4.15.2. Stereo-plotting instruments

Stereoscopic plotting instruments or stereo-plotters are designed to provide accurate solutions for object point positions from their corresponding image positions in a stereo-pair.

Stereo-plotting instruments, also known as stereo-plotters, are specialized devices used in photogrammetry to determine the 3D coordinates of points on the ground from overlapping photographic images. These instruments work by utilizing the concept of stereoscopy, where two slightly different perspectives of the same scene are combined to create a three-dimensional model. By analyzing where light rays from the left and right photographs intersect, we can derive accurate positions for various features on the terrain.

Think of stereo-plotting instruments like a pair of VR goggles that allow you to see 3D environments. Just as VR goggles present images from slightly different angles to create depth perception, stereo-plotters use two overlapping photos from aerial photography to create a three-dimensional representation of the landscape.

With the diapositives in place, light rays are projected through them; and when rays from corresponding images on the left and right diapositives intersect below, they create a stereo-model.

The creation of a stereo-model begins with placing two overlapping diapositives (transparencies) in the stereo-plotter. Light is projected through these images. When corresponding features from both images are aligned, the light rays intersect at a point below the instruments, representing a specific location on the ground. This intersection point is then used to determine the 3D coordinates (x, y, z) of that feature. The precision of this process relies on the correct orientation of the images.

Imagine you're using a pair of binoculars to look at a distant mountain peak. You might see the left view with the left eye and the right view with the right eye. By merging these views, you can perceive depth and accurately gauge distances. Stereo-plotting works similarly, allowing us to visualize and measure land features in 3D.

After relative orientation is completed, absolute orientation is performed. In this process, the stereo-model is brought to the desired scale and levelled with respect to a reference datum.

Once the stereo-model is created through relative orientation, the next step is absolute orientation. This involves scaling the model to ensure it accurately represents real-world distances and heights. It also requires leveling the images to a known reference point on the ground. The reference datum serves as a baseline to prevent errors related to irregular terrain or image distortion. Achieving accurate absolute orientation is crucial for producing reliable maps and models.

Think of setting up a miniature landscape for a model train. First, you adjust the train tracks to match the layout (relative orientation), then you ensure it’s all level and in the right proportions using a ruler and a marked base (absolute orientation). Just like that, stereo-plotters must align their models to real-world standards.

After completing the orientations, measurements from the 3-D model may be made and recorded, but it is now done in digital form.

After establishing the orientations, users can begin measuring distances and positions from the 3D model created within the stereo-plotter. The data collected is digitally recorded, which enhances accuracy and simplifies the process. Measurements can include horizontal (planimetric) positions as well as vertical (elevation) data, providing complete spatial information necessary for topographic mapping.

Imagine a 3D blueprint for a building. After setting the dimensions and layout, you pull out a digital tape measure to record distances directly from your plans. Similarly, after building the 3D model with a stereo-plotter, measurements are taken digitally for precision and ease.

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