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# Table of Contents
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[**5. Image Orthorectification**](#image-orthorectification)
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- [Choosing the Orthorectification Mode](#point_down-choosing-the-orthorectification-mode)
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- [Scaling](#straight_ruler-scaling)
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- [Resolution Definition](#resolution-definition)
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- [Real Reference Change](#real-reference-change)
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- [Defining Orthoimage Corners and Water Level](#defining-orthoimage-corners-and-water-level)
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- [Complete Orthorectification](#triangular_ruler-complete-orthorectification)
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- [Principle of Orthorectification](#principle-of-orthorectification)
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- [Positioning Reference Points](#positioning-reference-points)
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- [GRP Verification](#grp-verification)
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- [Transformation Parameters](#transformation-parameters)
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- [Transformation of All Images](#gear-transformation-of-all-images)
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- [Subsampling of Transformed Images](#chains-subsampling-of-transformed-images)
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#
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# :point_down: Choosing the Orthorectification Mode
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**_Fudaa-LSPIV_** offers two orthorectification modes:
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- **_Scaling_**: with images that are not distorted by perspective effects (taken with nadir viewing and without lens distortion), simple scaling in meters per pixel is sufficient.
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- **_Complete Orthorectification_**: with images distorted by perspective effects, complete orthorectification based on reference points is required.
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These modes can be accessed through the menu **_Orthorectification 🡪 Orthorectification Mode_**.
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Both of these image transformation modes allow for the implementation of 2D or 3D orthorectification, or simple image pixel scaling. These three transformation types and their prerequisites are summarized in the following graphic and described in the next sections.
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# :straight_ruler: Scaling
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> When the camera's point of view is vertical (from above), perspective effects can often be neglected, at least in the center of the image. In such cases, image orthorectification is not necessary. Scaling then simply involves providing the physical size of a pixel.
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The menu **_Orthorectification 🡪 Scaling 🡪 Parameters_** leads to the window for defining scaling parameters:
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### Resolution Definition
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In the **_Resolution_** tab, two methods are offered to input the image resolution (physical size of the pixels in meters):
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- Direct entry of the resolution if known: enter the resolution in m/pix in the dedicated field.
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- Entry of pairs of points defining segments of known size, assumed to be at an equivalent altitude to the water surface (see further for the general case):
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- Click on the positions of the endpoints of the first segment in the image. The image coordinates (I1,J1) and (I2,J2) of the two points will appear in the table.
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- Indicate the length of the segment (distance between the two points) in meters. The corresponding resolution is calculated.
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You can repeat this operation for as many known-size segments as you need. The average of the calculated resolutions will be displayed and will be the one used for the image transformation.
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- Alternatively, the actual coordinates of the segment endpoints can be entered instead of the segment lengths.
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- Select **_Coordinates_** instead of **_Distance_** in the dropdown menu.
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- When the known-size segments are not at an altitude equivalent to that of the water surface, perspective effects can be corrected:
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- Check the box **_Images taken by drone_**.
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- Enter the drone's flight altitude in the **_Drone Altitude (m)_** field using the same altimetric reference as the water level.
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- You will then need to input the altitude of each segment to allow for the calculation of the corresponding resolution.
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> :loudspeaker: _**Warning!**_
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>
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> Altitudes are absolute (not heights above the water level), and the resolution calculation depends on the water level (assumed to be 0 by default).
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## Real Reference Change
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The **_Transformation_** tab allows you to define the translation/rotation parameters to be applied to the real reference of the orthoimages. This is particularly useful if the velocity field needs to be compared with other data or if a bathymetric transect measured in this specific reference frame will later be used for flow rate calculations.
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By default, the **_No translation/rotation_** option is selected, and no reference change will be applied. The bottom left corner of the image will be the origin of coordinates (0,0), and the X and Y axes will be the horizontal and vertical axes of the starting image.
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Otherwise, by selecting **_Input of 2 points for translation/rotation_**, it is possible to define the reference change to be applied by entering the real coordinates (X1,Y1) and (X2,Y2) of two previously positioned points (by clicking) in the image: their image coordinates (I1,J1) and (I2,J2) will then appear in the table. The **_Apply_** button allows you to visualize the transformation of the first image in the sequence.
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## Defining Orthoimage Corners and Water Level
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The **_Others_** tab allows you to define the coordinates of the orthoimage corners: this is simply a sub-selection of the starting image.
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The **_Default Values_** proposed are the positions of the bottom-left and top-right corners of the entire image, calculated by multiplying the image dimensions (in pixels) by the resolution (in m/pix) previously calculated. This effectively retains the entirety of the starting images for transformation.
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You can also enter the water level (in m) in this **_Others_** tab. This value will not be used for the image transformation (unless the **_Images taken by drone_** option is selected) or for velocity calculation but will be used for flow rate calculation (to compute the wetted area).
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The **_Apply_** button allows you to transform the first image with the entered parameters to get an idea of the result. The **_Validate_** button saves the scaling parameters, displays the first transformed image, and allows you to move on to the next step (Transformation of all images). |
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