Errors in Profile Data-Collection Methods 



Every observed or measured quantity contains errors of unknown magnitude for a variety 

 of causes, and, iience, a measurement is never exact. One of tiie important functions of the 

 surveyor is to secure measurements tliat are correct within certain limits of error prescribed 

 by the nature and purpose of the survey. This requires knowledge of the sources of errors, 

 understanding of the effect of the various errors upon the observed quantities, and familiarity 

 with the procedure necessary to minimize errors to maintain a desired precision. 



Errors usually arise from three sources: operational errors, instrument errors, and external 

 errors. Operational errors include operator errors and limitations of the surveying procedure. 

 Instrument errors result from limitations of instruments or devices with which measurements 

 are taken. External errors arise from variations in natural phenomena such as temperature, 

 humidity, wind, and gravity. These errors, along with how they were identified and 

 removed, are discussed below. 



Operational errors 



Operational errors of the Zeiss survey system resulted from improper leveling of the 

 instrument, mis-aiming the instrument at the center of the prism cluster while taking the 

 measurement, an error in positioning the instrument in coordinate space (particularly in 

 elevation), and movement of the tripod during the survey. Improper leveling of the 

 instrument affected all the points measured and was not always easy to detect. Incorrectly 

 aiming the instrument also resulted in an error but only on individual points. This kind of 

 error occurred when the instrument was triggered to begin a reading prior to properly aiming 

 at the prisms. This resulted in sampling the azimuth and zenith angles at the time of 

 triggering. These incorrect angles were then used with the distance to the prisms to calculate 

 the CRAB location. Errors affecting single points were usually easy to detect and remove. 

 Errors from mispositioning the instrument and from movement of the tripod were eliminated 

 after August 1985 when two permanent fixed-instrument mounts were established on the pier 

 and on the roof of the facility. 



A different type of operational error resulted when topographically important points, such 

 as the bar crest or trough, were missed. Survey points were selected based on the timed 

 travel of the CRAB, with more points taken close to shore where the profile shape is more 

 complex. While it was possible for the Zeiss operator to follow the vertical movement of the 

 CRAB as it moved, small features and some peaks of significant features were sometimes 

 missed or inadequately defined. 



Use of the Geodimeter 140T greatly reduced operational errors. The auto-tracking system 

 eliminated errors caused by missing topographically important points and errors caused by 

 improperly aiming at the prisms. 



Chapter 2 Profile Data 



