"Feinberg, Harold" wrote:
>
> Can someone explain to me why translators like TFWREAD and TABWORLD import
> the .tfw file to tab but place all of the control points in the upper left
> corner?
Hi All!
Converting a TFW to a TAB is really just a question of rewriting info
from one file format to another.
Let's look at the ESRI tfw File Format:
It is used as an attribute file associated with corresponding .tif file.
a TFW file consists of 6 lines, denoting the following (comments in
parenthesis):
5.000000000000
(size of pixel in x direction)
0.000000000000
(rotation term for row)
0.000000000000
(rotation term for column)
-5.000000000000
(size of pixel in y direction)
492169.690845528910
(x coordinate of centre of upper left pixel in map units)
5426523.318065105000
(y coordinate of centre of upper left pixel in map units)
So a TFW file really has only one control point, and the rest is defined
by a rotation and the pixel size.
As you understand, only perfectly aligned and orthorectified images can
be treated like this, and they must have the same map projection as any
vector data.
MapInfo has another approach. By defining at least three
pixel-coordinate relationships, a temporary projection is computed and
then used by MapInfo to tweak the vector display to fit the image as
best it can. More control points, evenly distributed will increase
accuracy.
But for the case of the TFW file, all we really know is the position of
one pixel, the rest can be computed by pixelnumber x pixelsize. Since
MapInfo needs three points, we take the three corner pixels and
calculate the coordinates for these. No more accuracy would be achieved
by calculating the far corners of the image, even if it would look more
reliable.
When MapInfo opens an image 'for viewing only' a 'false' registration of
this kind is done, involving only three corner pixels (look in the tab
file to see this).
Hope this clarifies things
Cheers, Mats.E
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