Ground Control Points may optionally be added in the Pixel-to-Points tool to adjust the horizontal and vertical positioning of the point cloud. With no ground control points, the point cloud will be positioned using the input image metadata.
Ground Control Points identify recognizable features in the input images and align them to a known location (X, Y and Z coordinates). Typically the point cloud is aligned based on surveying of these identifiable features, however, the ground control point location could also be identified by loading other reference data, such as surveyed assets or reference orthorectified imagery, such as those available from Online datasources.
Enter ground control points on the left side of the Pixels to Points tool dialog, below the image list.
Specify the x, y and z location of the control points:
To load a set of control points, click on the File menu in the Pixels to Points dialog or right-click on the Ground Control Points section and choose Import Control Points From Loaded Point Layer or File...
To add a ground control point individually, press the New Point... button found below the Ground Control Points list in the Pixels to Points Tool dialog.
Enter the location of the Control Point using one of the options in the Location section of the Ground Control Point info dialog displayed to the right. Optionally specify the Elevation, name and custom symbol to use for the Ground Control Point.
Pixel Snap: If checked, this enables pixel snapping in the Pixels to Points preview window. Snapping is enabled by default. Uncheck to disable.
From the Input Image files list, select each of the input images, so that they display in the Image Preview. Images that likely contain the ground control point location are highlighted in green. Click on the Image Preview to zoom in on the clicked location.
When the Ground Control point is located press the Add Control Point to Image... button and click the Ground Control location on the input image preview. The mouse wheel may also be used to zoom in and out on the preview image, or clicked and dragged to pan the Image.
If the point location is incorrect, click the dot on the image to select it. Then drag to move the control point, or press the delete key to remove it.
Complete this process for each of the input images. Then add at least two more Ground Control Points, and locate them in each of the images.
When a Ground Control Point is selected from the list, all of the input images that contain that control point will display in blue text. Right-click on the Ground Control point list for additional options to move between input images. The Ground Control Points will also show the number of identified image matches in the # Points column.
Once the control points have been added, optionally right-click on the Ground Control Points list for additional options to check the specified points, including previewing them in the main map window.
The Image Preview/ Ground Control Points displays the image highlighted in the input image file list.
To zoom the preview image, locate the cursor over the part of the image that is of interest, and roll the mouse wheel. The image can also be zoomed by left-click and right-click of the mouse. A left-click will zoom in at the cursor location. A right-click will zoom out.
Click and drag the mouse wheel to pan the image.
If a control point has been added to the image, it is selectable by left-clicking. The cursor will change to an arrow when over the added point, allowing it to be selected. A red box will appear around the point indicating its selected state. Left-click and drag to move the selected control point.
Enter the known coordinates of the ground control point. Double-click the named point in the Ground Control Points list to bring this dialog back up for an already added Ground Control point.
Name- Specify a name to identify the ground control point.
Symbol - This is the point symbol that will be displayed on the image previews that contain this ground control point.
Elevation - The elevation value is important to vertically align the output point cloud. The default value will come from the image metadata but should be customized for improved vertical accuracy.
Location - Specify the coordinate location of the ground control point. The current projection is set from the Configuration display projection, and if no data is loaded into the map it will default to Geographic WGS84. The Select From Map... button will also be available in the location section, only when there is data loaded in the main map view.
Press the remove selected button to remove the entire Ground Control Point from the list of Ground Control points.
To remove the point from only one input image, press the point to select it. Then press the Delete key to remove the Ground Control Point instance.
Add Control Point to Image...
This button is located under the Image Preview. Press this button to add a control point to the current image preview. It may be necessary to first click on the image to zoom, or use the mouse wheel to zoom in on the image preview.
Once a point is added to the image, it can be updated. Click the point on the image to select it. Drag to move or press the Delete key on the keyboard to remove the point on the image.
Right-click on the Ground Control Points list to see the following options:
Edit Ground Control Points... (Double Click) - Use this option to modify the X,Y,Z of the Control Points, the Name, or the symbol used.
Move to Previous Image Containing Ground Control Points - This will move the image preview to the next image in the list that has a marked control point for the highlighted ground control point., if the Include Images that Likely Contain Ground Control Points when Moving option is enabled.
Move to Next Image Containing Ground Control Points - This will move the image preview to the previous image in the list that has a marked control point for the highlighted ground control point, if the Include Images that Likely Contain Ground Control Points when Moving option is enabled.
Include Images that Likely Contain Ground Control Points when Moving - This is a selectable setting that modified the behavior of the two above items to include the green suggested images in the navigation, or to only navigate through the blue control point added images.
Import Control Points from Loaded Point Layer or File... - Populate the Ground Control Point list from either a loaded point layer, or an external file. Specify the layer by name, or chose <Select File to Load> to pick an external file. Any vector format supported by Global Mapper can be used as long as it contains point features.
Load Control Points as Point Features in Main Map - This option will create a new Ground Control Point layer in the 2D map view, with labels and styles matching the points in the Pixels to Points dialog.
Save Control Points to Text File - This will save a text file of the Ground Control Point locations, as while as any images that have been marked as containing them, with the corresponding pixel X and pixel Y coordinates of the image. This can be loaded into a new Pixels to Points workspace to reuse just the control points.
Ground Control Report
Once the tool has been run with specified Ground Control Points, the Input Image list will populate with columns of additional statistics based on the fit of the input control points. This final complete operation dialog will also display the overall camera position fit error.