An optical square is a simple yet effective optical device used in surveying to accurately establish or verify a right angle (90 degrees) on the ground, often for offsetting points or setting out perpendicular lines. It achieves this by utilizing reflection to calculate and identify the right angle, as it allows the user to sight two points simultaneously: one directly and one through reflection, aligning them to form the precise perpendicular.
Understanding the Optical Square
At its core, an optical square is an optical device that uses reflection from internal mirrors or prisms to allow a surveyor to sight along a main baseline while simultaneously observing a point that forms a 90-degree angle to that baseline. This capability makes it indispensable for tasks requiring precise perpendicularity without the need for more complex total stations or theodolites for basic setups.
There are primarily two types of optical squares:
- Single-Prism or Mirror Optical Square: Uses one prism or mirror system. The observer sights directly along the baseline through one part of the mirror/prism, and simultaneously sees a reflected image of the perpendicular line's target.
- Double-Prism or Mirror Optical Square: Employs two prisms or mirrors, offering greater accuracy and ease of use as it allows for the simultaneous sighting of both the main line and the perpendicular line's target by aligning two reflected images.
Practical Applications in Surveying
Optical squares are highly versatile for various field tasks:
- Setting Out Perpendicular Lines: Establishing a line that is exactly 90 degrees to an existing baseline.
- Offsetting Points: Measuring a point's position by determining its perpendicular distance from a known line. This is crucial for plotting details that are not directly on a main survey line.
- Checking Squareness: Verifying if corners of structures, foundations, or plot boundaries are truly at right angles.
- Chain Surveying: Used to establish offset lines from the main chain line to locate details.
Step-by-Step Guide to Using an Optical Square
Using an optical square is straightforward, but requires careful alignment and observation. Here's a general procedure:
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Establish the Baseline:
- Define your main survey line (baseline) with two marked points, e.g., Point A and Point B. This is the line from which you want to create a perpendicular.
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Position the Observer (You):
- Stand on the baseline at the approximate point where you want to establish the perpendicular.
- Hold the optical square horizontally at eye level, aligning your eye with its sighting aperture.
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Sight Along the Baseline:
- Look through the direct vision port (or a specific section of the mirror/prism) of the optical square.
- Align your direct line of sight with a known distant point on your baseline (e.g., Point B). Ensure this target is perfectly centered in your view.
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Identify the Perpendicular Line (Using Reflection):
- While maintaining your direct sight on the baseline target, you will simultaneously see a reflected image in another part of the optical square's optics. This reflected image is from a point perpendicular to your direct line of sight.
- Have an assistant move a ranging pole or target along the approximate perpendicular line.
- Guide your assistant to move the target until its reflected image perfectly aligns with your direct sight of the baseline target.
- According to the reference, this process involves "identifying the line forming the right angle based on a reflection above the glass." You are essentially using the reflection to identify exactly where the 90-degree line intersects.
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Mark the Perpendicular Point:
- Once the reflected image of the perpendicular target precisely coincides with the direct image of your baseline target, the ranging pole held by your assistant is exactly on the perpendicular line.
- Mark this spot on the ground. This point, combined with your position on the baseline, forms a 90-degree angle with the baseline.
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Verify (Optional but Recommended):
- For critical work, it's good practice to re-check the angle from another point on the baseline or by swapping positions with your assistant.
Example: Offsetting a Point from a Baseline
Imagine you have a baseline (Line PQ) and you need to determine the exact position of a tree (T) by offsetting it perpendicularly from PQ.
| Step | Action |
|---|---|
| 1. Baseline Setup | Mark points P and Q on the ground to define your baseline. |
| 2. Observer Position | Stand on line PQ, approximately opposite the tree T. Let's call your standing point 'O'. |
| 3. Sight Baseline | Hold the optical square at O. Look through the direct view and align it perfectly with point Q (or P). |
| 4. Align Perpendicular | While sighting Q, observe the reflected image. Guide an assistant holding a ranging pole at the tree (T). Ask them to move the pole left or right along a rough perpendicular line until its reflected image in the optical square aligns precisely with your direct sight of point Q. |
| 5. Mark Perpendicular Point | Once aligned, the assistant's pole is exactly on the perpendicular line. Mark this point (let's call it T') on the baseline. |
| 6. Measure Offset | Now, measure the distance from O to T' (which should be zero, as O and T' are the same point if you're standing at the perpendicular intersection) and then measure the perpendicular distance from T' to T. This gives you the offset distance and the chainage (distance along the baseline) of the tree. |
Advantages and Considerations
Advantages:
- Portability: Small and lightweight, easy to carry in the field.
- Ease of Use: Relatively simple to operate with minimal training.
- Cost-Effective: Much more affordable than electronic surveying instruments.
- Quick Setup: Fast for setting out basic right angles on site.
Considerations:
- Accuracy: While good for short distances and general layout, it is less precise than a total station or theodolite for long distances or high-accuracy work.
- Line of Sight: Requires clear lines of sight for both direct and reflected views.
- Observer Skill: Accuracy depends on the user's ability to precisely align the images.
In summary, the optical square is an excellent tool for surveyors when a quick, reliable method for establishing or checking right angles is needed, leveraging the principle of reflection to precisely calculate and define perpendicular lines on the ground.
