Research On The Theory And Implementation Method Of Marine Seamless Vertical Reference Surface

In recent years, the implementation of seamless vertical datum is hot issue and not an easy task in marine surveying and mapping area. It is vertical initial benchmark for surveying in ocean, river, and costal zone or at the junction of water and land. Which has such characteristic:a) continuous and seamless; b) dose not vary significantly over time; c) conform to the human habits of marine surveying; d) be apt to convert with other vertical references. The establishment of modernization seamless vertical datum is internal demand of development of marine surveying and the national economic construction, which is inevitable trend, with the growing development and utilization of coastal zone, river shoal, and at the junction of water and land. It provides the theoretical basis and fundamental guarantee for coastline definition, the construction of digital costal areas and the national defense economy.At present, there is still a series of key problems in the establishment of seamless vertical datum. Such as, the choice of vertical datum to establish seamless vertical datum for best, and the pretreatment of the original data before the establishment, and how to establish and convert with other vertical datum etc. Therefore, there are many new ideas, new model and methods having been put forward after summarizing domestic and foreign research status, and careful analysis of existent key problems. Finally, it has realized that seamless vertical datum construction and mutual conversion of different vertical datum. The main work and innovative points reflect in:(1) The choice of vertical reference Firstly analysis and compare the commonly used vertical reference in marine surveying in detail, geoid and reference ellipsoid and mean sea level and depth datum. As land vertical reference, the geoid has continuous and seamless characteristics. But it is a gravity equipotential surface, and apt to be land vertical reference, not suitable as ocean vertical datum. Reference ellipsoid is geometric surface, although has simple mathematical model and continuous, seamless and smooth features, lack of physical significance. Moreover, it dose not accord with human in the habit of surveying and mapping activities. Mean sea level vary with the location and time difference, and can not guarantee the safe navigation. Although depth datum exists discrete, it is the standard vertical reference for chart height in marine surveying and mapping, for the ship security sailing providing protection. Therefore, the paper chooses depth datum to establish marine seamless vertical datum in physical meaning and human use, which both have actual physical significance and application value, and conform with habit of marine surveying and mapping.(2) The tides material pretreatment methodsThe determination of depth datum needs tidal observation, therefore, it is decisive importance to eliminate all kinds of different types of error in tidal observation accurately for precision controlling of depth datum. According to the characteristics of the error, they have been divided into:observation noise cause by the waves, the loss of tidal observation, gross error and system error four categories.a) The influence of waves.The FFT low pass filtering method has been used for filtering the observation noise, and the size of cut-off cycle is related to the cycle of tidal constituent. Through the analysis of the theory and experiment, it has got that the noise can be filtered out when the cut-off cycle is one hours. In order to prevent the active tidal ingredients filtered, the size of cut off cycle should not more than three hours.b) The loss of tidal observationAccording to tidal sequence matching related parameters, ratio of tidal ranges x, time difference of tide y, datum drift z, repair and acquire the missing tidal part. And the repairing can satisfy the90%of repaired point precision within10cm,80%within5cm. thus have already reached the tidal repairing precision standard of specifications for hydrographic survey.c) The gross errorFor the gross error in the long-term tidal observation sequence, this paper has put forward a new filtering method based on the tidal harmonic analysis. The filtering method is better than FFT low pass filtering method in accuracy, and overcome the problem of low accuracy and efficiency of artificial repair. It can satisfy more than90%of repaired point precision within20cm.d) The system errorFor the system error, which is mainly caused by tidal "zero" drift, have been studied. In the long-term and short-term tidal sequence two aspects, the new detection and repairing methods have been put forward respectively. The former detected system error using the chebyshev inequality principle in theory of probability, and repaired them with the mathematical expectation of mean sea, and it can satisfy precision better than15cm and90%of repaired point precision less than10cm. The latter, a self-segmented tidal sequence matching method which is based on the least squares has been put forward. Realize the system error effective detection and repair, the precision less than10cm.(3) The precision analysis and quality control of depth datumThe high accurate determination of depth datum is of great significance to the establishment of marine seamless vertical datum. The determination methods of depth datum can be divided into two classes, the first kind is to use the tidal harmonic constants acquired by tidal harmonic analysis to calculate the depth datum according to the depth datum models. The second type is according to the depth datum transfer methods to obtain. Aim at the first kind, this paper mainly from the relationship research between tidal depth datum stability, precision and number of tidal constituent, length of tidal sequence, has given how to use the number of tidal constituent and harmonic model under the different tidal observation length. For the second, firstly, concludes that the least squares transfer method is best through a large number of experiments of comparing several depth datum transfer methods. When synchronous observation time is more than three days, the transmission precision can reach up to5cm. With this method as an example, this paper put forward the tidal type difference value (HK1+HO1)/HM2between two places as an important transfer basis, and lots of experiments have been researched in the Yangtze estuary region. The conclusion is as follows:a) In a large of flat waters, when tidal type D-value is less than0.05and transmission distance less than80kilometers, the precision of depth datum can be within5cm. Whereas, the transmission distance is more than80km, the precision gradually decline and the decrease rate for about1cm/km or so.b) In the long and narrow rushing waters, when tidal type D-value is less than0.05and transmission distance less than40kilometers, the precision of depth datum can be within15cm. Or when the transmission distance is more than40km, the precision fall rapidly, and it is not suitable for depth datum transfer.c) Whether in a large or long and narrow waters, then tidal type D-value is more than0.05, the transmission precision is not very ideal, not suggestion for the transfer of depth datum. Lots of experiments have been tested in Yangtze estuary river, and it has been proved correct, the above conclusion is important to the precision controlling of depth datum transfer in the Yangtze estuary.(4) Research on the geometric implementation method of marine seamless vertical datumInverse distance weight, minimum curvature method, nearest neighbor interpolation, linear interpolation triangulations method, polynomial regression, moving average method, kriging method and radial basis function method, total eight kinds of commonly used geometric modeling methods, have been respectively used to establish marine seamless vertical datum in the local area of Wenzhou Oujiang river. Through the detailed comparison of the3d surface characteristics, isoline characteristics, outside precision aspects of the established seamless vertical datum, it has got a conclusion as follows:in the case of tidal stations arrangement not intensive and uniform distribution and the complex changes of depth datum height, inverse distance weight, kriging and radial basis function methods are suggested to be adopted to establish marine seamless vertical datum. The three kinds of methods can reflect the change trend of depth datum well, and the built surface has features of continuous and smooth.(5) Research on the physical modeling method of marine seamless vertical datumThe precision of Geometric interpolation method is affected largely by the distribution and number of interpolation node, and elevation changes of depth datum, and when geometric model pushed out, the precision is generally low. From the point of view of tidal physical transmission characteristics, the paper has put forward three kinds of physical interpolation methods which are to use tidal harmonic constant interpolation and the ratio of tidal range interpolation and the least square transfer parameters interpolation to determine depth datum. Through a large of experiments in the Yangzte Estuary River and Oujiang River, it has got that the three physical interpolation methods are more advantages in precision and stability. Among them, the tidal harmonic constant interpolation, whether interpolation or extrapolation, the precision can be in less than10cm, and has very high accuracy and stability, and is the best method to establish seamless vertical datum.(6) Transformation between land and ocean vertical reference and precision controlAccording to the relationship among reference ellipsoid, geoid and depth datum, the specific conversion models have been put forward among geodetic height and normal height and chart height. At the same time, detailed analyzing of error source of each parameter in the transformation model has been made, and has given error expression of elevation transformation model.