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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 learn about the concept of scale in photogrammetry. Scale is the ratio of a distance on a photograph to its corresponding distance on the ground. For example, if the scale is 1:25,000, then 1 cm on the photo represents 25,000 cm or 250 meters on the ground.
How do we actually calculate that scale from a photo?
Great question! The basic formula is S = ab/AB, where 'ab' is the distance between two points on the photograph, and 'AB' is the distance on the ground. This means we need to know both distances to determine the scale.
So, if we know the focal length and the height of the aircraft, we can also express scale differently, right?
Exactly! When we consider the focal length 'f' and the flying height 'H,' we find that scale can also be expressed as: S = f / H.
Is the scale always consistent across the photograph?
Not always. The scale can vary if the terrain has different elevations. For instance, if point A is at a higher elevation than point B, their scales will differ because the effective height to the camera changes.
That makes sense! Can we summarize what we learned today?
Sure! Remember that the scale is crucial in photogrammetry, and it can change based on elevation. The basic formula is S = ab/AB, while in relation to focal length and flying height, it’s S = f / H.
Now let’s look at how scale is affected when we have uneven terrain. For instance, if we have two points on a photograph with different elevations—let’s say point A and point B—how will this affect our calculations?
We have to adjust the scale based on their heights, right?
Exactly! The scale for point A would be calculated using S_A = f / (H - h_A), where h_A is the elevation of point A. The same goes for point B.
So in a real-world situation, we'd need to know the elevation of both points to accurately compute scale?
In most cases, yes! And when we have multiple elevations in a photo, we might even want an average scale, which helps ensure our overall measurements are accurate.
Can you give us an example of how that looks?
Sure! If you’d like to compute the average scale based on varying elevations, you would factor in the focal distance and various heights to derive your average scale based on the formula: S_avg = f / (H - h_average).
Got it! Let's summarize today's session.
Today we learned how terrain undulations affect the scale in photogrammetry. Always remember to consider elevation when calculating scales for varying points!
When you measure with scale, to the ground you must sail. Distance on photo, on Earth must prevail!
Imagine a photographer stands on a hill, taking photos of valleys below. The camera's height affects how distant valleys appear, making it essential to adjust the scale for accuracy.
Fabulous Flying Height: Focus on How Scale is computed. (Where F = Focal length, H = Height, S = Scale)
S.H.E. - Scale is Height affected by Elevation.
Term
What is Scale?
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Term
What factors affect Scale?
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The ratio of a distance on a photograph to the corresponding distance 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 of the aircraft above ground level when the photographs were taken.
The height of a point on the ground measured above mean sea level.
This section covers the determination of scale in aerial photographs and how it relates to the elevation of terrain points.
The scale of a vertical photograph is crucial in photogrammetry, as it represents the ratio of the distance between points on the photo to those on the ground. This section details the computation of scale under varying terrain conditions, emphasizing how elevation affects the accuracy of photogrammetric measurements.
Scale in photogrammetry refers to the ratio of distance between objects on a photograph to that on the ground. Understanding scale is vital for accurate measurements. The focus is on vertical photographs with a scale influenced by both the camera's focal length and the flying height above ground level.
Basic Scale Calculation: The formula for scale in a flat terrain situation is given by
where:
Complexity with Elevation: However, this scale changes with varying elevations, requiring adjustments in the calculations. When terrain adds undulations to the photography, the scale is computed differently, reflecting terrain elevations. The scale for undulating terrain can be expressed as:
where h_A and h_B represent different elevations on the ground.
Average Scale: If multiple elevations exist in a photograph, an average scale can be defined, influencing the overall measurement accuracy. This relationship is significant since it illustrates that scale varies across photographs, requiring careful attention to terrain characteristics during photo analysis.
Understanding these principles is essential for applications in mapping and modeling, ensuring that measurements derived from photographic images reflect true physical distances.