![]() 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 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:
Using
the Mosaic Window to fix a vector data Polynomial Transformations - control pointsUsing 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). 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. 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.
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