Solved Examples - 5.1 | 4. Graphical Radial Triangulation | Surveying and Geomatics
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Understanding Scale Calculation

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Teacher
Teacher

Let's begin our discussion on determining the scale of a photograph. The formula we generally use is Scale = f / (H - h), where 'f' is the focal length of the camera, 'H' is the height of the aircraft, and 'h' is the height of the terrain.

Student 1
Student 1

What happens if the terrain elevation changes while keeping the height of the aircraft constant?

Teacher
Teacher

Great question! If the terrain elevation changes, the scale of the photograph will also change. A higher terrain elevation—meaning 'h' increases—reduces the denominator, making the scale smaller or numerically larger. Keep this in mind as it directly impacts the image's accuracy.

Student 2
Student 2

Can you give us an example to illustrate this?

Teacher
Teacher

Sure! If the aircraft is at 1200 m above mean sea level, and the terrain elevation is 80 m, the scale would be calculated as 15/(1200-80), resulting in around 1:7467, meaning 1 cm on the photo equals 7467 cm on the ground. Remember the acronym 'H change, Scale range!' to help you recall how elevation affects scale.

Student 3
Student 3

Does this mean lower elevations give us larger scales – more detail?

Teacher
Teacher

Exactly! A smaller 'h' means a larger scale or more detail in your photograph.

Teacher
Teacher

To summarize, the scale of a photograph is inversely proportional to the difference in altitude; as the terrain elevation rises relative to your flying height, the scale diminishes.

Determining Flying Heights

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Teacher
Teacher

Next, we will look into determining the flying height needed to achieve a given photograph scale. Can anyone tell me what formula we would use?

Student 4
Student 4

Is it the same one for scale? Scale = f / (H - h)?

Teacher
Teacher

That's correct! But here, we need to rearrange it to find H. For example, if we want a scale of 1:10,000 with a terrain height of 1600 m, and the focal length is 20 cm, we calculate it as H = scale * focal length + terrain height.

Student 1
Student 1

Could you demonstrate that calculation?

Teacher
Teacher

Certainly! H = (10,000 * 0.20) + 1600 m gives us 3600 m for the required flying height.

Student 2
Student 2

So, is flying higher always better for capturing details?

Teacher
Teacher

Not necessarily! While a higher flight allows us to cover larger areas, it reduces detail, making strategic altitudes essential.

Teacher
Teacher

In summary, to achieve a specific scale, you can rearrange the formula to solve for H, taking both terrain height and scale into account.

Calculating Ground Distances

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Teacher
Teacher

Now let’s explore how we calculate ground distances between points A and B using their coordinates. The equation we'll use is X = x * (H - h) / f and Y = y * (H - h) / f.

Student 3
Student 3

Could you show us how we can apply this?

Teacher
Teacher

Absolutely! For example, if point A has photographic coordinates of (2.75, 1.39) cm, and B is at (-1.80, 3.72) cm, we first convert the coordinates to ground coordinates using the above formulas.

Student 4
Student 4

And then we would find the distance using the formula for calculating lengths, right?

Teacher
Teacher

Exactly! After obtaining coordinates for both points, we can use the distance formula to find AB = √[(X_A - X_B)² + (Y_A - Y_B)²].

Student 1
Student 1

Interesting! I see how each coordinate contributes to the final measurement.

Teacher
Teacher

To summarize, using photographic coordinates in conjunction with altitude and focal length allows us to derive exact ground distances.

Introduction & Overview

Read a summary of the section's main ideas. Choose from Basic, Medium, or Detailed.

Quick Overview

This section outlines a series of solved examples that demonstrate the principles and applications of photogrammetry, particularly focused on calculating scales, flying heights, and lengths of lines on photographs.

Standard

In this section, several solved examples provide practical applications of photogrammetric concepts. Key calculations include determining the scale of photographs based on focal length and elevation, flying heights necessary to achieve specified scales, and deriving ground distances from photographic measures. These examples solidify understanding of aerial mapping techniques.

