

 obtained along the breakwater with a Wild RC-8 aerial mapping camera (22.9-cm 

 by 22.9-cm (9-in. by 9-in.) format). The photos were secured from a helicopter 

 flying at an altitude of 9 1 m (300 ft), which resulted in high-resolution images and 

 contact prints with scales of 1:600. Photographic stereo pairs were obtained during 

 the flights. 



When aerial photography is planned and conducted so that each photo image 

 overlaps the next by 60 percent or more, the two photographs comprising the 

 overlap area can be positioned under an instrument called a stereoscope and viewed 

 in extremely sharp three-dimensional detail. If properly selected survey points on 



28 



Chapter 2 Monitoring Program 



