Newsgroup: it.comp.software.cad
Titolo: Autocad Map: georeferenziazione di un raster

Chiedo a qualcuno che utilizza AutocadMap se e come è possibile effettuare
la georefernziazione di un'immagine raster (conoscendo ovviamente le
coordinate di un certo n° di punti)?
Sapete indicarmi inoltre qualche guida-corso-tutorial sul programma in
oggetto?
Grazie  a tutti

 ha scritto nel messaggio
news:cHLG5.30427$Nz.9...@news.infostrada.it...


AutoCad MAP legge anche il file dati, associato al raster, contenente le
informazioni per la georeferziazione.
Quindi, ad esempio, se un file "pippo.tif" è assieme all'omologo
"pippo.tfw", lo posiziona correttamente nello spazio bidimensionale.
E' la stessa tecnica e lo stesso formato standard usato da ArcInfo, ArcView,
ecc. ecc.
Leonardo
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                 Leonardo Tagliavini
                 Via del Navile 1/3
                 40131 Bologna Italy
                 Tel. 051-6344416
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   (   )   Oooo. e-mail:  mailto:
lta...@iol.it
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

 Leonardo
Ti dispiacerebbe darmi qualche dettaglio maggiore: come è fatto un file TFW,
come devo fare per farglielo leggere, ecc.
Sperando di non chidere troppo...grazie

Ciao,
ti mando la pagina del Manuale ArcView dove spiega come farsi un file twf:
---------------------------- Inizio Help ------------------------------
Images are stored as raster data, where each cell in the image has a row and
column number. Shapefiles and ARC/INFO coverages are stored in real-world
coordinates. In order to display images with coverages or shapefiles, it is
necessary to establish an image-to-world transformation that converts the
image coordinates to real-world coordinates. This transformation information
is typically stored with the image.
Some image formats, such as ERDAS, IMAGINE, BSQ, BIL, BIP, GeoTIFF, and
grids, store the georeferencing information in the header of the image file.
ArcView uses this information if it is present. However, other image formats
store this information in a separate ASCII file. This file is generally
referred to as the world file, since it contains the real-world
transformation information used by the image. World files can  be created
with any editor. They can also be created using ARC/INFO's REGISTER command.
World file naming conventions
It's easy to identify the world file which should accompany an image file:
world files use the same name as the image, with a "w" appended. For
example, the world file for the image file mytown.tiff would be called
mytown.tiffw and the world file for redlands.rlc would be redlands.rlcw. For
workspaces that must adhere to the 8.3 naming convention, the first and
third characters of the image file's suffix and a final "w" are used for the
world file suffix. Therefore, if mytown.tif were in a an 8.3 format
workspace, the world file would be mytown.tfw. If redlands.rlc was in an 8.3
format workspace, its world file would be redlands.rcw.
For images that lack an extension, or have an extension that is shorter than
three characters, the "w" is added to the end of the file name without
altering it. Therefore the world file for the image file terrain would be
terrainw; the world file for the image file 
floorpln.rs
 would be
floorpln.rsw.
How the georeferencing information is accessed
The image-to-world transformation is accessed each time an image is
displayed (e.g., when you pan or zoom). The transformation is calculated
from one of the following sources, listed in order of priority:
· the world file
· the header file (if the image type supports one)
· from the row/column information of the image (an identity transformation)
Because a world file has higher priority, you can override the header file
transformation information by creating your own world file.
World file contents
The contents of the world file will look something like this:
                  20.17541308822119
                   0.00000000000000
                   0.00000000000000
                 -20.17541308822119
              424178.11472601280548
             4313415.90726399607956
When this file is present, ArcView performs the image-to-world
transformation. The image-to-world transformation is a six-parameter affine
transformation in the form of:
x1 = Ax + By + C
y1 = Dx + Ey + F
where
x1 = calculated x-coordinate of the pixel on the map
y1 =  calculated y-coordinate of the pixel on the map
x = column number of a pixel in the image
y = row number of a pixel in the image
A = x-scale; dimension of a pixel in map units in x direction
B, D = rotation terms
C, F = translation terms; x,y map coordinates of the center of the
upper-left pixel
E = negative of y-scale; dimension of a pixel in map units in y direction
Note  The y-scale (E) is negative because the origins of an image and a
geographic coordinate system are different. The origin of an image is
located in the upper-left corner, whereas the origin of the map coordinate
system is located in the lower-left corner. Row values in the image increase
from the origin downward, while y-coordinate values in the map increase from
the origin upward.
The transformation parameters are stored in the world file in this order:
                  20.17541308822119 - A
                   0.00000000000000 - D
                   0.00000000000000 - B
                 -20.17541308822119 - E
              424178.11472601280548 - C
             4313415.90726399607956 - F
Note    ArcView does not rotate, or warp, images. If your world file has
non-zero rotation terms (D and B parameters), the image may change its
position relative to vector data in the same view when you zoom or pan. Use
the ARC/INFO RECTIFY command to properly rotate the image.
--------- Fine Help -------------------------------
Nota finale:
Se un raster è "serio", cioè proveniente da un processo fatto correttamente,
allora le informazioni che ti servono per fare il tuo tfw debbono essere già
disponibili. In caso contrario, se il tuo Raster è "approssimativo" (es. una
fotocopia di una mappa) allora comunque non avrai una precisione altissima e
potresti anche provare a procedere a mano.



Provo a darci un'occhiata, intanto ti ringrazio.