Information Layer - Vector Data Registration

The Vector Registration of an information layer is a geometrical transformation relating plane coordinates from a vector data, which belongs to an information layer, to a project coordinates reference. In the SPRING this reference system is, in the worst case, the planar coordinates for a certain cartographical projection. As any cartographical projection has a well defined relationship with geographical coordinates, it is possible to say that the Vector Registration of an information layer establish a relationship between the planar coordinates from an information layer and geographical coordinates.

The Vector Registration might be useful when digital information is created without being concerned with any attachment to project or geographical coordinates. It is common, for instance, to have digital maps developed in CAD systems, which have their coordinates referred to the devices used for digitizing them. Thus, instead of projection coordinates, in meters, these digital maps can have coordinates in millimeters, because when using a digitizing table the function to relate the table coordinates to a map projection coordinates is missing.

The Vector Registration can also be used to improve the vector data georeferenced already inserted in the project context. Some causes for a low quality of georeferencing are calibration errors between table coordinates and projection coordinates, errors when defining some projection parameters (for instance, the incorrect definition of a central meridian in the UTM projection), errors while importing digital data with incomplete documentation, or even errors from mistakes in the unit definition for the planar coordinates for the projection considered.

The first step in the Vector Registration procedure is the database activation which has a Reference Project, that is a project that has data with reliable georeferencing. Next, if the vector data to be fixed is not yet in an information layer, thus in any project under the same Database, the user should import the vector data to project it in the active Database. This project can even be the reference project. In the vector data case where the coordinates are not attached to a projection system, it is suggested that they are inserted in a NO-PROJECTION project, where the involving rectangle can be defined in the same original unit as the vector data being considered. If a street maps, developed in the AutoCAD, exported in the DXF format, did not consider the projection coordinates and has an original involving rectangle going from 0 to 1000 mm in X and Y, the user should create a NO-PROJECTION project in the SPRING, with an involving rectangle using the same values in mm.

The Vector Registration itself starts with the reference project activation and the selection of an information layer that has good references (well defined terrain attributes) that can be used as control points. The user, then, has to call the Register function, click on the Information Layer button to select and draw in window 5 the vector data to be fixed, and start the control points definition. At this moment, although the standard procedure considers the Window mode for the control points acquisition, the user can change the acquisition mode to keyboard or Table. The control points creation, edition, and selection procedures and the polynomial mapping function definition is similar to the one used in image registration.

The next step is the Save operation for the control points selected by the user. Differently then the image registration, where the control points are saved in the image file itself (*.grb), the vector register requires that the user save the control points in an external file with a *.pro extension. In this way, the Vector Registration output has an information layer to be corrected and a *.pro file with a list of acquired control points. It is good to remember that the Control Points button in the Registration main window can be used to select and open an already existing file with the control points (*.pro) to, for instance, allow their updates through the insertion of new control points.

The last phase is to use the Mosaic window to remap the information layer used in the vector registration for an output information layer created and activated in the Reference project. In order to get the control points file (*.pro) called and used in the remapping it is required that the user click on the Adjust selection button, leaving it selected. Thus, the Mosaic execution allows the user to say to SPRING which control points file (*.pro) should be used to remap the vector data.

The user can get more information at:

 

Operation at the SPRING Registration window

See also:

Using the Mosaic Window to fix a vector data


Polynomial Transformations - control points

Using the polynomial transformations is a common procedure in image Registration. The polynomial transformations make the connection between the image coordinates and the reference coordinate system through the control points. Control points have attributes identifiable in the image and in the terrain, that is, they are confirmed attributes and their coordinates are known in the image and in the reference system. Road crossing, airport runways, river junctions are natural candidates for control points.

The parameters determination for the selected polynomial transformation is performed through the solution of an equation system. In order to assemble the equation system the control points coordinates has to be known in both, the image and in the reference system. The image coordinates (row, column) are obtained when the user clicks on the image attribute. The reference coordinate are usually obtained through reliable maps which also has the attributes used as control points (in the Table mode in the Registration window). The SPRING also accepts measurements performed directly on the terrain using a GPS (keyboard mode). Existing vector data and georeferenced images can also be used as an extraction source for reference coordinates (Window mode).

Once the n control points are determined and the polynomial transformation is selected, a system with 2n equations is defined to solve 6, 12, or 20 parameters, depending on the polynomial's degree (1o, 2o or 3o degree). Thus, it is possible to conclude that the minimum number of control points is 3 for the 1o degree polynomial, 6 for the 2o degree polynomial and 10 for the 3o degree polynomial (see the equations for a 1o and 2o degrees polynomials in the figure below).

 

 

registro_esq.gif - 6063 Bytes

 

The minimum number of control points represents the situation of a determined equation system, where the number of equations coincide with the number of unknown variables to be computed. However, as the control points measured coordinates are subjected to errors, it is suggested to use a number of points higher than the minimum. In this case, the equation system is over determined, that is the number of equations is larger than the number of unknown variables. This procedure allows to distribute the measure errors of the control points. In practical terms it is advised to use 6 control points for the 1o degree polynomial, 10 control points for the 2o degree polynomial and 14 control points for the 3o degree polynomial.

One has to keep in mind also that the control points distribution over the area to be registered is very important, because the polynomial transformations usually works well only for the area where the control points were defined.

 

Register


Logical Sequence to Perform a Vector Registration

Next an example is presented of the logical sequence that the user should follow to georeference an information layer using as a reference an already georeferenced data from an active project. The example supposes there are a Database and a Project.

 

Step

Dialog Box or Menu

Result

1 - Activate a Database


Database


Active Database.

2 - Activate the project that has the area of the vector data to be corrected

Projects

Active Project.

3 - Importing a vector data to an IL

Importing

Available IL in the control panel.

4 - Activate the Reference Project (which can be the same as the one in item 2)

Projects

Active Project.

5 - Getting control points

Vector Registration

Selecting an Information Layer

Several Control Points.

6 - Select points for Registration and error analysis.

Vector Registration

Best Points Selected.

7 - Save the control points for a file Registration.

Vector Registration

File with a *.pro extension.

8 - Remap the IL with vector data for the reference Project

Mosaic

IL with the georeferenced vector data.

 

About Registration