怎樣計算黃浦江的寬度？

先係ｂ岸ｇｅ地方
選一點ｃ
令ａ，ｂ，ｃ三點
成為三角形
設ｂ，ｃ兩點都係浦東岸邊
咁ｂｃ的長度就可以直接量出呢

如題量到ｂｃ＝５２１．２１米
∠ａ同∠ｃ也可以用精密ｇｅ經緯儀
測出呢

如果測到
∠ａ＝４５∘１６’４２”
∠ｃ＝４６∘４３’１２”

係三角形ａｂｃ中
∠ａ，∠ｃ，ｂｃ都測出呢
利用正弦定理計出ａｂ的長度

由正弦定理

sin∠ｃ」ａｂ＝sin∠ａ」ｂｃ

１．０２４６＊５２１．２１
＝５３３．９４米

誤差大約３ｃｍ

要用經緯儀 (theodolite)。 ...........
Two persons are position in the opposite bank of the river with the SAME sea level ALONG the banks.
Let the one with theodolite be A and the one with the flag at the opposite bank be B. The position of A is fixed. Firstly, B stands at the original position (say, O) and A observes B and then set the theodolite to be zero.
After setting zero, B moves STRAIGHT towards right and then stop in a distance, say, d1. A then records the angle ( say, w1 ) observed.
B then moves STRAIGHT backward and then stop in a distance d2 FROM the original position O.
The river width is then computed by d1, d2, w1 and w2.

Points to be notice:
1) A and B should be in the same sea level otherwise the level difference at EACH points MUST be known in the computation.
2) B must move in a straight line. This is easily be done by methods such as 拉緊漁絲
3) B need not to move backward. B may also move straight in the second move. Moving backward is just a practice to reduce the reading error in setting the theodolite. However, B must always move in a straight line.
4) May take more measurement for comparison.
5) In order to reduce the percentage error, it would be better to take d1 and d2 approximately equal to the estimated river width. However, in practice, 50m, 100m or 1000m are used for ease of computation.

I am sorry that my chinese input is horrible and so I just typed the answer in english. I will be glad if anyone agree with my answer and translate it into chinese.

2006-12-21 22:58:05 補充：
Is ab the river width? Impossible! You even did not point out the correct position of the theodolite!!!