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子午线的测定,一般分测星座和太阳两种。观测星座,又以观测北极附近的北极星为便。但利用北极星在上下中天和东西大距两个位置要在晚上进行观侧,很为不便。现在,我要介绍的是太阳高度法。太阳在白天就能看见,在测量工作中,随时都可以观测,不过受着折光和经纬仪的限制,最好当太阳在仰角15°至20°左右时进行观测,容易得到满意的结果。它的计算公式如下:
Meridian determination, generally divided into two types of constellation and the sun. Observation constellation, but also to observe the Polaris near the North Pole for the convenience. However, the use of Polaris up and down in the sky and things from two locations in the evening to view side, very inconvenient. Now, I want to introduce the solar height method. The sun can be seen during the daytime and can be observed anytime during the survey. However, subject to the limitations of refraction and theodolite, it is best to observe the sun at elevation angles of 15 ° to 20 ° to get satisfactory results. It is calculated as follows: