To perform the registration

The Register interface allows two basic procedures: the Image Registration and the Vector Registration of an Infolayer. The Image Registration can be used for georeferencing and later insertion in the GRIB format image database (image file with the .grb extension). The Vector Registration performs the georeferencing of Infolayers that contain vector data and that belong to any Project linked to the current Database.

The registration function is available in SPRING's main module, after the activation of a Database. In the case of Image Registration it is not necessary to load a Project when the control points are acquired in the Table or Keyboard modes. Load a Project only if the control points were obtained with the Screen mode. In the case of the vector registration of an infolayer one must work with a loaded Project, that contains the data used as a reference for registration (Screen mode). However, it is also possible to change the acquisition mode to Table or Keyboard.

After recording in a remote sensing image, it is important to evaluate, or validate, the positional quality of the recorded image for different sensors from different satellite platforms.


See about the Image Registration in SPRING

See about the Vector Registration in SPRING

See about the Validation in SPRING

With the objective of allowing a better understanding the registration operation was divided in the following items:

How to select an Image for Registration

In the beginning of registration one must select an image file (GRIB format only). The control points that will be defined later will be stored in the image file itself.

To select an image for registration:

  • run the module SPRING and load a Database;
  • click on File - Image Registration... in the main menu;
  • note that the main window, the "Image Registration", and another window, "Image Selection", that should be used to choose which GRIB file will be registered, will be opened; if you need to select another image for registration click on Image... .

NOTE: If no Project is loaded, it will be necessary to inform in the window "Projection" the projection and Earth model (datum)  that will be used to register the image.

NOTE
: If there are no image files (*.grb) in the directory that is selected, click on the button Directory..., to open the window "File Selection" unix.gif - 943 Bytesor "Directory Selection" windows.gif - 1353 Bytes.

NOTE:
For images whose bands are in different files it will be necessary to select all the files, for the control points will only be stored in the files that were selected.

Registration Operations


How to select an infolayer for the vector registration

The vector registration starts with the selection of an infolayer. The control points that will be used in the vector registration will be saved in a separate file, with the extension *.pro.

To select an infolayer for the vector registration:

  • run the module SPRING and load a Database;
  • load a reference Project from your Database ( atalho_projeto.gif - 1036 Bytes);
  • click on File - Image Registration... in the main menu;
  • note that the main window, "Image Registration", and another window, "Image Selection", that should be closed, will be opened. Click on Infolayer... to open the window "Infolayer Selection". Select a Project, a Category and the Infolayer that will be used for the vector registration.
  • click on Control Points... to select and open an existing file of control points (*.pro). That could be necessary, for example, to update this file with the insertion of new control points.

Registration Operations


Acquisition of Control Points

With the image to be registered drawn on Display 5, the user is ready to start the acquisition of control points in the window "Image Registration".

Point Acquisition

As was mentioned above, the user can acquire the points in three different ways: using a map in the digitizing table ( Digitizer mode), using an already georeferenced infolayer (Screen mode), or informing the coordinates on the keyboard ( Keyboard mode). See each of these modes below:

Digitizer Mode

To acquire the points with the Digitizing Table:

  • click on Digitizer on the radio buttons in Acquisition;

NOTE:  If the digitizer is not calibrated the system will present the message "The Digitizer was not calibrated"; thus, place the reference map on the digitizer and start the calibration procedure.

  • identify the homologous points, that is, points that correspond to the same features on the image and on the map before starting the acquisition of those. Use the Zoom feature in the "Control Panel" for a better precision in locating the points;
  • click on Create in the radio buttons in Operation;
  • click in the text box Name:  and type in a name for the point that will be acquired;
  • click on the button CR so that the command is transferred from the mouse to the digitizer. At this moment a message will request that you click with any digitizer button over the desired point on the map;
  • a point represented by a green cross is presented on the image in Display 5; drag and drop the green cross over the point of the image that corresponds to the same point in the map;
  • repeat the procedures starting from the second step above to in order to acquire other points.

