Ground surveys of the concrete armor unit targets were conducted during the 

 period 9-12 September 1996. Target coordinates were established using standard 

 surveying techniques. The purpose of armor unit targeting and target surveys was 

 to generate a set of control data by which the accuracy of the photogrammetric sur- 

 vey work could be validated and defined. Ground survey data obtained for the 

 armor unit targets are presented later in this report, where they are compared to the 

 photogrammetric survey data results. 



Aerial Photography 



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. 



Aerial photography was obtained along the Humboldt Bay jetties with a Wild 

 RC-8 aerial mapping camera (9-in. by 9-in. format). Black and white mapping pho- 

 tos were secured from a helicopter flying at low altitude (91 .4 m (300 ft)), which 

 resulted in high-resolution images and contact prints with scales of 2.54 cm (1 in.) 

 on the photograph equal to 15.2 m (50 ft) in the prototype. Photographic stereo 

 pairs were obtained during the flights. Flight lines were run parallel to the center- 

 lines of the structures. Typical stereo pairs secured for the north and south jetty 

 heads are shown in Figures 13 and 14, respectively. Aerial photography was 

 obtained on 15 September 1996, three days after the ground survey was completed. 



Photogrammetric Analysis of Armor Unit Targets 



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 over- 

 lap 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 overlapping photogra- 

 phy, very accurate measurements of any point appearing in the photographs can be 

 obtained. This technique is called photogrammetry. 



The stereo pair images obtained during aerial photography for the Humboldt Bay 

 jetties were viewed in an Analytical Stereoplotter, and stereomodels were oriented to 

 the control point data previously obtained. In the stereomodel, very accurate hori- 

 zontal and vertical measurements can be made of any point on any armor unit 

 appearing in the print. The stereomodel was used for all photogrammetric compila- 

 tion and the development of orthophotography. 



Orthophotos combine the image characteristics of a photo with the geometric 

 qualities of a map. The digital orthophoto is created by scanning an aerial photo 

 with a precision image scanner. The scanned data file is digitally rectified to an 



20 Chapter 2 Monitoring Plan and Data 