Detailed

Detailed Summary

This section encompasses a variety of solved examples that are instrumental in understanding fundamental principles of aerial photogrammetry. The main focus of the examples includes:

  1. Scale Calculation: Examples demonstrate how to determine the scale of photographs at varying altitudes and terrain elevations using the formula Scale = f / (H - h), where f is the focal length, H is the flying height, and h is the terrain height.
  2. Flying Height Determination: Some examples involve calculating the required flying height of an aircraft to achieve photographs at specific scales, involving the same scale formula rearranged to yield H.
  3. Ground Distance Calculation: Example problems help ascertain the length of lines between points on the ground based on their photographic coordinates, utilizing the relationships established through the earlier formulae.
  4. Application of Controlled and Uncontrolled Scales: Several examples highlight how overlap and dimensions can influence the overall mapping scale and operational parameters for aerial photography, underscoring the importance of meticulous planning in photogrammetric surveys.

These examples, rooted in practical computation, serve as essential exercises to reinforce the knowledge of aerial mapping and triangulation methods introduced earlier in the chapter.

Audio Book

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Example 4.1: Scale Calculation for Vertical Photograph

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A vertical photograph was taken at an altitude of 1200 m above msl. Find out the scale of photograph for a terrain lying at an average elevation of (i) 80 m and (ii) 300 m, if the focal length of the camera is 15 cm.

Solution:
H = 1200 m, f = 15 cm, and (i) h = 80 m, (ii) h = 300 m

The scale of a photograph is given by = f / (H - h)

(i) Scale = 15 / (1200 - 80) = 1 / 7467. It means 1 cm on the photo is equal to 7467 cm on the ground.

(ii) Scale = 15 / (1200 - 300) = 1 / 6000.

Detailed Explanation

In this example, we are asked to find the scale of a photograph taken from a height of 1200 meters above mean sea level (msl). The scale formula in photogrammetry relates the focal length of the camera to the difference in altitude between the camera and the terrain.

  1. Explore the Variables: We identify the altitude of the photograph (H = 1200 m), the focal length of the camera (f = 15 cm), and the terrain elevations (h = 80 m and h = 300 m).
  2. Apply the Formula: The scale of the photograph can be calculated using the formula: \(Scale = \frac{f}{(H - h)}\). This equation calculates how the image size relates to its real-world size based on elevation.
  3. Calculate the Scale:
  4. For h = 80 m: \(Scale = \frac{15 cm}{(1200 m - 80 m)} = \frac{15 cm}{1120 m} \approx 1 : 7467\) which tells us that 1 cm on the photo represents 7467 cm on the ground.
  5. For h = 300 m: \(Scale = \frac{15 cm}{(1200 m - 300 m)} = \frac{15 cm}{900 m} \approx 1 : 6000\). This means 1 cm on the photo corresponds to 6000 cm on the ground.

Thus, the calculation for elevation variations demonstrates how scale changes with differences in height.

Examples & Analogies

Imagine you are trying to measure the distance of a soccer field being viewed from an airplane at a certain altitude. If you're 1200 meters high, the view is vastly different depending on where the soccer field sits. If the field is at sea level (0m), you’ll see it larger than if it’s nestled in a valley (at 300m). Similar to a zoom lens, the further you are from the ground (higher altitude), the less detail you can perceive directly, which is what affects the scale of the images you're capturing.

Example 4.2: Scale for Vertical Photograph at Flat Terrain

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A camera of 152 mm focal length lens is used to take a vertical photograph from a flying height of 2780 m above mean sea level. If the terrain is flat having an elevation of 500 m above msl, determine the scale of the photograph.

Solution:
f = 152 mm, H = 2780 m, h = 500 m

S = f / (H – h) = (152 / 1000) / (2780 – 500) = 0.152 / 2280 = 1 / 15000.

Detailed Explanation

In this example, we're tasked with determining the scale of a photograph taken from a high altitude over flat terrain.

  1. Identify Given Values: We know the focal length (f = 152 mm), the flying height (H = 2780 m), and the elevation of the flat terrain (h = 500 m).
  2. Use the Scale Formula: The scale is derived from the same formula: \(S = \frac{f}{(H - h)}\).
  3. Convert Units: Ensure the focal length is in the same unit as the flying height, so we convert 152 mm into meters, which equals 0.152 m.
  4. Calculate the Scale: Plugging the numbers in:
    \[S = \frac{0.152}{(2780 - 500)} = \frac{0.152}{2280} \approx 1 : 15000\]. Here, the scale signifies that 1 cm in the photograph depicts 15000 cm on the ground.

