4.27. Scaling in Arundel’s method

Graphical radial triangulation is performed with simple instruments, such as mirror stereoscope, a ruler, a pencil and a tracing sheet. The steps are given below:

In Arundel's method of mapping, radial triangulation is used, which is a technique for determining the positions of points from photographs. This method makes use of simple tools like a mirror stereoscope, rulers, and pencils, making it accessible for manual use. The process involves several steps designed for accurate identification and connection of control points.

Imagine using a simple camera to take pictures of a landscape. After capturing the images, you use a ruler and pencil to find the heights of hills and locations of trees on a tracing sheet, similar to how you'd sketch a map while looking at landscape photos.

1. The photographs are laid out in strips and all GCPs are identified on the photographs and their numbers marked on the photographs.

The first step in the radial triangulation process involves organizing the photographs into strips. Ground control points (GCPs) are critical locations on the ground whose coordinates are known. These points are identified on each photograph and marked with numbers. This allows for precise reference when aligning the photos.

Think of this like indexing chapters in a book where each chapter represents a photograph. By labeling key pages (GCPs) you can quickly find important information when creating an overall picture of the story.

2. The principal point is obtained for each photograph. The principal points are then stereoscopically transferred to the adjoining photographs as conjugate principal points.

The principal point in a photograph is the central point from which all measurements are made. Once the principal point is identified, it can be transferred to adjacent photographs to create a conjugate principal point. This ensures that the images are aligned accurately with one another.

Imagine taking a picture and then trying to take another photo next to it. By marking the center of the first photo, you can easily align the second, ensuring they fit together like pieces of a puzzle.

3. Now two points, called minor control points (MCPs), also called pass points or wing points, are selected on both sides of the principal point of photo, at about 2 cm from the upper and lower edge of the photograph, fulfilling the following conditions: a. The two points should be as nearly at the same elevation as the principal point. b. The points should be at a distance from the principal point, which is equal to twice the mean base of the adjoining photographs. c. The points should lie approximately on the bisector of the base angle on either side, and d. The point should serve as lateral point as well.

In selecting minor control points (MCPs), specific criteria must be followed to ensure accurate mapping. The MCPs are selected close to the principal point in accordance with elevation, distance from the principal point, and angular considerations. This careful selection helps to maintain a uniform scale across the photographs.

Consider these MCPs as markers on a running track that indicate key distances. You want them placed at equal intervals (distance from the principal point) and at the same level on the track (elevation) to ensure everyone runs the same race.

4. The MCPs are selected and transferred stereoscopically to adjoining photographs.

Once the minor control points have been chosen, they need to be transferred to the adjacent photographs using a stereoscopic method. This technique creates a three-dimensional effect that allows for accurate placement of the MCPs in relation to the photographs.

Imagine creating a 3D model of a building by taking multiple photos and sketching on top. Transferring MCPs is like accurately projecting one layer of your model onto another so they align perfectly.

5. The lateral control points (LCPs) are selected in the centre of lateral overlaps of adjacent strips to serve as connecting points between different strips. These LCPs are selected at least at the beginning and the end of the strip and on every third photo of the strip. These are then stereoscopically transferred to photographs of adjoining strips.

Lateral control points (LCPs) act as connection points between different strips of photographs. These points are strategically chosen to ensure that a coherent map can be built by connecting segments properly. The LCPs are located in areas where photographs overlap.

Think of lateral control points as the junctions in a railway system where two train lines meet. These junctions help ensure that trains can move smoothly from one track to another without losing direction.

6. The radial directions from the principal point to all minor control, lateral control and ground control points appearing on the photograph are drawn, through the points.

In this step, lines (radials) are drawn from the principal point to all identified control points. This helps in visualizing the relationship between these points and positions them accurately on the photographs. Once completed, it aids in understanding the scale and layout of the area being mapped.

Imagine drawing lines on a map from a central park (principal point) to various landmarks (control points). This not only shows distances but also helps you see the layout of the city in relation to the park.

Due to elevation differences of terrain and variations in the flying height of aircraft, the scale of photographs generally varies considerably. The photographs of a strip are required to be brought to a common scale through graphical triangulation.

Background differences such as terrain elevation and aircraft flying height lead to variations in photo scale. Therefore, it is crucial to adjust these photographs to a common scale, which is achieved using graphical triangulation.

Imagine trying to fit together puzzle pieces of different sizes; scaling brings them to the same size so they can fit perfectly, ensuring the final image is coherent.

Each strip is plotted on a transparent sheet to facilitate the drawing. The photographs are laid out in their correct relative directions so that the plotting is carried out in the right direction.

Plotting photographs on a transparent sheet allows the cartographer to accurately trace and draw over the images, maintaining their correct orientation. This step is essential for aligning all photographs correctly before finalizing the map.

Think of this step as using tracing paper to overlay your favorite design. By following the lines accurately, you ensure that your final piece looks as intended without mistakes in alignment.

The scale is then carried out by plotting the actual distance AB between the two ground control points, between which the scaling is to be done, on a straight line drawn on a separate sheet.

In this critical step, the actual distances between ground control points are measured and plotted, establishing a reliable scale for the entire map. This ensures that all elements are to the same proportion and can be accurately referenced.

It's like measuring the length of a table (AB) to ensure that when you draw its representation on paper, it reflects the same length proportionately, making the drawing realistic.

After scaling all the points of minor control plots, these points are pricked through the tracing sheet. The positions of principal points, minor control points and lateral control points of all the strips are likewise pricked on the graph sheet and adjusted...

Final adjustments ensure that all control points are accurately aligned on the graph sheet. The process involves pricking points through the tracing sheets to leave imprints, allowing adjustments to minimize any errors that may have occurred.

Think of this as finalizing a layout for a garden. After marking where each flower should go, you may need to tweak their positions slightly to ensure they look good together—this is what these adjustments do for the mapping process.

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