📚 Three-Figure Bearings: Concepts and Calculation Techniques | 三数方位角的概念与计算技巧
In navigation, surveying, and many IB Mathematics applications, the three-figure bearing is the standard method for specifying direction. Understanding how to interpret, convert, and calculate three-figure bearings is essential for solving problems involving trigonometry, vectors, and coordinate geometry.
在导航、测绘及IB数学的许多应用问题中，三数方位角是描述方向的标准方法。理解三数方位角的读法、转换与计算技巧，是解决三角函数、向量及坐标几何相关问题的关键基础。
1. Definition and Notation | 定义与记法
A three-figure bearing is the angle measured clockwise from true north to the direction of interest. It is always written with three digits, ranging from 000° to 360°.
三数方位角是从正北方向起，按顺时针方向量至目标方向所成的角度。它始终用三位数表示，范围从000°到360°。
North is 000°, East is 090°, South is 180°, West is 270°.
正北为000°，正东为090°，正南为180°，正西为270°。
A bearing of 000° means the direction is exactly north; a bearing of 360° is equivalent to 000° and is typically not used.
方位角为000°表示方向恰为正北；360°与000°等价，但通常不采用。
The angle is always measured clockwise, never anticlockwise.
方位角恒为顺时针量度，绝不采用逆时针方向。
The three-digit format avoids ambiguity: for example, 047° clearly indicates a direction forty-seven degrees clockwise from north, while the number 47 alone could be mistaken for an unnormalised angle.
三位数制式可避免歧义：例如047°清楚地表示从正北顺时针旋转四十七度的方向，而单独写47则可能被误解为一个未规范化的角度。
2. General Direction Description | 一般方向的描述
When a direction is described using compass notation such as "N30°E" or "S50°W", we can convert it into a three-figure bearing by following a simple rule.
当方向以"N30°E"或"S50°W"等罗经方位表示时，可按简单规则将其转换为三数方位角。
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Express the direction relative to North or South first, then towards East or West.
先以正北或正南为基准，再说明偏向东或西。
From North: bearing = 000° + offset to East, or 360° − offset to West
From South: bearing = 180° − offset to East, or 180° + offset to West
The table below summarises the conversions:
下表汇总了常见转换关系：
| Compass notation | Bearing | 罗经写法 | 方位角 |
| N30°E | 030° | 北偏东30° | 030° |
| N60°W | 300° | 北偏西60° | 300° |
| S40°E | 140° | 南偏东40° | 140° |
| S25°W | 205° | 南偏西25° | 205° |
Notice that "N30°E" means from North, turn 30° towards East, giving 030° directly. "N60°W" means from North, turn 60° towards West, which is equivalent to 360° − 60° = 300°.
注意："N30°E"表示从正北向东转30°，直接得030°；"N60°W"表示从正北向西转60°，等价于360°−60°=300°。
3. Back Bearings (Reverse Bearings) | 反方位角
The back bearing or reverse bearing is the bearing of the return direction, exactly opposite to the original direction. It is obtained by adding or subtracting 180°.
反方位角是与原方向恰好相反的返回方向所对应的方位角。其求法为在原方位角基础上加上或减去180°。
If bearing b ≤ 180°, back bearing = b + 180°
If bearing b > 180°, back bearing = b − 180°
Equivalently, you can add 180° and then subtract or add 360° if the result exceeds 360°.
等价地，可以先加180°，若结果超过360°则再减去360°。
For example, if a boat travels on a bearing of 065°, the return journey would be on a bearing of 245°. If it travels on a bearing of 300°, the back bearing is 120°.
例如，一艘船按065°方位角航行，返程方位角则为245°。若按300°航行，则反方位角为120°。
4. Drawing a Bearing Diagram | 绘制方位角示意图
Accurate diagram construction is a core skill for bearing problems. Always start by drawing a vertical line representing north from the observation point.
准确作图是解决方位角问题的核心技能。务必首先从观测点画一条竖直的线代表正北方向。
Step 1: Mark the observation point O, draw the north line upward.
第一步：标出观测点O，向上画出北向线。
Step 2: Use a protractor to measure the bearing clockwise from the north line.
第二步：用量角器从北向线起顺时针量取方位角。
Step 3: Draw the ray from O in that direction and label the angle.
第三步：从O沿该方向画出射线，并标注角度。
For triangle problems, all vertices have their own north lines parallel to each other. This enables the use of alternate angles to find unknown angles inside triangles.
在三角形问题中，每个顶点处都画有彼此平行的北向线。这样可利用内错角关系求三角形内未知的角度。
When connecting two points A and B, the bearing of B from A and the bearing of A from B differ by 180°, providing a useful check on your diagram.
