Digitisation and analysis of historical vertical deflections in India

We describe the [somewhat tedious] process of digitising from a 1955 report that lists over 1000 vertical deflections in India and some surrounding countries. It involved error-checking with closed-loop tests and resolution of an ambiguity surrounding the meridional vertical deflection at the Kalianpur origin of the datum. We transformed these Kalianpur coordinates to geodetic coordinates on geocentric datum to compute absolute vertical deflections. However, due to many changes to the Everest spheroid due to different feet to metre conversions and readjustments of the Kalianpur datum, we were restricted to using the abridged Molodensky transformation parameters for the 1975 Kalianpur datum and Everest 1956 spheroid based on only seven common points from the WGS84 technical manual. We compared these transformed absolute vertical deflections with EGM2008 and GGMplus (for both models: meridional standard deviation: ∼±2″; prime vertical standard deviation: ∼±3″), showing that the effort of digitisation and scrutiny of historical geodetic data is indeed worthwhile.

本文详述了一项略显繁琐的数字化工作流程：针对1955年一份记载了印度及周边国家超1000个垂线偏差（vertical deflections）的历史报告开展数据数字化处理。该流程包含闭合环测试误差校验，以及针对卡利安普尔基准原点处子午圈垂线偏差相关歧义的消解工作。我们将这些卡利安普尔坐标系下的坐标转换至地心基准面下的大地坐标，以此计算绝对垂线偏差。然而，由于埃弗雷特椭球（Everest spheroid）因不同英尺-米单位换算及卡利安普尔基准的多次重新平差发生了诸多变更，我们仅能依托WGS84技术手册中的7个公共点，使用适配1975年卡利安普尔基准的简化莫洛坚斯基转换参数，以及1956年版埃弗雷特椭球完成坐标转换。我们将转换得到的绝对垂线偏差与EGM2008、GGMplus两类模型的计算结果进行对比，两类模型的子午圈分量标准差均约为±2″，卯酉圈分量标准差约为±3″，对比结果表明，对历史大地测量数据进行数字化与严谨核查的工作确实颇具价值。