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Interactive Audio Lesson
Listen to a student-teacher conversation explaining the topic in a relatable way.
Introduction to Parallax
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Today, we will discuss parallax, which refers to the apparent difference in the position of an object viewed from different angles. Can anyone tell me why this concept is important in aerial photography?
It's important because it helps us measure elevations of objects.
Exactly! The greater the height of an object, the greater the parallax we will observe. This is why we use stereo pairs of photographs to see the same object from slightly different angles.
So how does this relate to the actual measurements we take?
Great question! We will use a tool called the parallax bar to help us measure these differences accurately.
What exactly does the parallax bar do?
The parallax bar measures the absolute parallax difference between two points. It's vital for calculating elevations accurately.
Can you give us an example of how we would use it?
Certainly! We will dive into practical exercises, but remember, the key point is that higher elevations indicate greater parallax!
To summarize, parallax refers to the observable difference in position from various viewpoints, critical for height measurements in aerial photography.
The Parallax Bar
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Let's move on to the parallax bar itself. Who can describe its structure?
It's a metallic rod with a fixed bar and a movable bar for measurement.
That's right! The movable bar has a micrometer drum for precise adjustments. Now, when you set the parallax bar over a stereo pair, what should you look for?
We need to align the floating marks with the same terrain point on both images.
Correct! This ensures that the measurements are accurate when we read the main scale and micrometer reading.
How do we know we're measuring the right point?
It's crucial to always backtrack to the principal points and ensure alignment during measurements. If everything is correctly aligned, your readings will provide the parallax needed for further calculations.
And what's the result of this measurement?
It allows us to determine the elevation of a point based on its parallax difference relative to known reference points. Remember, measuring accurately is vital as errors can accumulate!
To recap, the parallax bar aids in measuring the difference in parallax between points on stereo photographs, critical for assessing object heights.
Height Determination
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Now that we've discussed the methodology of using the parallax bar, how do we actually compute the height of unknown points?
We relate the parallax difference to known elevations, right?
Absolutely! For instance, if we know the height of one point, we can calculate the height of another point using the formula we discussed.
Could you show us how these calculations look?
Sure! If we know the elevation of point A and we have the parallax difference, we can apply the formula to compute the height of point B using its parallax.
Should we be concerned about potential errors in these measurements?
Yes! Factors like improper orientation of photographs and measurement inaccuracies can lead to significant errors.
What's an example of an error we could encounter?
An example would be a calibration error in the parallax bar itself. If the readings are off, all subsequent calculations will also be affected.
In conclusion, determining height from parallax involves precise measurements, awareness of potential errors, and application of the correct formulas.
Introduction & Overview
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Quick Overview
Standard
The section introduces the parallax bar as a tool for measuring the parallax between points in stereo aerial photographs. It outlines the relationship between parallax, height, and distance from the principal point, emphasizing the importance of this measurement in determining object heights.
Detailed
Measurements by Parallax Bar
In aerial photography, the change in position of images due to the motion of the aircraft is known as stereoscopic parallax or simply, parallax. This phenomenon is essential in accurately gauging the elevation of points represented in stereo pairs of photographs. The relationship between parallax and the height of the measured points is notably proportional; higher points exhibit greater parallax than lower points.
The Parallax Bar
The parallax bar is a vital device for measuring differences in parallax between two points in stereo photographs. It comprises a graduated metallic rod with two bars: a fixed bar and a movable bar equipped with a micrometer drum that adjusts for fine measurements. When using the parallax bar, floating marks engraved on glass graticules help align images taken from different vantage points.
Measurement Process
To measure parallax, the parallax bar is positioned so that floating marks overlap precisely at the same terrain point in stereo mode. The readings from both the main scale and micrometer give the total parallax reading. The absolute parallax is calculated as the algebraic difference of distances to a point, measured from the principal points of the images.
Height Determination
The elevation of a point can be approximated by relating the parallax difference with known elevations. This methodology allows for estimating the height of other points based on the known heights of one or more control points in the common area. However, accuracy depends on various factors, including precise measuring techniques and consistent calibration of the equipment.
Audio Book
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Definition of Parallax
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The change in position of an image from one photo to the next due to aircraft’s motion is called stereoscopic parallax, x-parallax, or simply parallax. The parallax is directly related to the elevation of the point, and is greater for high points than for low points.
Detailed Explanation
Parallax refers to the apparent shift in position of an object when viewed from different angles or perspectives. In the context of aerial photography, parallax occurs when the aircraft moves, causing the image of the same land point to appear in different positions in successive photographs. This effect is important because it helps measure the height of terrain features. High points create a larger parallax than low points, meaning they will appear to move more between two images, revealing their elevation above the reference level.
Examples & Analogies
Imagine watching a car drive past you while standing on a sidewalk. If you shift your position just a little to the left or right, the car appears to move against the background. The larger the distance between your view and the car's position, the more apparent the change is. This is similar to how we measure terrain heights from different aerial views.
Overview of the Parallax Bar
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The parallax bar is a device used to measure the difference of parallax between any two points on the stereo-photographs, more precisely. The parallax bar consists of a graduated metallic rod (in mm) attached with two bars, as shown in Figure 4.19.
