Ground Control Points (GCP)
Function Overview
Using Ground Control Points (GCPs) can improve the quality and absolute accuracy of image alignment. After editing the GCPs, you can use the update alignment function to update the image alignment results to the GCP coordinates. It is recommended to perform control point editing operations on projects that have already completed image alignment.
Usage
The basic process of marking ground control points is as follows:
- Click the Ground Control Point button and select the image project file to be edited in the pop-up dialog. Only image project files that have been added to the software can be selected.
- Click OK to start two windows: the control point list window and the image window.
- Import or mark the data objects in the window to obtain the coordinates of the control points. All imported or marked control points will be listed in the control point list window.
- Select a control point from the list, choose an image from the candidate images, and click the corresponding observation position of the control point in the single image window to complete a marking.
- Repeat the process to complete the marking of all control points. Click the save button to save the marking results into the image project file for subsequent operations.
Control Point Prediction
For image projects that have completed image alignment, when a control point is selected, the images and their predicted points where the control point might appear will be added to the image window based on the control point coordinates, alignment results, and existing observations for marking.
Note: The coordinate system of the control points must be set correctly, otherwise, prediction failure or incorrect subsequent alignment updates may occur.
Control Point List Window
Import Control Points
Two types of control point files are supported: control point coordinate files (.txt or .csv) and control point observation export results (.xml). The control point observation export result is generated through the export function and can directly import control point coordinates, attributes, and observations together. For control point coordinate files, select the control point file in the import interface, and set the format in the pop-up dialog.
- Coordinate System: Set the coordinate system of the imported control points.
- Skip Rows: Skip the number of invalid rows at the beginning of the file (such as comments), default is no skip.
- Column Header: Click the column header, and specify the attribute of each column in the drop-down list, such as serial number, latitude and longitude, elevation.
- Separator: Effectively split the data using the separator. Supports four types of separators: space, tab, comma, and semicolon, and their combined separators. The default is the combined separator. If the file separator is not one of the above four types, you can enter a custom symbol in the separator text box.
Note: Importing control points will clear all control points in the current control point list.
Export Control Points
Export the existing control points and their attributes, observations, and other information to an XML file, which can be imported again using the import function.
Save
Save the existing control points and their attributes into the image project file.
Select Point
In the connected window, select the position of various data objects, and use the coordinate position of the selected point to update the coordinates of the currently selected control point.
Add Point
Add an empty control point.
Delete Point
Delete the currently selected control point.
Coordinate System
Set the coordinate system and mode of the control points. To modify the coordinate system, select more in the drop-down box, and choose the corresponding coordinate system in the pop-up dialog. Control point modes are divided into rigid and non-rigid. High-precision control points are recommended to use the non-rigid mode, while low-precision control points are recommended to use the rigid mode.
Linked Viewer
The select point function requires setting a connected window to operate under that window. Therefore, when there are multiple windows, you need to select the window here.
Target Type
Perform image target detection on the selected control points. In the drop-down box, you can select the type of target to be detected. Click the detect button to start detection. Successfully detected images will directly add the results to the control point observations. Note: The target detection function searches near the predicted value of the control point and may fail to detect.
Show All Images
Check this option to add all images as candidates to the image window.
Control Point List
The control point list lists all current control points. Selecting a row will jump to the coordinate position of that control point. Double-click the point name cell, coordinate cell, or plane elevation accuracy cell to modify the corresponding value. The drop-down boxes under the identification and type columns can switch the identification and type.
Image Viewer
In the image window, candidate images are listed in the upper half of the window, including images with prediction information (red dot in the upper right corner) and images with observation information (green dot in the upper right corner). The lower half is the image display window, where the red crosshair indicates the predicted point coordinates. Clicking on the image will add the observation value, displayed as a green crosshair. Right-clicking on the image display window can clear the current observation.
Control Point Explanation
A control point contains the following information:
- Name: The unique ID information of the control point.
- Coordinates: Distinguish based on the selected coordinate system. For local coordinate systems or projected coordinate systems, it is X, Y, Z; for geographic coordinate systems, it is latitude, longitude, and height.
- Observations: The number of image observation points manually added or identified through target detection.
- Control Piont Flag: Divided into control points and check points. Control points are included in calculations during subsequent alignment updates, while check points are only used for accuracy checks in the report generation.
- Type: Divided into XYZ, XY, and Z, which are plane elevation control points, plane control points, and elevation control points, respectively.
- Horizontal and Vertical Accuracy: Set the horizontal and vertical accuracy, which affects the weight of the control point during calculations.
- Error: The distance between the control point coordinates and the intersection point of the observation values.
Note: An effective control point requires at least two observations. The update alignment function requires at least four control points.