4.26.2. Common scale in triangulation

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:

Graphical radial triangulation uses basic instruments to create a scaled map by referencing multiple photographs. The important tools involved include a mirror stereoscope for viewing images in 3D, a ruler for measuring distances, a tracing sheet to plot points and lines, and a pencil for marking on the tracing sheet. This technique is used to ensure that images from aerial photographs can be transformed into a two-dimensional map that accurately reflects the three-dimensional features of the terrain.

Think of it like piecing together a puzzle where each photograph represents a piece. You need to ensure that each piece fits correctly to form the complete picture, which is achieved through careful measurement and alignment.

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

In the first step, photographs are arranged in a sequence or 'strips' where critical points on the ground, known as Ground Control Points (GCPs), are identified and labeled. GCPs are locations in the photographs where the exact coordinates are known (e.g., intersections, building corners). Marking these points helps ensure that when the photographs are processed, they can be accurately aligned with real-world locations.

Imagine using a map with landmarks. If you can spot known locations, you can determine your position relative to those landmarks, making it easier to navigate or reconstruct the area with precision.

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 is the center of the photograph that serves as a reference for measuring distances and angles. Once this point is identified on each photo, it is transferred to adjacent photos using stereoscopic techniques, ensuring that the photos are aligned based on this reference. This step is crucial for maintaining consistency across the photographs.

Think of a principal point as a central hub in a network. Just like how you would connect different points in a map to this central hub, we connect each photograph to a central point to keep all images aligned correctly.

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: ...

Minor Control Points (MCPs) are selected close to the principal point to help further refine the accuracy of the triangulation. The selection process ensures that these points meet specific criteria related to elevation, distance from the principal point, and their positioning relative to other elements in the photograph. This careful selection is crucial for establishing a stable control system across multiple photographs.

It’s like choosing points on a trampoline where you need a few stable spots (MCPs) around the center to create a balanced jump. The better the positioning of those points, the better the overall performance of the jump (or triangulation).

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

Once the MCPs are selected, these points are then carefully transferred to adjacent photographs using the same stereoscopic technique applied to the principal points. This step helps in establishing continuity and connectivity between the photographs, which is necessary for accurate mapping.

Imagine a relay race: each runner (photo) must accurately hand off the baton (MCP) to the next runner to keep the race smooth. In triangulation, ensuring that points transfer correctly allows the mapping process to flow without disruption.

5. The lateral control points (LCPs) are selected in the centre of lateral overlaps of adjacent strips to serve as connecting points.

Lateral control points (LCPs) are chosen at locations where the photographs overlap. These points serve to link adjoining strips of photographs together, facilitating a seamless integration of images into a single mosaic. Effective selection of LCPs is vital for producing maps that accurately represent the entire surveyed area.

Think of LCPs as pieces of tape keeping the edges of two posters together. If the tape (LCP) is placed well, the two posters (photos) will join seamlessly, creating a bigger and more coherent display.

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 called radial directions are drawn from the principal point to all selected control points on the photograph. These radials represent the direction to each control point and serve as visual guides when plotting the points on the mapping interface, ensuring that relative positions are maintained.

Imagine you are standing at the center of a pie with various types of fruit (control points) extending outward. Drawing lines from the center (principal point) to each type of fruit (point) helps you visualize where each piece is located.

Due to elevation differences of terrain and variations in the flying height of aircraft,...

In aerial photography, variations in terrain elevation and differences in flying heights can cause the scale of images to differ. Therefore, it’s necessary to standardize all images to a common scale. This is achieved through graphical triangulation, which involves manipulating the photographs so they can be accurately compared and combined into one coherent map.

This process is akin to adjusting different pieces of clothing that are all made of different fabrics and sizes so that they match a single outfit (the common scale), allowing for a cohesive look.

After scaling all the points of minor control plots, these points are pricked through the tracing sheet...

Once all control points are scaled, they are marked on a tracing sheet for final adjustments. This involves ensuring that all ground control points match their plotted positions accurately, minimizing discrepancies between photographs. The aim is to create a final map that reflects the real-world terrain with high precision.

It’s like a painter reviewing a finished canvas and making sure that all parts match the vision they had; they go back and touch up any areas that look out of place to achieve the best overall piece.

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