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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 explore the concept of scale in vertical photographs. Can anyone tell me what the scale represents?
Is it the ratio of distances in the photograph to the actual distances on the ground?
Exactly! The scale is calculated as the distance between two points on the photograph compared to the distance between the same points on the ground. So, if we refer to the formula S = ab / AB, what do each of these variables stand for?
ab is the distance on the photo and AB is the distance on the ground, right?
Correct! Great job! This fundamental understanding will help us apply scale calculations effectively.
Let's calculate scale using focal length and flying height. Who remembers the basic formula?
Is it S = f / H, where f is the focal length and H is the flying height?
Exactly! The scale increases with a higher focal length or a lower flying height. Can someone give me an example of how changing these values affects the scale?
If the focal length increases, while maintaining the same height, the scale will get larger, allowing for more detail in the photograph!
Absolutely right! Your understanding of how varying these factors impacts the resulting imagery is crucial.
Now let’s talk about how scale changes when photographing undulating terrain. Can anyone explain why that is?
Because different points on the ground can be at different elevations?
Exactly! If the elevation changes, it can skew the scale for each point. What happens to the scale as elevation increases?
The scale decreases, because the flying height is effectively higher the further above sea level a point is.
Great work! Thus, the average scale calculation also needs to consider this variation. Can anyone provide the formula for the average scale?
S_avg = f / (H - h_avg), where h_avg is the mean elevation of the area.
Perfect! Knowing how to compute this average scale is essential in aerial photography.
The ratio of distance on a photo to the actual distance on the ground.
A key variable that affects image scale.
The height at which the camera captures images, affecting scale.
The scale may differ for points at varying elevations.
Overall scale computation considering different elevations.
If a photograph has a focal length of 100mm and is taken from 1000 meters above ground, the scale is calculated as S = f / H = 100mm / 1000m = 1:10,000.
For two points at different heights (h_a = 50m and h_b = 70m), their individual scales from the same height (H = 1000m) would be S_A = f / (H - h_a) and S_B = f / (H - h_b).
Higher the height, smaller the sight, lower the flight, more detail in sight!
Imagine a photographer capturing landscapes from a hot air balloon. As they float higher, the details below shrink. But when they hover low, every detail pops into view. This illustrates how scale shifts with flying height.
F Like Focal length; H Like Height - Remember: Scale is F/H.
SHF - Scale equals Height (focal) divided by Flight height.
Term
Scale of a Photograph
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Term
Focal Length
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Term
Flying Height
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The ratio of the distance between two objects on a photograph to the distance between the same points on the ground.
The distance from the optical center of the lens to the focal plane when the camera is focused at infinity.
The altitude at which the aircraft flies above mean sea level during photography.
The height of a point above a fixed reference, typically mean sea level.
The effective scale for a photograph averaged over various elevations within the coverage area.
This section explains the concept of scale in vertical photographs, detailing how it is calculated based on focal length and flying height above ground.
In this section, the scale of a vertical photograph is defined as the ratio of distance between objects on the photograph to the distance on the ground. The section elaborates on how to compute this scale using simple geometric principles, showing its dependence on camera focal length and flying height, as well as addressing variations in scale due to ground relief.
The scale of a vertical photograph is a crucial concept in photogrammetry, allowing for accurate representation and measurements of objects in aerial imagery. In a vertical photograph, the scale (S) is defined by the ratio of the distance between two points on the photograph (ab) to the corresponding distance on the ground (AB), expressed mathematically as:
In a flat terrain scenario, this relationship can also be articulated using focal length (f) and flying height (H) from the camera to the ground:
This indicates that the scale is directly proportional to the focal length of the camera lens and inversely proportional to the flying height, illustrating how camera settings influence the scale when capturing images from an aircraft. However, it is important to consider that as the ground is rarely perfectly flat, variations in elevation must be taken into account, especially when points on the ground are at different heights.
For points A and B at various elevations (h_a and h_b) above mean sea level, the scale must be adjusted accordingly:
Overall, the scale of a vertical photograph is important for developers and planners as it aids in determining accurate measurements and analyses, especially in mapping and surveying applications. If all points in a photograph were at the same elevation, uniform scale would apply. However, given that variations in elevation exist, the average scale across a photography can be represented as:
This general principle reinforces the necessity of understanding how scale varies with terrain features for accurate photogrammetric results.