Figure 23. Example of target established on St. Paul breakwater 



the ground have previously been targeted and are visible in the overlapping photo- 

 graphy, very accurate measurements of any point appejiring in the photographs can 

 be obtained. This technique is called photogrammetry. The low-altitude stereo pair 

 images obtained during aerial photography at St. Paul Harbor were viewed in a 

 stereoscope and stereomodels were oriented to the monument and target data pre- 

 viously obtained. In the stereomodel, very accurate horizontal and vertical mea- 

 surements can be made of any point on any armor stone appearing in the print. The 

 stereomodel was used for all photogrammetric compilation and development of 

 orthophotography. 



Orthophotos combine the image characteristics of a photograph with the geom- 

 etric quaUties of a map. The digital orthophoto is created by scanning an aerial 

 photograph with a precision imaging scanner. The scanned data file is digitally 

 rectified to an orthographic projection by processing each image pixel. Orthophotos 

 were prepared for the St. Paul Harbor main breakwater. Precise horizontal measure- 

 ments may be obtained from the orthophotos using an engineer scale since the image 

 has been rectified and is free from skewness and distortion. 



In addition to digital orthophotos, point plot maps, contour maps, and cross 

 sections were developed for the main breakwater using the digital terrain model 

 (DTM). Point plot maps consisted of an approximately 0.5-m (1 .5-ft) grid pattern 

 overlaid on the structure. Precise vertical and horizontal measurements were 

 obtained at the intersections of the grid. Contour maps of the breakwater, developed 

 from the DTM, for a 0.3-m (1-ft) contour interval also were obtained. In addition, 

 using the analytical stereoplotter and DTM grid, cross sections were developed 

 along the breakwater at 30.5-m (100-ft) intervals. 



Chapter 2 Monitoring Program 



29 



