A Case Study on Hydrographic Survey Processing with Ellipsoidal Altitude

The ability to clearly image and map the sea bottom depends very much on a capability to accurately acquire and apply measurements of vessel altitude and/or vertical displacement. These measurements include vertical acceleration, three dimensional GPS (Global Positioning System) position (in x,y and Z) , tide levels and dynamic draft. As the capability of modern swath echo sounders has improved, so has the need for an increased attention to these positional hydrographic data attributes. The focus of this paper will be a case study of a survey area where tides and water level were quickly changing and difficult to measure or predict. The paper will serve to identify the effect of using Ellipsoidal Altitude in place of Heave (vertical displacement) in such an environment. The case study in this paper will show that the efforts in hydrographic data processing can be greatly reduced if Altitude is used in place of Heave. IHO (International Hydrographic Office) standards when applied to data sets where Altitude is used instead of Heave will also be examined. The IHO standard for Heave is typically specified at 5 cm or 5% of wave height and the requirement for accuracy in Altitude will be no less stringent. A method for achieving this level of accuracy is known as IAPPK (Inertially Aided Post Processed Kinematic). The IAPPK method will be described and applied to the data set under examination. The IAPPK method produces reliable high bandwidth Altitude with sub 5 centimeter accuracy. The IAPPK method is not routinely used in Hydrographic survey. IAPPK advantages and work flow will be described. The bathymetric data processing technique which will be applied in the case study will be the most modern available. It will employ TPE (Total Propagated Error) and CUBE (Combined Uncertainty Bathymetric Estimator) methodology. TPE in particular, relies heavily on the height and vertical displacement attributes, in particular the expectation of error estimation in these measurement- s. Error estimates of tide and dynamic draft are difficult to predict and model. This paper will show visible benefits when Heave is replaced with post processed Ellipsoidal Altitude. The uncertainty for Altitude is computed by the Inertial Navigator and can be input into TPE. The uncertainty for tide and dynamic draft fall out of the equation. The remaining uncertainty will be in the transformation of ellipsoidal Altitude to the chart datum. This paper will examine the error sources associated with measurements of vertical displacement and Ellipsoidal Altitude. In order to obtain the total variance for the final depth of an individual sounding, one must have knowledge of the variances that are associated with depth uncertainties. The case study will show that these error sources in the position domain can be applied as a-priori or dynamic values and that the final sea bottom image and/or depths can be affected these instructions give you basic guidelines for preparing papers for conference proceedings.