QMap - Geodetic Control Point (GCP) Geometric fine correction
The purpose of geometric correction is to map an image without geodetic coordinates to a given coordinate system. Through mapping transformation, it can eliminate the deviation of image geometric position caused by various factors, such as extrusion, stretching, distortion and offset, etc. It is especially suitable for remote sensing images and stitching of various super large images. Through geometric correction, precise positioning images of various geographic information can be established, which makes the analysis of historical changes with time as the axis very convenient.
Now according to the software function interface, the introduction is as follows:
Click the "Load" button to load the selected geodetic control points with features. The control points to be selected are listed in the list box at the top of the dialog.
2. Match Control Points
Click an item in the list box at the top, and the details are listed in the edit boxes for "Logo", "Note", "GCP(x,y)" coordinates, etc. Open an image to be corrected, select the screen position corresponding to the geodetic coordinates, right-click the mouse, and fill in the screen coordinates and image name into the "Img (x, y)" and "ImgName" in the dialog box to complete a control point registration. Click the "Insert" button to insert the control point into the list box below.
3. Control point edit
In the list box in the middle of the dialog box, select an item, all the information of the item will be listed in "Logo", "Note", "GCP(x,y)", "Img(x,y)" and "ImgName" etc. are displayed and reinserted after modification. In addition, double-click a data in the list box with the left mouse button, and its content can also be modified.
4. Display of control points
By selecting "GCP display size", you can change the size of the control point crosshair on the image to suit the observation.
"Lock control point display and image name" selection means to bind control points to the image, and control points that do not belong to the image will not be displayed. Conversely, control points can be displayed on different images.
5. Geometry Correction Algorithm Selection
a) Binary Linear: The number of control points must be equal to 3, which is the easiest algorithm to satisfy the conditions.
b) Binary Quadratic Nonlinear: The number of control points is greater than or equal to 6, which has satisfactory results for images with moderate position shift.
c) Binary Cubic High-order: The number of control points is greater than or equal to 10, which has better effect on images with large position offset.
The default is Binary linear linearity.
6. Resolution selection after geometric correction
Map resolution (Meter)： 0.5M，1M，5M，10M。默认为1M。
7. Sampling span selection
In the coordinate system transformation, the pixel values of the image are resampled. A sampling span of 100% means that the corrected area is replaced with a pixel value of the original image. And 50% means that the corrected area is replaced by the mean of the 4 pixel values of the original image. Default is 100%.
8. Map Origin selection
Since the origin of the computer screen coordinate system is the upper left corner, the origin of the general coordinate system is the lower left corner. Therefore, the origin of the coordinate system of the transformed geometrically corrected image is optional, and the default is the lower left corner.
9. Coordinate xy swap selection
In the test or processing, the latitude and longitude or xy coordinates of the geodetic coordinates can be exchanged, and the default is not exchanged.
10. Type selection
Type refers to the type of coordinates, there are "Latitude and Longitude" and "Plane Coordinates", the default is latitude and longitude. The coordinate type should be selected correctly, otherwise, the operation will be incorrect.
11. Check deviation
Check deviation can detect the deviation of each geodetic control point corresponding to the coordinates on the screen. Click the "Check deviation" button, under the dSX and dSY columns in the control point matching list box, the relative deviation values of the corresponding points under the SX and SY columns will be listed. If the deviation of a certain point is large, the coordinates (SX, SY) of the point must be adjusted. Double-click the SX or SY value of an item to edit and modify it immediately. Click the "Check deviation" button repeatedly to modify the SX or SY values of different item, so that the overall dSX and dSY are relatively small and reasonable.
12. Geometry Correction Test and Process
Click the "Test" button, if the deviation of the control point is large, a prompt box will pop up to terminate the test. If the control point error is within the allowable range, the geometric correction distribution pattern will be gradually displayed on the original screen image for easy judgment.
After the test is satisfied, you can click the "Process" button, select the geometrically corrected image name (.gmap), and complete the processing. The image name suffixed with "gmap" is the unique file form of this software.
Selecting "" will pop up a prompt box during each trial or process, informing the "If it is very slow, you can press Tab key to force a break" operation, so as to deal with various problems.
13. Save or Load Plan
Plan can save entire processing parameters and data to a file, which can be loaded to replicate experiments and processing when needed.
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