NOTE: After dragging the point to its final position on the image, the one deemed correct, it will change into a yellow cross. Note that from the third point on the subsequent points are placed very near to their exact position, for the system already uses the previous points to calculate the correct position of the point being acquired.

OBS
: It is important that the points be well distributed over the image area, on the region that will be part of the project. On the contrary, you cannot guarantee a good geometric correction of the image on the regions that are far away from the set of control points. You should also avoid that the control points be distributed over a straight line. Such distribution will usually generate a wrong solution for the polynomial function that was chosen.

OBS
: We recommend that you click on Save in order to save the points already acquired, even if you haven't acquired enough points to correct the image or vector data.

Screen Mode

NOTE: The acquisition of the control points via Screen is only possible with a loaded project. Thus before performing the registration in the Screen mode make sure to load a project from your database ( atalho_projeto.gif - 1036 Bytes).


To acquire points via screen:

  • in the "Control Panel" choose the infolayer that will be used as a reference for the registration of the image that is presented in Display 5 (for example, an already digitized road map or drainage network, or else another image already georeferenced in the loaded project);
  • click on Screen on the radio buttons Acquisition;
  • identify the points that are homologous, that is, points that correspond to the same image features, or in the vector adjustment data, and also on the map or on the reference image, before starting the acquisition of the points. Use the Zoom: feature in the "Control Panel". It will allow you a greater precision in the positioning of the points.
  • click on Create in the radio buttons Operation;
  • click on the text box Name: and type in a name for the point that will be acquired;
  • click on CR so that the message "Select a reference point !" is presented; click on OK in the message box and then point the cursor over the point on the reference image, clicking with the left button of the mouse again;
  • a point represented by a green cross shows up on the image or vector data in Display 5; drag anddrop the green cross until the same geographic point chosen on the reference screen;
  • repeat the procedures starting from step three in order to acquire other points.

OBS: Note that in the Reference Coordinates sector in the "Image Registration" dialog, the exact coordinates of the point acquired from the infolayer of the loaded project are presented.

OBS
: It is recommended to click on Save in order to record the points that were already acquired, even though enough points have not been acquired so far to correct the image or vector data.

Keyboard Mode

To acquire points via keyboard:

  • click on Keyboard in the radio buttons Acquisition;
  • click on Create in the radio buttons Operation;
  • click on the text box Name: and type in a name for the point that will be acquired;
  • click on CR so that the message "Type the reference coordinates !" is presented; click on OK in the message box;
  • in the Reference Coordinates sector type the Geographic Coordinates in the text boxes Lat and Long or the Plane coordinates in the text boxes X: and Y: ;
  • click on CR in the sector Reference Coordinates;
  • a point represented by a green cross shows up on the image or vector data in Display 5; drag anddrop the green cross until the same geographic feature chosen;
  • repeat the procedures starting from step two in order to acquire other points.

NOTE: The coordinates of points, typed in on the keyboard, should usually be acquired by topographic or geodetic means (GPS, for example). This type of input is useful when there is no availability of a cartographic base to register an image or vector data.

NOTE: In this type of control point input it is not necessary to have a project loaded, since the coordinates of the points will be typed on the keyboard. It is enough to inform what projection and Earth model (datum) will be used.

OBS
: It is recommended to click on Save in order to record the points that were already acquired, even though you have not acquired enough points to correct the image or vector data.

Registration Operations


Other operations on control points

See following other operations for the manipulation of control points on the image or vector data to be registered.

Delete Control Point

To delete a control point:

  • click on Delete in the radio buttons Operation;
  • click on the name of the point that will be deleted in the list box Control Points;
  • click on CR to execute the deletion;
  • confirm in the message "Confirm deletion? Control point -> xxxx" with a YES to delete or a NO to cancel. Note that the name of the point is removed from the list box.

NOTE: The point deleted is still visible in Display 5 until you click on Execute - Draw ( atalho_desenhar.gif - 941 Bytes) in the active display.

