Orthometric corrections from leveling, gravity, density and elevation data: A case study in Taiwan

A new orthometric correction (OC) formula is presented and tested with various mean gravity reduction methods using leveling, gravity, elevation, and density data. For mean gravity computations, the Helmert method, a modified Helmert method with variable density and gravity anomaly gradient, and a modified Mader method were used. An improved method of terrain correction computation based on Gaussian quadrature is used in the modified Mader method. These methods produce different results and yield OCs that are greater than 10 cm between adjacent benchmarks (separated by ∼2 km) at elevations over 3000 m. Applying OC reduces misclosures at closed leveling circuits and improves the results of leveling network adjustments. Variable density yields variation of OC at millimeter level everywhere, while gravity anomaly gradient introduces variation of OC of greater than 10 cm at higher elevations, suggesting that these quantities must be considered in OC. The modified Mader method is recommended for computing OC.