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| GeoCommunity Mailing List |
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| Mailing List Archives |
| Subject: | GISList: SUM: Create World Files ? |
| Date: |
11/09/2002 09:21:36 PM |
| From: |
Douglas Culbert |
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Hello Lists,
Thank-you for your response.
Special thanks goes out to:
Calin Nitu
Paul Ramsey
Litea Biukoto
Kim Ollivier
Paul Lohr
Michael Morin
Tyler Kleykamp
and any others I may have forgotten.
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Original Question:
I'm wondering how to generate world files.
I have a number of hard copy aerial photos. They have been scanned and
saved them in *.tif format. From the hard copy photos I was able to obtain
:
Flight Path #
Tile #
Central coordinate (center pixel Latitude and Longitude)
Scale
Angle of Flight Path
I entered the coordinates into a database and generated a point shapefile
for the centroids of the photos.
Is there a way to develop a world file with this limited information?
Or do I have to go and collect GCP's to georeference these images?
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Answers:
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It is possible. Its much easier if the photos are regular and rectangular
because the rotation
terms are zero, so all you need is the top left pixel centre and the pixel
size,
not the centre of the photo.
If each one is different, there are less shortcuts, but it may still be
worthwhile.
Here is what I do:
Create an index polygon cover, not just a centroid point.
This may be easy from the information you have already.
Create a simple box around each image in "page units" ie no world file.
Transform each box to the index polygon. I used a simple aml loop here.
Save the transform parameters and scrape off the summary results at the top
because they are the world file parameters that you need.
Write them into a world file and hey presto there they are.
Since I had several hundred, it was worth the development and very fast to
run.
Kim Ollivier
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The proffesional way must be aerotraingulation (GCP creation) and
orthorectification (rectification using GCPs - for planimetric & a DTM from
that area - for altimetric rectification).
Also you can develop a *.twf (World file) from your data, but the resultes
will be under espectation.Knowing the flight path angle with the NS axis and
the dimmension of the aerial photo compute the photo corners coordinates -
ERRORS.
Calin Nitu
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To create world files you need to know a corner tick coordinate, which you
would have to derivce from the scale, center point, and flight angle.
Probably, it would not be super accurate. Good enough for very rough
display, but not good for anything else.
Paul Ramsey
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Made a search @ http://support.esri.com/. You may have to do a few
calculations to get the upper left (x,y). Also edited the text a bit, long
winded explanation.
The world file is an ASCII text file associated with an image and contains
the following lines:
Line 1: x-dimension of a pixel in map units
Line 2: rotation parameter (=0)
Line 3: rotation parameter (=0)
Line 4: NEGATIVE of y-dimension of a pixel in map units (Tip:negative number
unless in the southern hemisphere)
Line 5: x-coordinate of center of upper left pixel
Line 6: y-coordinate of center of upper left pixel
Create a world file, using a text editor. This is generally practical only
when the image does not require any rotation or rectification to be properly
georeferenced, meaning:
lines 2 and 3 should be zero.
Line 4 is negative to convert from image row numbering (increasing from the
top down) to map coordinates (increasing from the bottom up). For a
rectified image, line 4 = -(line 1).
The ArcInfo commands REGISTER and RECTIFY, as well as GRIDIMAGE and
CONVERTIMAGE create a world file.
Naming convention - If the image file name has a 3-character extension
(image1.tif), the world file has the same name followed by a tfw extension
(image1.tfw).
Used the same procedure and it worked.
Litea Biukoto
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you need to register and rectify....
Michael Morin
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You might be able to calculate the values of the world
file, but the images will probably need to be
rubbersheeted to improve the accuracy of the images.
To further improve the accuracy, you could go one step
further and provide information to orthorectify the
image. Usually specialized imagery software is
required to this since it involves inputting info such
as the plane's tilt, and the camera's information.
THe ArcView 3.2 help provides a good overview of how a
world file is compiled. You would need to calculate
the value of the upper left pixel, and find out the
scale of the image. Then you co
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