Abstract: Combining neighboring swaths is an important step in swath sounding data processing.The error structure of ping in swath sounding is brought forward.Then the adjustment defect model is given for the swath combination net,and we poin out the lack of datum data is main factor to make the adjustment mode defect.To solve the large equation based pings,the formulae is derived based on the theory of adjustment division.A practical example for detecting systematic errors is given.Experimertal results show that the defect model can detect the systematic errors on a ping,or on a swath even on the whole surveying area.