Examples & Analogies

Think of taking a photograph of a park from a tall radio tower. The higher you go, the more the details blend together, making them appear smaller in the image. A photo taken from a height of 2780 m with minimal elevation change (like a flat terrain park) allows the camera to capture broader areas, but those areas are shown much smaller in the image compared to how they would appear from a lower viewpoint.

Example 4.3: Calculating Flight Height for Desired Scale

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A camera of focal length 20 cm is used to take vertical photographs of a terrain having an average elevation of 1600 m above msl. At what height above msl must an aircraft fly to take the photographs at 1:10,000 scale.

Solution:
f = 20 cm, h = 1600 m, and scale = 1:10,000

S = f / (H - h)
H – h = Sf or H = Sf + h
H = (20 / 100) (10,000) + 1600 m = 3600 m.

Detailed Explanation

This example revolves around finding the necessary height for an aircraft to achieve a specific photographic scale.

  1. Given Data: We acknowledge the focal length (f = 20 cm), average elevation of terrain (h = 1600 m), and the desired scale (1:10,000).
  2. Understanding Scale Requirement: The scale indicates how much ground is represented in the photograph compared to its actual size. In this case, a scale of 1:10,000 means that 1 cm on the photograph corresponds to 10,000 cm on the actual ground.
  3. Convert Scale to Fraction: Convert the scale into a formula: \(S = \frac{20/100}{(H - 1600)}\).
  4. Rearranging: Rearranging gives us the height we need: \(H = Sf + h\). Plugging in the numbers:
    \[H = (0.2) (10000) + 1600 = 3600 m\]. Thus, to achieve the desired photographic scale, the aircraft must fly at an altitude of 3600 m.

Examples & Analogies

Imagine you are mapping a city block in detail from the sky. If you want everything to appear clearly and measureable at a scale of 1:10,000, you must establish how high you need to fly. Think of it like a painter needing to determine the right distance to accurately portray all the tiny features of the city on a canvas. The higher the kite flies, the broader the view but lesser the detail retained!

Example 4.4: Ground Measurements from a Photograph

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A line AB 300 m long, lying at an elevation of 600 m above msl, measures 9 cm on a vertical photograph. The focal length of the camera lens is 30 cm. Determine the average scale of the photograph if the average elevation of the terrain is 700 m above msl.

Solution:
Ground distance = 300 m, map distance = 9 cm, f = 30 cm, h = 600 m

Map distance / Ground distance = S = f / (H-h)
0.09 / 300 = 0.30 / (H - 600)
H - 600 = (0.30 * 300) / 0.09
H = 1000 + 600 = 1600 m
Now S = f / (H – h)
av av
S = 0.30 / (1600 - 700)
av
= 1 / 3000.

Detailed Explanation

In this scenario, we examine how to establish the scale of a photograph based on specific measurements provided.

  1. Analyze the Inputs: We have a line of known ground distance (300 m) that appears as 9 cm on an aerial photograph, with a camera focal length of 30 cm and an elevation of 600 m.
  2. Formula Utilization: First, we set up the relationships to find flying height (H). We'll rearrange the relationship: \(\frac{0.09}{300} = \frac{0.30}{(H - 600)}\).
  3. Solve for H: By multiplying both sides, we find that: \(H - 600 = \frac{0.30 * 300}{0.09}\) yields \(H = 1600 m\).
  4. Calculate Scale: Finally, with determined height, the average scale is found using \(S = \frac{f}{(H - h)}\): \(S = \frac{0.30}{(1600 - 700)} = 1 : 3000\).

Examples & Analogies

Imagine a real estate surveyor trying to find the dimensions of a large lot using an aerial image. By knowing how long a property line is (300 m), they can measure the corresponding line segment on their photograph and use it to figure out the height at which the photograph was taken. This is akin to using a map to gauge a journey's path based on notable landmarks - you compare what's on the map with what exists on the ground!

