Abstract: The paper discusses whethter the doubled half-thread reading value read from a theodolite can be taken as a sight-length. After strict derivation in math the mathematic model error value of between the doubled half-thread reading and the full-thread reading is achieved; comparing the drawn error value curve with the accuracy of aurveyed sight-length numbers the paper gives clear conclusions and suggestions

Key words: sight-length, theodolite, doubled half-thread

摘要： 笔者针对采用经纬仪二倍半丝读数能否作为视距的问题进行讨论.经过严密的数学推导，得出了采用二倍半 丝与采用全丝读数视距差值的数学模型;通过绘制差值的变化曲线和视距测量的精度进行比较，对该问题做出了 明确的结论和建议

关键词: 视距测量, 经纬仪, 二倍半丝法