In this workspace you collect ground control points (GCPs) for the input image. Corresponding reference ground control points (GCPs) are used to compute transformations in order to resample or calibrate the image.
This workspace opens when you select a geometric model and click OK in Set Geometric Model - Ribbon Workspace dialog, or load a saved model. The GCP Tool Reference Setup dialog also opens, where you select your source for reference GCP points.
The Multipoint Geometric Correction workspace consists of:
- Menu Bar
- GCP CellArray
- Status Bar
Open Input Layer Click to add a layer to the Input Views. The File Selector opens.
All Types is available in the Files of type list to allow you to choose the collecting reference points. Select an image, and then open any file to add as the reference image in the Multipoint Geometric Correction dialog.
Load Input GCPsClick to load an existing input .gcc file into the Multipoint Geometric Correction workspace. The File Selector opens. Loading input GCPs appends the GCPs from the file to the current input list in the Multipoint Geometric Correction workspace. If the map projections are different, you are prompted to reproject the points.
Save Input to Image Node Click to save the loaded GCPs to a file. By default, the GCPs are saved as a node in the image file.
Save Input As Click to save the loaded input GCPs to new file. The File Selector opens. This allows the GCPs to be saved to a separate .gcc file rather than into the image file. This may be useful if these GCPs have an application other than to the input image.
Open Reference Layer Click to add a layer to the Reference Views. The File Selector opens.
All Types is available in the Files of type list to choose the collecting reference points. Select an image, and then open any file to add as the reference image in the Multipoint Geometric Correction dialog.
Open Reference Base Map Click to add a base map layer to the Reference Views.
Load Reference GCPsClick to load an existing reference .gcc file into the Multipoint Geometric Correction workspace. The File Selector opens.
Save Reference Click to save the loaded reference GCPs to a .gcc file.
Save Reference As Click to save the loaded reference GCPs to a new file. The File Selector opens.
New External Viewer Click to open a new External Viewer to spatially link separate images to the View panes in the Multipoint Geometric Correction workspace to use as visual reference or contextual guide.
Exit Click to close the Multipoint Geometric Correction workspace.
View Only Selected GCPs When this option is checked, only the selected GCPs (highlighted in the CellArray) are shown in the Main View pane described in this section. You can also select a GCP in the CellArray, and the Main View will drive to that GCP.
To select a GCP, click in the Point # column of the CellArray.
Show Selected GCP in Table When this option is checked, you can select a GCP in the Main View to highlight the respective GCP in the CellArray. Select a graphic GCP icon by clicking on it in the Main View. You may have to scroll down in the CellArray to locate the highlighted GCP.
Set Point Type Click to select the type of points to use. GCPs may be designated as control points or check points.
Control Click to set point type to Control. Control points are used to calculate the geometric transformation model.
Check Click to set point type to Check. Check points are not used in the calculation, but used to independently evaluate the error in the transformation.
For both of these, the point type is set for the current selection in the CellArray.
Reset Reference Source Click to change the Reference image for your GCP points. The GCP Tool Reference Setup dialog opens. This allows you to collect reference points from multiple sources (for example, a Viewer or a .gcc file). If a reference map projection conflict occurs, you are required to select the desired projection.
Reference Map Projection Click to change the Reference map projection and units. The Reference Map Information dialog opens.
Point Prediction Click to calculate the transformation of reference points. The GCP Prediction dialog opens. Point prediction only works once a valid geometric transform exists. For example, when there is already a minimum set of points defined and solved. At this point, the Multipoint Geometric Correction workspace predicts the position corresponding to the GCP using the transform. This does not add any new information to the transform, but is a useful convenience feature to put the corresponding point in its approximate position.
Point Matching Click to compare spectral values. The GCP Matching dialog opens. Point matching has the same requirements as point prediction and also requires the reference be associated with an image. Point matching uses the transformation to predict the corresponding point position, then matches pixel patches in the two images to determine the precise, subpixel position where the cross correlation coefficient is maximized.
