documents. The frequency of calibration is the major distinguishing factor 

 between the classes of survey, and directly affects the accuracy and adequacy 

 of the final results. With the increasing use of GIS for analysis and manipu- 

 lation of data, high standards of accuracy are imperative. Calibrations are 

 time-consuming and reduce actual data collection time. Nevertheless, this 

 must be countered with the economic impact that low quality data may be 

 useless or may even lead to erroneous conclusions (leading, in turn, to incor- 

 rectly designed projects and possible litigation). 



The maximum allowable tolerances for each class of survey are shown in 

 Table 6. 



Table 6 



Maximum Allowable Errors for Hydrographic Surveys 



Type of Error 



Survey Classification 



1 

 Contract Payment 



2 



Project Condition 



3 



Reconnaissance 



Resultant Two-Dimensional One- 

 Sigma RMS Positional Error Not to 

 Exceed 



3 m 



6 m 



100 m 



Resultant Vertical Depth 

 Measurement One-Sigma Standard 

 Error Not to Exceed 



± .152 m 

 (± 0.5 ft) 



± .305 m 

 (±1.0 ft) 



± .457 m 

 (±1.5 ft) 



(From HQUSACE (1991)) 



Table 7 depicts positioning systems that are considered suitable for each 

 class of survey. The table presumes that the typical project is located within 

 40 km (25 miles) of a coastline or shoreline reference point. Surveys further 

 offshore should conform to the standards in the NOAA Hydrographic Manual 

 (National Oceanic and Atmospheric Administration 1976). Planning and suc- 

 cessful implementation of offshore surveys are sophisticated activities and 

 should be carried out by personnel or contractors with considerable experience 

 and a successful record in achieving the accuracies specified for the particular 

 surveys. 



Geophysical Techniques 



Geophysical survey techniques, involving the use of sound waves and high 

 quality positioning systems on ocean vessels, are widely used for gathering 

 subsurface geological and geotechnical data in terrestrial and subaqueous 

 coastal environments. Geophysical procedures provide indirect subsurface 

 data as opposed to the direct methods such as coring and trenching. The use 

 of geophysical methods can assist in locating and correlating geologic materi- 

 als and features by determining acoustic transparency, diffraction patterns, 

 configuration and continuity of reflectors, and apparent bedding patterns. 



56 



Chapter 3 Field Data Collection and Observation 