Detailed Explanation
A parallax bar is a specialized instrument designed for precise measurements in photogrammetry. It consists of a main scaled rod that measures in millimeters and has two bars attached to it. One bar is fixed, while the other is adjustable using a micrometer drum, allowing for fine-tuning and very accurate readings of parallax. The parallax bar enables researchers to take measurements between selected points on stereo-pair photographs, which helps in calculating the elevations of those points through triangulation methods.
Examples & Analogies
Think of the parallax bar like a ruler with a sliding marker. When you are trying to measure a distance on a drawing or model, you can slide the marker to align it perfectly with your point of interest. This process ensures you get a precise measurement of the distance, similar to how the parallax bar aligns markers with features in aerial images.
Using the Parallax Bar for Measurements
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The points/objects on stereo-pair are selected whose parallax bar readings are to be taken for the computation of heights. The oriented stereo-pair is now viewed in stereo mode, and the floating marks of graticules are kept at a point/object.
Detailed Explanation
To use the parallax bar effectively, first identify the objects whose heights you want to measure from the stereo-pour photographs. Once identified, the stereo-pair images are viewed through the stereoscope to create a 3D effect. The floating marks on the graticules (thin glass plates) are then placed over the corresponding points on the left and right images. The goal is to adjust their alignment so that they appear to fuse into one mark, indicating the precise height of the object above the datum.
Examples & Analogies
Imagine playing a game where you have to line up two images perfectly to create an illusion—like a magic trick. In this case, the images are the aerial photographs, and the floating marks act as your magic wands that you adjust until the images blend seamlessly. This gives you the precise height measurement you need for each object.
Understanding Absolute Parallax
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The absolute parallax of a point on a stereo-pair is determined as the algebraic difference between two distances which are measured from the corresponding principal points, parallel to the direction of flight (air base). It is also called x-parallax.
Detailed Explanation
Absolute parallax, or x-parallax, is mathematically represented by the difference in horizontal distance between the same point observed in two different aerial images. These distances are measured from the principal point of each photograph, along the line of flight. This algebraic difference provides information about the elevation of the terrain relative to a known reference point, which is crucial for calculating the height of various objects in the images.
Examples & Analogies
Think of absolute parallax like tracking two points on a moving car as you stand on the sidewalk. If you measure the distances from your own viewpoint (the principal point) to each of the two points (the front bumper and the back bumper of the car) in two snapshots from different angles, you can determine how far the car has moved and how high it is above the ground compared to where you are standing.
Height Determination Using Parallax Measurements
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The elevation of unknown point B can be determined using parallax bar measurements based on the known height of point A. Since, it is difficult to measure the coordinates of all the objects on the photos, so these coordinates can be replaced by the parallax bar readings (P and P ) at points a and b.
Detailed Explanation
Once the parallax readings from the bar are obtained, estimations of heights at different points can be made using these measurements. The elevation at one known point serves as a reference. The relationship derived from the parallax readings allows for the computation of other heights based on the parallax difference and known measurements. This method helps in bridging gaps where direct measurements are challenging.
Examples & Analogies
This is like using a known building height to estimate the heights of nearby trees. If you know the height of the building, you can then apply similar calculations for the trees, using their positions and distances to guess their heights. The building's known height acts as a benchmark for making additional calculations.
Definitions & Key Concepts
Learn essential terms and foundational ideas that form the basis of the topic.
Key Concepts
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Relief Displacement: The displacement of objects from their true positions in an aerial photograph due to their height.
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Radial Displacement: The direction of relief displacement, which is always radial from the principal point.
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Height Estimation: Height can be computed using parallax measurements and known reference heights.
Examples & Real-Life Applications
See how the concepts apply in real-world scenarios to understand their practical implications.
Examples
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A high building may show significant displacement in a parallax measurement, indicating its height is greater than that of surrounding structures.
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If a known control point has an elevation of 100 meters, and the parallax readings indicate a difference with another point, we can compute its height accurately.
Memory Aids
Use mnemonics, acronyms, or visual cues to help remember key information more easily.
🎵 Rhymes Time
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To find a height, you measure parallax, take care of errors to avoid a tax!
📖 Fascinating Stories
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Imagine two friends trying to view a tall tree from different angles, one on a hill and the other in a valley. The friend on the hill sees the tree more displaced than the friend in the valley—this illustrates how elevation affects visibility through parallax.
🧠 Other Memory Gems
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Remember 'PARA' for Parallax—Position, Angle, Reference, and Adjustment.
🎯 Super Acronyms
HARM
- Height Affects Readings of Measurement
- helps recall that height influences parallax readings.
Flash Cards
Review key concepts with flashcards.
Glossary of Terms
Review the Definitions for terms.
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Term: Parallax
Definition:
The apparent difference in the position of an object when viewed from different angles, crucial for elevation measurements.
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Term: Parallax Bar
Definition:
A device used to measure the difference in parallax between points on stereo photographs.
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Term: Absolute Parallax
Definition:
The algebraic difference in distance of a point measured from the principal points of two stereo photographs.
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Term: Elevation
Definition:
The height of a point relative to a reference level, often used in the context of determining ground features from aerial photos.