连接两点A与B时，B相对于A的方位角和A相对于B的方位角相差180°，这为检查图形提供了有效依据。
5. Converting Bearing to Displacement Components | 方位角与位移分量的转换
To work with bearings analytically, we often resolve a distance travelled into north and east components. Let d be the distance travelled on bearing θ.
为对方位角进行解析运算，常将航行距离分解为北向与东向分量。设沿方位角θ航行的距离为d。
North displacement = d × cos θ
East displacement = d × sin θ
The convention matches the unit circle where 0° is North, 90° is East, so the usual x-y trigonometric definitions are rotated appropriately.
该约定与单位圆中0°为正北、90°为正东的定义一致，因此通常的x-y三角定义相应作了旋转。
For example, a ship travels 120 km on a bearing of 035°. The northward displacement is 120 cos 35°, approximately 98.3 km, and the eastward displacement is 120 sin 35°, approximately 68.8 km.
例如，一艘船沿035°方位角航行120 km。北向位移为120 cos 35°，约98.3 km；东向位移为120 sin 35°，约68.8 km。
6. Finding the Bearing from Coordinates | 由坐标求方位角
Given two points with known coordinates, we can compute the bearing from one point to another using trigonometry, taking care with the quadrant.
已知两点的坐标时，可通过三角函数计算一点到另一点的方位角，并务必注意象限的判断。
Let ΔE be the eastward difference and ΔN be the northward difference from point P to point Q. Define the acute reference angle α as:
设从点P到点Q的东向差为ΔE，北向差为ΔN。定义锐角参考角α为：
α = arctan(|ΔE| / |ΔN|)
Then the bearing θ is determined by the signs of ΔE and ΔN:
方位角θ则由ΔE与ΔN的符号决定：
| Signs (ΔE, ΔN) | Quadrant | Bearing θ | 符号（东，北） | 象限 | 方位角θ |
| (+, +) | NE | α | (+, +) | 东北 | α |
| (+, −) | SE | 180° − α | (+, −) | 东南 | 180° − α |
| (−, −) | SW | 180° + α | (−, −) | 西南 | 180° + α |
| (−, +) | NW | 360° − α | (−, +) | 西北 | 360° − α |
bearing is 090° for ΔE > 0, or 270° for ΔE < 0. If ΔE = 0 and ΔN < 0, the bearing is 180°.
若ΔN=0，则ΔE>0时方位角为090°，ΔE<0时为270°。若ΔE=0且ΔN<0，则方位角为180°。
7. Worked Example: Bearings in a Triangle | 例题：三角形中的方位角
e bearing of C from B is 120°. If AB = 80 km and BC = 60 km, find the bearing of C from A.
考虑三角形ABC，其中B相对于A的方位角为040°，C相对于B的方位角为120°。若AB=80 km，BC=60 km，求C相对于A的方位角。
+ 40° = 220°, but as an interior angle with the north line, it is easier to use geometry.
在B点画北向线。B处北向线与BA所成的方向角为180°+40°=220°，但利用几何求内角更为简便。
of BA (from B to A) is the back bearing of 040°, which is 220°. The interior angle at B is |220° − 120°| = 100°.
在B处，BC方向与北向夹角为120°。BA（从B到A）的方向是040°的反方位角，即220°。因此B处的内角为|220°−120°|=100°。
Using the cosine rule in triangle ABC:
在三角形ABC中使用余弦定理：
AC² = 80² + 60² − 2 × 80 × 60 × cos 100°
AC² ≈ 6400 + 3600 + 1667.6 = 11667.6, so AC ≈ 108.0 km.
AC² ≈ 6400 + 3600 + 1667.6 = 11667.6，故AC ≈ 108.0 km。
By the sine rule, sin ∠BAC ÷ 60 = sin 100° ÷ 108, giving ∠BAC ≈ 33.2°.
由正弦定理，sin ∠BAC ÷ 60 = sin 100° ÷ 108，得∠BAC ≈ 33.2°。
A is 040° and C lies to the east of AB, the bearing of C from A is 040° + 33.2° = 073.2°.
因B相对于A的方位角为040°，而C位于AB的东侧，故C相对于A的方位角为040°+33.2°=073.2°。
8. Triangulation and Resection | 三角测量与后方交会
Triangulation uses bearings from two known points to locate an unknown point. If two observers at fixed locations measure bearings to the same target, the intersection of the two bearing lines determines the target's position.
三角测量利用两个已知点的方位角来确定未知点的位置。若两个固定观测点测得同一目标的方位角，两条方位线的交点即为目标位置。
Suppose observer P sees a ship on a bearing of 060° and observer Q sees the same ship on a bearing of 150°. If P and Q are 40 km apart, the triangle formed has a known side and two known angles.
