Abstract: This paper analyzes the effect of two kinds of altitudes provided by barometer and digital map on the position precision,and deduces the error expressions.Many simulation results show that in the two cases,the effects of the same altitude errors on the positioning precision is identical,and the rules that the two altitude errors affect positioning precision can be expressed that altitude error has little effect on the east-west position error,but has large effect on the south-north position error,the lower latitude the larger effect,the higher latitude the less effect.