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 the ground have previously been targeted and are visible in the over- 
lapping photography, very accurate measurements of any point appearing in the 
photographs can be obtained. This technique is called photogrammetry. 
Due to logistical problems mentioned previously, low-altitude, high-quality, 
stereo pair images were not obtained for the Ofu Harbor breakwater. The aerial 
photographs obtained from the fixed-wing aircraft during the October 1996 
deployment, however, were analyzed in an analytical stereoplotter using conver- 
gent photogrammetric techniques. These procedures produce results that are not 
as precise as the vertical stereo pair analysis, but provided acceptable and useful 
data. The oblique images were oriented to control point data in the stereo model, 
where measurements could be obtained for the targeted tribars. The stereo 
model was used for all photogrammetric compilation. 
A photogrammetric analysis of the armor unit targets was conducted and x, y, 
and z (easting, northing, and el) coordinates were obtained. These data were 
compared to those obtained during the October 1996 ground surveys to establish 
the accuracy of the photogrammetric work. Position and elevation data obtained 
for the tribar targets as a result of the photogrammetric analysis are presented in 
Table 5. In addition, the absolute values of the differences in position and eleva- 
tion as opposed to the ground survey are also shown in Table 5. Differences 
between the values ranged from 0.0 to 12.2 cm (0.0 to 0.4 ft) in the horizontal 
directions and from 0.0 to 10.1 cm (0.0 to 0.33 ft) in the vertical direction. The 
average of the differences between the x, y, and z coordinates was 3.3, 4.1, and 
2.3 cm (0.108, 0.134, and 0.077 ft), respectively. Differences in the vertical 
positions were closer than the horizontal positions. Based on the analysis, only 
22 percent of the target differences were less than 1.5 cm (0.05 ft) in the horizon- 
tal directions, and 35 percent of the differences in the vertical direction were less 
than 1.5 cm (0.05 ft). Of all horizontal positions, 51 percent were within 3 cm 
(0.1 ft), as were 77 percent of all vertical positions. 
Full-scale hard copies of aerial photographs are on file at the authors' offices 
at WES and CEPOD. In addition, all photogrammetric compilations and analy- 
ses have been stored on diskettes in Intergraph files for future use. In summary, 
detailed information relative to the armor unit positions for the Ofu Harbor 
breakwater have been captured by means of aerial photography and photogram- 
metric analysis. Data are stored and can be retrieved and compared against data 
obtained during subsequent monitoring. Thus, armor unit movement may con- 
tinue to be quantified in future years. 
Chapter 2 Monitoring Plan and Data 25 