Example 4.5: Finding Aircraft Height from Aerial Measurement

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A line AB measures 12.70 cm on an aerial photograph and the corresponding line measures 2.54 cm on a map at a scale of 1/50,000. The terrain has an average elevation of 200 m above msl. Calculate the flying height of the aircraft, above msl, if the photograph was taken with a camera of 16 cm focal length.

Solution:
Photo distance = 12.70 cm, map distance = 2.54 cm, map scale = 1 / 50,000, h = 200 m, f = 16 cm

Photo distance / map distance = Photo scale / map scale
12.70 / 2.54 = Photo scale / (1 / 50000)
5.0 = 50000 * Photo scale
Photo scale = 1 / 10,000.

Now S = f / (H - h)
1/ 10,000 = 0.16 / (H – 200)
(H - 200) = 0.16 * 10,000
H = 1600 + 200
H = 1800 m.

Detailed Explanation

This example calculates how high an airplane must fly to capture an aerial photograph with specific measurements adjusted for terrain elevation.

  1. Interpret Given Quantities: The measurements established reveal a 12.70 cm line on the photo correlating to a 2.54 cm length on a map whose scale is given as 1/50,000 with an average elevation of 200 m.
  2. Set Up Ratios: The relationship between photograph distance, map distance, and scales provides a means to determine the photo scale. Setting equalities shows us the photo scale: \(5.0 = 50000 * Photo scale\) which simplifies to \(Photo scale = 1/10,000\).
  3. Final Step - Height Determination: By substituting the values back into the scale equation: \(S = \frac{f}{(H - h)}\), the needed height H can be evaluated leading to the conclusion \(H = 1800 m\).

Examples & Analogies

When you take a picture of a family picnic from a camper, you want to ensure the whole scene fits the frame. Similarly, in aviation surveying, a pilot must calculate an optimal height to efficiently capture the terrain for mapping accurately. This process is much like a child adjusting their camera's distance to ensure Grandma's apple pie isn't cut off at the side - precise measurements matter!

Definitions & Key Concepts

Learn essential terms and foundational ideas that form the basis of the topic.

Key Concepts

  • Scale: The ratio of image distance to ground distance, fundamental in photogrammetric calculations.

  • Flying Height: The height at which an aircraft must fly to achieve the desired scale and detail in aerial photographs.

  • Ground Distance: The actual distance between geographic points derived from photographic measurements.

  • Photographic Coordinates: The coordinates used to determine the location of points on a photograph, essential for distance calculations.

Examples & Real-Life Applications

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Examples

  • Example of calculating the scale of a photograph taken at an altitude of 1200 m with terrain elevation at 80 m using a 15 cm focal length.

  • Calculating the required flying height for a scale of 1:10,000 with a focal length of 20 cm and terrain elevation of 1600 m.

Memory Aids

Use mnemonics, acronyms, or visual cues to help remember key information more easily.

🎵 Rhymes Time

  • To find the scale, just do not fail, divide focal length by height - prevails!

📖 Fascinating Stories

  • Imagine flying high like a bird, noticing landscapes below as blurred; but with focus tight, your photos delight, revealing the world in exacting sight.

🧠 Other Memory Gems

  • Remember: Fly high, Focus low, Scale grows. H - h over f is the key, let it flow!

🎯 Super Acronyms

F.H.S.E. - Focal length, Height, Scale, Elevation - key terms to remember.

Flash Cards

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Glossary of Terms

Review the Definitions for terms.

  • Term: Altitude (H)

    Definition:

    The height of the aircraft above mean sea level.

  • Term: Focal Length (f)

    Definition:

    The distance from the lens to the image sensor in a camera, affecting image scale.

  • Term: Ground Control Point (GCP)

    Definition:

    Point with a known location used to help georeference images during mapping.

  • Term: Minor Control Point (MCP)

    Definition:

    Points used to establish connections in photogrammetry for better adjustment.

  • Term: Scale

    Definition:

    A ratio that expresses the relationship between a distance on a photograph and the corresponding ground distance.