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Warping Maps for High
Accuracy
PixEdit supplies a
correction function that compensates for many of the typical errors
introduced in the scanning process. Most of these distortions are not
critical for engineering applications, but such errors may be
unacceptable for most mapping applications. Roller scanners and scanners
with multiple cameras are in general not suitable for mapping
applications without post corrections of the data. Flatbed scanners are
much more accurate, but even these types of scanners are likely to
introduce minor distortion. Even if the scanner is nearly perfect, the
map media itself may change its size and shape depending on media type,
temperature, air moisture and storage method.
The following text is not meant to be a complete description of
scanner-introduced distortions, but it covers the most basic problems
found in roller-based scanners. The most common distortions are:
Variable Stretch and compression in a vertical direction: A
constant stretch or compression error is most likely present. In
addition, due to the fact that the map has a certain weight, the map is
fed in slower in the beginning of the scanning process, and faster at
the end, (if the map has to be lifted by the scanner). It is therefore
recommended to let the map rest horizontally both on its way into and
out of the scanner, but the mechanical construction of the scanner does
not always permit this. The results of this error is stretching in the
first part of the map, and compression in the lower part of the map.
Variable Stretch and compression in a horizontal direction: Since
camera optics are not a 100% linear, the image is stretched at the edge
of the visible field of each camera. For scanners with a single fixed
sensor instead of cameras, horizontal distortion is a minor problem.
Ordinary skew: It is difficult to perfectly adjust a document
in-line with the strip of sensors in most scanners. An ordinary skew is
most likely to be present in the map after scanning.
Un-parallel sides: Due to the above-mentioned problems, each side
of the map will not be 100% parallel to each other. Instead, the scanned
result resembles a four-sided polygon on a microscopic scale.
There are also other sources of inaccuracies in scanners, but the common
distortions mentioned above need correcting in mapping applications.
The basic correction function found in Warp accepts four accurately
digitized corners of the area to be corrected. It then transforms this
area into a perfect rectangle with a specified size. This type of
transformation is typically called a four-point correction. Before
executing the Warp (previously known as the map correction function in
early versions of PixEdit), each of the four corners of the scanned map
(or document) must be defined. This is done by inserting correction
marks. Click on the Warp tool and insert correction marks in each of the
four corners of your document border. For best results, always select a
zoom factor of at least 1:1 or use the blowup window with AccuPix turned
on, so that your correction marks can be inserted exactly in the center
of each corner.
If you need to edit the position of an inserted correction mark, click
the mark once more to delete it. Then insert a new one by following the
above-described procedure. After careful and accurate digitizing of each
of the four corners, use the following procedure to correct document
deformation:
Instead of correcting the map immediately by clicking the Warp button,
you may choose to click the Generate Macro button. This option enables
you to continue with inserting correction marks in more maps without
waiting for the process to complete. PixEdit will, in this case, start
the correction job when you use TOOLS, PROCESS GENERATED MACROS.
Combining corrected maps
In some cases, you may find it useful to combine several maps together,
forming one large, seamless map. After having corrected, for example,
nine maps by the Warp function, create a new blank sheet using the FILE
NEW command. Set the size of the new document large enough to contain
all of the nine maps that have been corrected.
Insert the upper left map into the blank document by using the FIGURE
INSERT function in the EDIT menu. Click outside the blue frame to
permanently insert the first map. Then follow the same procedure,
selecting a new map to be inserted to the right of the previous map, and
position it to the correct position. Use the VIEW menu and select the
ZOOM 1:1 option or, even better, turn on the Grid and the Grid Snap
function with a grid size equal to the size of your maps. Continue to
move the map so it fits together with the previous map. Since the
correction function splits the surrounding frame of each map to half the
thickness, the maps should fit perfectly together. Click outside the
blue lines of the selected area to join the two maps permanently
together. Continue to join the rest of the maps in a similar way.
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