On the Error Observed in Electronically Digitized T-7 XBT Data

Abstract

Recent studies in the Gulf of Mexico have generated datasets that permit comparisons of CTD data with electronically digitized T-7 XBT data. This paper presents the results of two such comparisons involving the use of the BathySystems SA-810 and the Sippican Mk-9 digitizers and makes a further comparison with the findings of other investigators. A single, general, linear depth adjustment and an alternate third-order polynomial adjustment to the Sippican drop-rate equation are determined for the combined XBT dataset for application at depths greater than 100 meters. At shallower depths, the standard Sippican equation alone is applied. It is noted that the slope of the calculated linear adjustment is nearly identical to that calculated by Heinmiller et al. for the lower part of the water column (below 325 m) in the Sargasso Sea. The alternate polynomial adjustment was found to agree within 4.5% (1.1 meters) of the depth adjustment at 750 meters calculated by Hanawa and Yoritaka for data collected in the northwestern part of the North Pacific subtropical gyre off Japan.