Horizontal positions are based on the American Samoa Plane Coordinate System 
and all elevations are referenced to mlw datum. The initial ground survey work 
was conducted during the period 17-18 October 1996. 
A total of 33 tribars were selected for detailed study. They represent an even 
distribution throughout the tribar field, and occupy positions ranging from near 
the waterline to close to the rib cap at the crest of the structure. These armor 
units (tribars 101-133) were painted with three targets (Figure 15). The target 
number is followed by an A, B, or C. Three targets on an individual tribar allow 
for very precise measurements depicting individual armor unit movement. 
Twenty additional tribars (units 19-38) were painted with a single target to serve 
as photogrammetric control points, as well as to be used to detect armor unit 
movement during future ground surveys. A 0.64-cm (1/4-in.) hole was drilled at 
the center of each target to mark the survey points for subsequent surveys. Loca- 
tions of the targeted tribars are shown in Figures 16 and 17. Positions and eleva- 
tions of the targeted tribars obtained during the October 1996 survey are pre- 
sented in Table 1. 
Figure 15. View of a targeted tribar (three targets established) 
For the tribars with three targets, a more in-depth analysis was conducted. 
With the x, y, and z (easting, northing, and el) coordinates defined for each tar- 
get on the various armor units, the centroid of each targeted tribar was computed. 
In addition, the position of each armor unit relative to the x, y, and z axes was 
determined. Figure 18 shows the orientation of representative tribars to the three 
axes. The centroid of each targeted tribar and each armor unit's orientation (rota- 
tion angle relative to x, y, and z) are presented in Table 2. Computations were 
made based on the October 1996 ground survey. These are base level conditions 
from which comparisons can be made in future surveys. 
Chapter 2 Monitoring Plan and Data 
17 