Help for GCP Tool Click to open this Help document.
Pseudo Map Coordinate System
The Multipoint Geometric Correction workspace uses a pseudo map coordinate system rather than a file coordinate system of the image when the image is non-georeferenced.
The file coordinates for a pixel value in an image are in the Column and Row indices used to locate that value in the raster array of the image. A non-georeferenced image is normally displayed so that the upper-left corner of the image represents file coordinate (0,0). The positive Column (X) direction is to the right, and the positive Row (Y) direction is downwards.
In ERDAS IMAGINE V8.3 and above, the file coordinate system definition is unchanged (the Inquire Cursor in File coordinate mode reports coordinates with the positive Y direction being downwards from a 0,0 origin at the top left corner of the image), but the Multipoint Geometric Correction workspace references an image using the pseudo map coordinate system if it is non-georeferenced (or its real map coordinate system if it is georeferenced). In this system, the origin (0,0) is still at the top left of the image, but the positive Y direction is upwards. Consequently, Y coordinates for non-georeferenced images are now negative, since the image rows are, by definition, below the origin of the coordinate system.
Click to open the Model Properties dialog for the model you are using with your image. From this dialog, you can specify changes to the model properties displayed for the image in the Viewer.
Click to turn off and on Automatic GCP Editing mode. Other icons in the GCP toolbar are also activated when using Automatic GCP Editing mode. The Automatic GCP Editing mode turns on all the automated tools at once; Point Prediction (or Point Matching if image to image), auto computes for the Transformation calculation, and View to selected point. This also allows you to quickly gather points, keep the Transformation calculation current, and always see what is going on in the Views. Automatic GCP Editing mode becomes active when you collect the fourth GCP in the Input image.
Click to solve the geometric model with control points.
Click to set automatic Transformation calculation. Automatic Transformation calculation recomputes the transformation whenever a GCP is added, deleted, or modified.
Click to compute the error for check points. This does not compute a new transformation, but updates the RMS error fields for the check points as well as the RMS totals.
Click to reset the image to a zoom factor of 1.0.
Click to fit the image in the active View to the smallest View dimension.
Click to use the selection cursor to select GCPs in the View.
Click for zoom recenter, box zoom, and real-time zoom features. For zoom recenter, click in the View to indicate the center of the enlarged area. For Box zoom, click and drag a box in the View. For real-time zoom, middle-hold and scroll in View.
Click for zoom recenter, box zoom and real-time zoom features. For zoom recenter, click in the View to indicate the center of the reduced area. For Box zoom, click and drag a box in the View. For real-time zoom, middle-hold and scroll in View.
Click and move mouse to roam the image. Click and drag over the image to pan up, down, left, and right.
Click to create new GCPs. The cursor will turn into a cross-hair in the View. This tool must be locked to create GCPs repeatedly without having to click on this again.
Click to unlock the tool currently in use. Once unlocked, you can enable different tools.
Click to lock the tool currently in use.
Click to find a selected point in the input image. The Input Main View (later in this section) and Input Zoom View drive to the point.
Click to find a selected point in the reference image. The Reference Main View and Reference Zoom View drive to the point.
Click to update Z values for selected GCPs.
Click to set automatic Z value updating for all GCPs.
Some geometric models support Z values (elevation) in the calculation. If applicable, these buttons are enabled to allow Z values to be updated for GCPs.
The source of the Z values is determined in the geometric model properties dialog. Normally, they are either defined from a Digital Elevation Model (DEM), or are defined as a constant.
Updating the Z values causes the GCP tool to reset the Z values for each selected GCP by reacquiring it from its source. Auto Z value updating reacquires the Z value whenever a GCP's position is modified.
Click to open the Resample dialog to specify the output file, resample method, and output information.
Click to calibrate the image. The Calibrate Image warning dialog opens.