设观测点P测得船在060°方位，观测点Q测得船在150°方位。若P与Q相距40 km，则所成三角形已知一边与两角。
Let the ship be at S. The interior angle at P is 60° from the north line to PS. The interior angle at Q relates to the bearing 150°; the direction from Q to P is the back bearing of the line PQ, which depends on the relative placement of P and Q.
设船位于S。P处的内角为北向线与PS之间的60°。Q处内角与150°方位角相关；从Q到P的方向是直线PQ的反方位角，其值取决于P与Q的相对位置。
The sine rule then gives the distances PS and QS. This technique is fundamental in navigation, surveying, and search-and-rescue operations.
随后利用正弦定理即可求得PS与QS的距离。该技术在导航、测绘及搜救作业中具有基础性地位。
9. Common Mistakes and Pitfalls | 常见错误与陷阱
Students frequently make predictable errors when dealing with three-figure bearings. Being aware of these pitfalls improves accuracy.
学生在处理三数方位角时常犯一些典型错误。了解这些陷阱有助于提高准确率。
Measuring anticlockwise: bearings are always measured clockwise from north.
逆时针量角：方位角必须从正北顺时针量取。
Forgetting leading zeros: 040° is not the same as 40°, and examiners penalise missing zeros.
遗漏前导零：040°不同于40°，考官会对缺失的零扣分。
Adding 180° incorrectly for back bearings: check whether to add or subtract depending on whether the original bearing is below or above 180°.
反方位角加减180°时出错：应根据原方位角是小于还是大于180°决定加或减。
Using cos for east displacement: the east component is distance × sin θ, not cos θ.
用cos求东向分量：东向分量应为距离×sin θ，而非cos θ。
Neglecting quadrant signs when computing bearings from vectors: arctan alone does not distinguish between opposite directions.
由向量求方位角时忽略象限符号：仅用arctan无法区分相反方向。
10. Tips for Speed and Accuracy | 快速准确的技巧
Mastering a few practical habits can dramatically reduce errors and save time in examinations.
掌握几个实用习惯可以显著减少错误并节省考试时间。
Always draw a clear north line at every observation point.
在每个观测点处务必画出清晰的北向线。
Label alternate angles using the parallel north lines to transfer directions around the diagram.
利用平行的北向线标出内错角，以便在图中传递方向信息。
When using the sine or cosine rule, draw the triangle separately from the bearing diagram to avoid clutter.
使用正弦或余弦定理时，将三角形单独画出，避免与方位角示意图混杂。
Estimate the final bearing by visual intuition: a bearing between 000° and 090° should point roughly northeast, which helps catch mistakes.
通过直观估计最终方位角：介于000°与090°之间的方位角应大致指向东北，有助于发现错误。
Write bearings with exactly three digits, including leading zeros, in both working and final answers.
在计算过程及最终答案中，方位角一律写成三位数，包括前导零。
11. Connection to Vectors and Complex Numbers | 与向量和复数的联系
In vector form, a bearing θ may be represented by a unit direction vector. The unit vector pointing in the direction of bearing θ is:
在向量形式中，方位角θ可用单位方向向量表示。指向方位角θ方向的单位向量为：
u = (sin θ, cos θ)
Here the first coordinate is the eastward component and the second is the northward component. This matches the standard conversion already discussed.
这里第一个坐标为东向分量，第二个坐标为北向分量。这与前述标准转换完全吻合。
This vector representation is especially useful when adding displacements. Two legs of a journey can be written as position vectors, added componentwise, and then converted back to a single bearing and distance.
该向量表示在位移叠加时尤为有用。一段航行的两段位移可写成位置向量，按分量相加，再转换回单一的方位角与距离。
12. Revision Summary | 复习总结
The three-figure bearing is a compact, unambiguous system for describing direction, built around clockwise measurement from north using exactly three digits. Key operations include converting from compass notation, calculating back bearings, resolving into north and east components, and determining bearings from coordinates or from triangle geometry.
三数方位角是一种简洁而无歧义的方向描述体系，它以正北为基准、按顺时针方向量度并以三位数表示。核心操作包括罗经写法的转换、反方位角计算、北向与东向分量的分解，以及由坐标或三角形几何求方位角。
Always sketch a diagram first, mark parallel north lines, and check the quadrant before finalising an answer. With consistent practice, bearing problems become straightforward applications of basic trigonometry and geometry.
解题时务必先画示意图、标出平行北向线，并在得出最终答案前检查象限。通过持续练习，方位角问题将化为三角函数与几何的基础应用。
Published by TutorHao | IB Mathematics Revision Series | aleveler.com
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