Move the Control Point

To move a control point:

  • click on Edit/Move in the radio buttons Operation;
  • click on the name of the point that will be moved in the list box Control Points; note that the point selected is presented in green on Display 5;
  • position the cursor over the selected point, click upon it with the left button and drag it to its new position.

NOTE: The point moved keeps its original position until you click on Execute - Draw ( atalho_desenhar.gif - 941 Bytes) on Display 5.

Change the name of the Control Point

To change the name of the control point:

  • click on Rename in the radio buttons Operation;
  • click on the name of the point that will be changed in the list box Control Points; note that the point selected in the box is also presented in the text box Name: below;
  • click on the text box Name:, change the name there and click on CR.

Registration Operations


To acquire a feature

NOTE: NOTE: Feature collation and editing can only occur in Screen.


To acquire points via screen:

  • in the "Control Panel" choose the infolayer that will be used as a reference for the registration of the image that is presented in Display 5 (for example, an already digitized road map or drainage network, or else another image already georeferenced in the loaded project);
  • click on Screen on the radio buttons Acquisition;
  • identify the points that are homologous, that is, points that correspond to the same image features, or in the vector adjustment data, and also on the map or on the reference image, before starting the acquisition of the points. Use the Zoom: feature in the "Control Panel" . It will allow you a greater precision in the positioning of the points.
  • click on Create in the radio buttons Operation;
  • click on the text box Name: and type in a name for the point that will be acquired;
  • click on CR so that the message "Select a reference point !" is presented; then point the cursor over the point on the reference image, clicking with the left button of the mouse again;
  • a point represented by a green cross shows up on the image or vector data in Display 5; drag anddrop the green cross until the same geographic point chosen on the reference screen;
  • repeat the procedures starting from step three in order to acquire other features.

OBS: Note that in the Reference Coordinates sector in the "Image Registration" dialog, the exact coordinates of the point acquired from the infolayer of the loaded project are presented.

OBS
: It is recommended to click on Save in order to record the points that were already acquired, even though enough points have not been acquired so far to correct the image or vector data.

Refining the position of a control point

When we use another image as a reference for the acquisition of points in the Screen mode, we can use a feature to refine the position of a point that has already been inserted in the image to be adjusted (image in Display 5).

To refine the position of a point the system uses an area of 17 x 17 pixels on the reference image and of 65 x 65 pixels in the image to be adjusted, in order to calculate the position through the analysis of an autocorrelation matrix. The maximum value in this matrix is 0.7; above that the system moves the point to a new position.

The refinement of a point depends on some parameters:

    • Degree of the polynomial : 1st or 2nd degree;
    • Number of control points : at least 3 for first degree and 6 for second degree;
    • Presence of system correction : if the image has no system correction the dependence of the two items above will apply; if the image has system correction, one point only will be enough to refine the translation of the navigation equation of the system correction.

The figures below show a point on the reference image and the same point both before and after the refinement in its position.

         REFERENCE IMAGE                   IMAGE FOR ADJUSTMENT

     registro_ref01.gif - 18918 Bytes       registro_ref02.gif - 18341 Bytes

      IMAGE WITH REFINED POINT

     registro_ref03.gif - 18884 Bytes

Refining the position of a control point:

  • click on Select in the radio buttons Operation (make sure that only one point is selected);
  • click on the name of the point in the list box Control Points;
  • click on Refine to correct the position of the point. Note that the point moves to a new position.

NOTE: If the point in the image is off the limits defined by the area of 65 x 65 pixels, or below the maximum value for the autocorrelation, the Refine button will not cause any effect.

Registration Operations


Exporting to ASCII

After the acquisition of the control points the user has the option of saving the control points into a file of SPRING's ASCII format.

To save control points into the file:

  • click on Export for ASCII... ;
  • in the dialog "Save As" type in the destination directory, a file name, and click on Save.

Registration Operations

 


Change the Image for Adjustment

If the band or color composition used as image for adjustment is not sufficiently clear for the acquisition of the points, the user can change the bands since the points that have been acquired are the same for the image file (*.grb).

