Georeferencing tiff by creating tfw file
Delfos, Jacob
jacob.delfos at MAUNSELL.COM
Thu Nov 17 03:03:26 EST 2005
Dejan,
I think you are mixing up scale and resolution: scale is ratio between
the real world and whatever you are looking at (hardcopy, screen, etc).
So 1:25 means 1 unit on the screen equals 25 units in the real world
(whichever units, as you said).
Resolution expresses how many pixels go into a fixed length (expressed
in a unit). It is necessary to have a unit, because obviously the result
between 300 pixels in a metre, or 300 pixels in an inch, will be
different. So "metres/pixel" is a resolution, NOT a scale. Metres/pixel
tells you have many metres a pixel is, either on your screen or on a
hardcopy. This does NOT tell you how many metres it is in the real world
(geographic space). You need scale AND resolution to determine that.
Hope I didn't confuse you further.
Jacob
> -----Original Message-----
> From: UMN MapServer Users List
> [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On Behalf Of Gambin Dejan
> Sent: 17 November 2005 15:39
> To: MAPSERVER-USERS at LISTS.UMN.EDU
> Subject: Re: [UMN_MAPSERVER-USERS] Georeferencing tiff by
> creating tfw file
>
> Thanks Ed, thanks Jacob,
>
> That is the thing I have missed - the scale reported is in
> "meters/pixel" and this gives the correct result....but correct me
> please - isn't the scale "unitsless" parameter? 1inch on map=25inch in
> world, 1m on map=25m in world? I was looking in gdalinfo
> output and the
> TIFFTAG_RESOLUTIONUNIT that is set to pixel/inch, so I did 25/300
> thinking it is the pixel size in inches that needs to be converted to
> meters...obviously wrong
>
> regards, dejan
>
> > -----Original Message-----
> > From: UMN MapServer Users List
> > [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] On Behalf Of Ed McNierney
> > Sent: Thursday, November 17, 2005 12:43 AM
> > To: MAPSERVER-USERS at LISTS.UMN.EDU
> > Subject: Re: [UMN_MAPSERVER-USERS] Georeferencing tiff by
> > creating tfw file
> >
> >
> > Dejan -
> >
> > "..if the map is in 1:25 scale, that means that 1 inch on the
> > map is 25 inches in the real world....if the map was scanned
> > on 300dpi then the pixel size is 25/300 in inches, then
> > converted to meters"
> >
> > Yes, that's all there is to it. There's nothing more to
> > understand. If the scale you're reporting in CAD Overlay is
> > in units of "meters per pixel", then one pixel is
> > 0.08467878514170 meters. If that image were scanned at 600
> > DPI, then each inch on the printed map source would produce
> > 600 pixels or 50.80 meters. 50.80 meters is 2,000 feet, so
> > that would correspond to a scale of 1" = 2,000' or 1:24,000
> > scale. Or it could be a 1:12,000 scale map scanned at 300
> > DPI - or a 1:48,000 scale map scanned at 1,200 DPI. They
> > would all come out the same - the image resolution in units
> > per pixel is a product of the source scale and the scanning
> > resolution, and you can't distinguish the two.
> >
> > - Ed
> >
> > Ed McNierney
> > President and Chief Mapmaker
> > TopoZone.com / Maps a la carte, Inc.
> > 73 Princeton Street, Suite 305
> > North Chelmsford, MA 01863
> > Phone: +1 (978) 251-4242
> > Fax: +1 (978) 251-1396
> > ed at topozone.com
> >
> > -----Original Message-----
> > From: Dejan Gambin [mailto:dejan.gambin at pula.hr]
> > Sent: Wednesday, November 16, 2005 6:07 PM
> > To: Ed McNierney
> > Cc: MAPSERVER-USERS at LISTS.UMN.EDU
> > Subject: RE: [UMN_MAPSERVER-USERS] Georeferencing tiff by
> > creating tfw file
> >
> > Sorry I forgot to set the CC..
> >
> > I have received this tif file from a colleague who wanted to
> > "georeference" it and place it on the existing mapserver map.
> > He just sent me the upper left coordinates (real world
> > coordinates in meters) and this scale. Suppose he scanned the
> > paper map that is in 1:25 scale and gave me the coordinates
> > of the upper left corner. So I thought:
> >
> > ..if the map is in 1:25 scale, that means that 1 inch on the
> > map is 25 inches in the real world....if the map was scanned
> > on 300dpi then the pixel size is 25/300 in inches, then
> > converted to meters. But it is not and obviously I am pretty
> > newbie in this geo-math and I am missing some important things.
> >
> > Anyway, after a while he sent me a tfw file saying he has
> > built it with CAD Overlay...that is all I know and this tiff
> > is now positioned well on the map...
> >
> > ...Sorry, I am just curious in how this is
> > theoretically/mathematically calculated so I asked....
> >
> > This is also interesting to me because we have some
> > non-referenced tiff maps with some known world coordinates on
> > them and I would like to georeference them in the future, if
> > this is possible...
> >
> > regards, dejan
> >
> > > -----Original Message-----
> > > From: Ed McNierney [mailto:ed at topozone.com]
> > > Sent: Wednesday, November 16, 2005 10:54 PM
> > > To: Dejan Gambin; MAPSERVER-USERS at LISTS.UMN.EDU
> > > Subject: RE: [UMN_MAPSERVER-USERS] Georeferencing tiff by
> > creating tfw
> >
> > > file
> > >
> > >
> > > Dejan -
> > >
> > > Well, that's not the GeoTIFF output you posted the first time <g>!
