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A short class with Sarah and her students. The other ways are under More.
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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.
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.
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.
The displacement of objects from their true positions in an aerial photograph due to their height.
The direction of relief displacement, which is always radial from the principal point.
Height can be computed using parallax measurements and known reference heights.
A high building may show significant displacement in a parallax measurement, indicating its height is greater than that of surrounding structures.
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.
To find a height, you measure parallax, take care of errors to avoid a tax!
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.
Remember 'PARA' for Parallax—Position, Angle, Reference, and Adjustment.
HARM: Height Affects Readings of Measurement, helps recall that height influences parallax readings.
Term
What is Relief Displacement?
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Term
What does the parallax bar measure?
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Term
What is Absolute Parallax?
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The apparent difference in the position of an object when viewed from different angles, crucial for elevation measurements.
A device used to measure the difference in parallax between points on stereo photographs.
The algebraic difference in distance of a point measured from the principal points of two stereo photographs.
The height of a point relative to a reference level, often used in the context of determining ground features from aerial photos.
This section discusses the concept of parallax in aerial photography, explaining how relief displacement of objects affects their measurement in stereo-pairs.
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.
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 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.
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.
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.