To change the band presented on Display 5:

  • click on the button Image... at section Selection of the Adjusting Data in the lower part of the "Image Registration" dialog to open the dialog "Image Selection";
  • click on the image(s) in the text box Bands and define the visualization in monochromatic mode (M) or color composition (R, G, and B); click on Execute - Draw on ( atalho_desenhar.gif - 941 Bytes) on Display 5.

Registration Operations


Selecting the Points for Registration

After the acquisition of control points the user is ready to define which points will really be used for the definition of the mapping equation. The window "Image Registration" can be used to define the best points, which are those with the smaller errors, considering the choice of the degree of the polynomial that will be used to register an image.

To assess the mapping equation we recommend that 6 control points be acquired for a first degree polynomial (minimum of 3 added of 3 more to calculate the errors) and 10 control points for 2nd degree polynomials (minimum of 6, plus 4 more to calculate the errors).


To select the points for registration:

  • click on Select in the radio buttons Operation;
  • click on the name of the points in the Control Points list box;
  • click on one button in the Polynomial Degrees radio buttons;
  • click on the System Correction check box if the image was system corrected.

As the points are selected the errors are calculated (in units of resolution, pixel, for the case of image registration, and meters in the case of vector registration) and automatically presented to the side of the control points. The calculations are also performed for the points not selected, which serve as test points, that is, they allow a non-biased assessment of the registration results.

NOTE
: Remember that when 3 points are selected, for a polynomial of the 1st degree, the errors are zero, since in this case we have 6 simultaneous equations with 6 variables. The same holds for a 2nd degree polynomial when 6 control points are selected.

Note that underneath the Polynomial Degree radio buttons the total error (in pixels, for images, or in meters for vector data) of the Test Points (points that were not selected) and the Control Points (points that were actually used for registration).

The ideal situation is to work with control points that are precise and evenly distributed.

The user should note the value presented in Error of the Control Points, for it should be used to control the desired precision. In urban areas, for example, one could consider a 0.5 pixel error for a 30 meter resolution. In forested areas we could accept an error of 3 pixels for the same resolution, due to the difficulties in finding good control points. That will depend on the user's application.

Another parameter used in the error analysis is a function of the scale that one is working. For a mapping in the 1:50.000 scale, for example, the acceptable registration error is about 25 meters. Thus,  a two pixel error in the 10 meter resolution, that is 20 meters, would be acceptable in this work scale.

Repeat, whenever needed, the control and test point selection operation, until a satisfactory result is achieved. You could test with other points, other polynomial, or even correct the position of some point.

Registration Operations


Saving the Parameters of a Registration

After choosing the best points for a registration and defining the mapping equation, all of these parameters should be saved for further use. In the case of an image registration, the control points and the  mapping equation are saved in the image file itself (*.grb). In the case of a vector registration, these parameters are saved in another file with the extension *.pro. SPRING uses the Import GRIB function to insert a registered image into a project and benefits from the Mosaic function to remap a registered vector data into a project.

 

To save the registration parameters:

  • click on Save in the "Image Registration" dialog in order to save the points that were acquired and selected.

NOTE: If the user tries to close the point acquisition window without saving them the system will present the message : "Control points nor saved. Do you wish to save them? Yes / No"

  • click on "Yes" and the points will be saved as if the Save button had been pressed;
  • click on "No" and the newly acquired control points will not be saved.

After this step the image can be imported into a SPRING project (see " Registering an image"), provided its bounding box contains the geographic area of the image. In the same way, a registered vector data can be mapped into a SPRING project (see Mosaic of Infolayers).


Registration Operations

 


Validating control points or features.

After choosing the best points and features that will be used in the registry and defining the mapping equation, the validation parameters can be chosen. Note that the field in the interface will be active if there are dots and / or features. You can choose one parameter at a time or both together. A report will be displayed showing the results of various calculations and can be saved in a file (.txt).


Registration Operations

 

See also:
Importing GRIB (Images) files into a Project.
To Execute a Mosaic.
The Logical Sequence to perform the registration.
About Digital Images.