@article{ning_2015,  title={USING SURFACE FITTING AND BUFFER ANALYSIS TO ESTIMATE REGIONAL GEOIDAL UNDULATION},  volume={21},  ISSN={1982-2170},  url={https://doi.org/10.1590/S1982-21702015000300035},  DOI={10.1590/S1982-21702015000300035},  number={3},  journal={Boletim de Ciências Geodésicas},  publisher={Universidade Federal do Paraná},  author={NING, FANG-SHII},  year={2015},  month={Jul},  pages={624–636} }