Gravimetry Theories And Methods Of The Survey Line Network Adjustment

Airborne gravimetry is a new kind of technique for surveying the gravity acceleration over the earth's surface and is advanced in gravimetry history, which basing on aircraft, including gravimeter, GPS, altimeter and attitude sensor etc.. Airborne gravimetry is applied to rapidly get extensively and precisely well-distributed information of the earth gravity field in order to meet the needs of Geodesy, Geophysics, Oceanography, Exploration, Space-Science and the other fields.Airborne gravimetry includes scalar airborne gravimetry and vector airborne gravimetry. Scalar airborne gravimetry is used to get the value of the gravity acceleration, and vector airborne gravimetry can get not only the value but also the direction of the gravity acceleration. Airborne gravimetry which is discussed in this paper denote scalar airborne gravimetry only.In order to estimate the accuracy of airborne gravity measurements, the tracks of an airborne gravity survery are generally planed to join each other. Discrepancies between measurements on different tracks may exist at the intersection because airborne gravity data are unavoidably affected by different kinds of error source. Furthermore, the performances of these errors may appear systematic or random. Based on an analysis of the sources of errors in airborne gravity measurements, an error model is studied to constructed mathematically which can characterize the change of systematic errors, with which the network adjustment is carried out and the model parameters are determined simultaneously. And finally, the compensation of systematic errors is realized. The network adjustment is one of key techniques for airborne gravimetry, it is also an important component in this paper. Consequently, the paper investigates the following aspects about the network adjustment theory and practice for airborne gravimetry, moreover, some questions correlated with solving the adjustment problem are discussed in detail.1. Based on actualities of airborne gravimetry, two practical methods of searching crossovers are proposed, i. e. the skip-searching method and the graphic aid-searching method. A simulate network is used to test the efficiency and the reliability of these methods.Result demonstrates cross- points can be searched out rapidly and exactly by using these two methods.Compared with searching crossovers one by one, the skip-searching method increases the efficiency 91 times, and the graphic aid-searching method increases the efficiency 176 times.2. According to the characteristic of airborne gravity survey, several statistic variables are derived from theory of probability and mathematical statistics combined with knowledge of errors principles. These statistic variables are applie to the significance test of systematic errors in every surveying line. Two practical applications prove their efficiency.3. Using knowledge of adjustment with additional systematic parameters, based on an analysis of the sources of errors in airborne gravity measurements, an error model is studied to constructed mathematically which can characterize the change of systematic errors and with which three kinds of new crossover adjustment models are presented. Two practical survey data sets are used as a case study to verify the efficiency and reliability of the compensation method. The results show that the precision of the compensated network has been increased from 2.74mgal to 7.11magal.4. Based on an M-estimation principle, a robust estimator of parameters of systematic errors model is introduced in every surveying line. Comparison of the estimator from the least-squares method with the robust method shows that the estimator from the robust estimator is more reliable.