How to execute the Vector to Raster transformation
The Vector-Raster transformation creates a Thematic Image
representation from the classes in the IL. An IL may have only a Thematic
Image. If there is any change in the vectors and classes, the transformation
has to be performed again.
This transformation generates an image in the raster format from data
represented vectorially. There are two cases to be considered: the linear
element and the polygon.
For a linear element the transformation can be performed joining the vector or
linear element, to a raster matrix. This transformation identifies what raster
elements are crossing a line and codes them with attributes or class values
associated to the line.
Example:
For polygonal elements, the area to be converted is initially defined
(typically the rectangle involving the set of polygons) and the "pixel"
size. These information define a grid which is placed over the original polygon
map. A class has to be associated to each "pixel" (or an
attribute value). There are several solutions for this problem. The most common
consists in computing the "pixel" center and verifies inside what
polygon is placed, associating to the "pixel
" the class value defined for the polygon.
The figure below shows the vector-raster transformation procedure.
The vector to raster transformation adds distortion in the data original
geometry. These distortions increase with the "pixel
" size and with the polygons boundary complexity. "Pixels" located at the
boundary will be mapped to the closest polygon.
The smaller the cell size ("pixel
"), the higher the transformed image fidelity, however, the thematic image
requires more storage space.
Vector Raster Transformation:
-
activate the Database (
) and the desired Project (
);
-
select using the "Control Panel"
the IL of the Thematic
category which will have its classes represented in the raster format;
-
click on Thematic - Vector -> Raster...
in the main menu;
-
in the Active IL: it is informed whatever was selected in the
"Control Panel";
-
in Output Image Resolution: type the Horizontal and Vertical
resolution for the image that will be generated;
-
click on Apply to execute the
transformation;
NOTE
: An IL may have only one thematic image. If another resolution was desired,
the transformation has to be executed again. The old thematic image will be
changed. The image resolution has to be the best possible to represent the
vector details present in the IL. This resolution is limited to the available
storage space. It is suggested a maximum resolution compatible to the map
precision limit, that is 0.5 mm times the map scale. For a given scale, say
1:100.000, the 50m resolution is adequate.
NOTE: Check in the "Control Panel"
that the thematic information layer will have an image representation.
See
about the vector and raster representation in the SPRING
See also:
Vector- Raster Transformation
|