A nonlinear incremental measurement equation under poor observation condition was established

and its linearization method was given. Based on this equation

an extended incremental Kalman filter(EIKF)model under poor observation condition and its recursive calculative steps were proposed. Due to environmental factors and the instability of measurement equipments

the measurement data had unknown system errors in engineering. Classical extended Kalman filter(EKF)method cannot compensate and correct the unknown system errors

which produced considerable filter errors and even led to diverge. The presented EIKF method can successfully eliminate these unknown system errors and effectively improve the accuracy of nonlinear filtering. The method is simple to calculate and easy to apply in engineering.