Based on a statistics analysis of the experimental data of trigonometric leveling networks
at four extra-long bridges such as the Humen bridge, etc., comparisons are made in forward and
backward altitude differences, loop closures, the third-and the fourth-grade-leveling surveys
difference limitations, and the altitude differences between trigonometric height survey and order
leveling at common points. It is verified that in networks of rivers, using trigonometric height survey to
replace the fourth-grade-leveling is completely feasible for the leveling measurement of crossing rivers, but
when using it to replace the third-grade-leveling, the reliability is only about 75%.