The Multipoint Geometric Correction workspace provides you with groups of View panes in which you can collect or display ground control points, view images, and visually perform output data verification. The View panes are grouped as follows:
- Input Views - Group of three linked Views displaying the Input image
- Reference Views - Group of three linked Views displaying the Reference image
Each group of three Input Views and group of three Reference Views contain these types of Views:
The Main View is the largest work area. One Main View displays the input image, and the other Main View displays the reference image. In the Main View, ground control points are shown. You can visually perform output data verification.
The Over View displays the entire input or reference image. The Over View displays a Geographic Link Box to indicate the portion of the overall image that is currently displayed in the Main View. The link box may also be used to pan and zoom the display in the Main View.
The Zoom View displays a zoomed-in view of the area in the Main View. You can collect ground control points in this View. A Geographic Link Box in the Main View indicates the portion of the image currently displayed in the Zoom View.
The GCP CellArray lists the GCP X and Y coordinates for the Input (image to be rectified) and Reference images. If Z values are used, they are listed here as well. The X and Y residuals and RMS error values are also listed.
Right-click in a row column to open the Row Selection menu. Right-click in any column heading (except the first) to open the Column Options menu.
Point # Reports the Ground Control Point number. Point numbers are arranged sequentially.
Point ID Reports a numeric identifier for each point.
> (Region indicator) A filled arrowhead symbol in this field indicates the selected point in the CellArray. Click in this field to drive to the indicated GCP.
Color Color patch showing the current color for the specific Input GCP displayed over the imagery in the Views. To change the color of a point, click inside the cell and select a color from the menu.
X Input Reports the X coordinate of the corresponding GCP in the Input image.
Y Input Reports the Y coordinate of the corresponding GCP in the Input image.
Color Color patch showing the current color for the specific Reference GCP displayed over the imagery in the Views. To change the color of a point, click inside the cell and select a color from the menu.
X Ref. Reports the X coordinate of the corresponding GCP in the Reference image. You can manually enter the coordinate if desired.
Y Ref. Reports the Y coordinate of the corresponding GCP in the Reference image. You can manually enter the coordinate if desired.
Z Ref. Reports the Z coordinate of the corresponding GCP in the Reference image, if available. You can manually enter the coordinate if desired.
Type Reports whether the point is type Control or Check. Control points are used to calculate the geometric transformation model. Check points are not used in the calculation, but used to independently evaluate the error in the transformation.
X Residual Reports the difference between the original X coordinate of a Full or Horizontal control point and the X coordinate estimated for the point using the current model. This field is not editable.
Y Residual Reports the difference between the original Y coordinate of a Full or Horizontal control point and the Y coordinate estimated for the point using the current model. This field is not editable.
RMS Error Reports the RMS error for the respective point based on that point’s X Residual and Y Residual. Error is expressed in pixels of the input image. This field is not editable.
Contrib. Reports the normalized values representing each point's RMS error in relation to the total RMS error. This field is not editable.
For more details about X and Y Residuals and RMS Errors, see RMS Error Calculation.
Match When Automatic GCP Editing mode is active, reports the quality of the match in the range of -1.0 to 1.0. A larger match number indicates a better match for the point pair. The match is calculated using Point Prediction or Point Matching. This field is not editable.
The status bar located at the bottom of the Multipoint Geometric Correction workspace reports the coordinates at the current location of the mouse. As you roam over the image in a View, the location of the mouse cursor reports here. As you move the mouse over the Toolbar buttons, a brief description of the button displays in the left side of the status bar.
On the right side of the status bar, the following information is reported:
RMSE Reports RMS Error. RMSE is expressed in pixels of the input image.
Error Std. Dev. Reports the standard deviation (variation) of the Error values. Error is expressed in pixels of the input image.
Input Projection Reports what the resulting output projection will be, and indicates the projection source as input, reference, or specified, according to the project settings and available data.
For more information about transformation matrices see Transformation Matrix.
For information about how the RMS Error is calculated see RMS Error Calculation.