> > >
> > > What is the "scale" you have (the value that's 25.40363554)?
> > > What is the definition of that value, and its units (if any)?
> > > And what are the definition and the units of the "CAD Overlay"
> > > output, too? It's hard to work with dimensionless,
> > undefined numbers
> > > - where did they come from? We need some more details to
> be able to
> > > help. Thanks!
> > >
> > > - Ed
> > >
> > > P.S. Please always reply to the whole list, not just to the first
> > > responder.
> > >
> > > Ed McNierney
> > > President and Chief Mapmaker
> > > TopoZone.com / Maps a la carte, Inc.
> > > 73 Princeton Street, Suite 305
> > > North Chelmsford, MA 01863
> > > ed at topozone.com
> > > (978) 251-4242
> > >
> > > -----Original Message-----
> > > From: Dejan Gambin [mailto:dejan.gambin at pula.hr]
> > > Sent: Wednesday, November 16, 2005 4:31 PM
> > > To: Ed McNierney
> > > Subject: RE: [UMN_MAPSERVER-USERS] Georeferencing tiff by
> > creating tfw
> >
> > > file
> > >
> > > Ed,
> > >
> > > Sorry, I have run gdalinfo on the tiff with already
> > existing tfw file
> > > that I have calculated and that is WRONG...
> > >
> > > Running gdalinfo on this tif (that has no associated tfw)
> > results in:
> > >
> > > Driver: GTiff/GeoTIFF
> > > Size is 8857, 5905
> > > Coordinate System is `'
> > > Metadata:
> > > TIFFTAG_XRESOLUTION=300
> > > TIFFTAG_YRESOLUTION=300
> > > TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch) Corner Coordinates:
> > > Upper Left ( 0.0, 0.0)
> > > Lower Left ( 0.0, 5905.0)
> > > Upper Right ( 8857.0, 0.0)
> > > Lower Right ( 8857.0, 5905.0)
> > > Center ( 4428.5, 2952.5)
> > > Band 1 Block=8857x5905 Type=Byte, ColorInterp=Palette
> > > Color Table (RGB with 2 entries)
> > > 0: 255,255,255,255
> > > 1: 0,0,0,255
> > >
> > > So, I have this output, I have xy coordinates (in meters):
> > > 5455750 and 5021000 and I have the scale of 25.40363554.
> > The question
> > > remains - how do I calculate the "pixel size" (first and fourth
> > > parameter) of tfw file that I need to create.
> > >
> > > Using CAD Overlay I get the value of 0.08467878514170 that
> > is correct
> > > but I need to know how to get this value by formula, not
> with util
> > > like CAD Overlay
> > >
> > > Sorra again
> > >
> > > regards, dejan
> > >
> > > > -----Original Message-----
> > > > From: UMN MapServer Users List
> > > [mailto:MAPSERVER-USERS at LISTS.UMN.EDU]
> > > > On Behalf Of Ed McNierney
> > > > Sent: Wednesday, November 16, 2005 4:51 PM
> > > > To: MAPSERVER-USERS at LISTS.UMN.EDU
> > > > Subject: Re: [UMN_MAPSERVER-USERS] Georeferencing tiff by
> > > creating tfw
> > >
> > > > file
> > > >
> > > >
> > > > Dejan -
> > > >
> > > > The gdalinfo utility is telling you the pixel size. The
> > fourth line
> >
> > > > of the output below, beginning with "Origin = ", ends with
> > > "Pixel Size
> > >
> > > > = ". Those are the very same values you would use in
> the first and
> > > > fourth lines, respectively, of a TFW file. It looks like
> > > this is the
> > > > same value you calculated, and it appears to be correct.
> > > >
> > > > - Ed
> > > >
> > > > Ed McNierney
> > > > President and Chief Mapmaker
> > > > TopoZone.com / Maps a la carte, Inc.
> > > > 73 Princeton Street, Suite 305
> > > > North Chelmsford, MA 01863
> > > > ed at topozone.com
> > > > (978) 251-4242
> > > >
> > > > -----Original Message-----
> > > > From: UMN MapServer Users List
> > > [mailto:MAPSERVER-USERS at LISTS.UMN.EDU]
> > > > On Behalf Of Gambin Dejan
> > > > Sent: Wednesday, November 16, 2005 8:59 AM
> > > > To: MAPSERVER-USERS at LISTS.UMN.EDU
> > > > Subject: [UMN_MAPSERVER-USERS] Georeferencing tiff by
> creating tfw
> > > > file
> > > >
> > > > Hi,
> > > >
> > > > Even there are probably some utils I can use, I would
> > still like to
> > > > know how can I calculate the "pixel size" parameter in
> > tfw file (so
> > > > the first and fourth parameter) from the existing tiff
> > file and the
> > > > following
> > > > informations:
> > > >
> > > > For example, "gdalinfo my.tif" says
> > > >
> > > > Driver: GTiff/GeoTIFF
> > > > Size is 8857, 5905
> > > > Coordinate System is `'
> > > > Origin = (5455749.998925,5021000.001075) Pixel Size =
> > > > (0.00215084,-0.00215084)
> > > > Metadata:
> > > > TIFFTAG_XRESOLUTION=300
> > > > TIFFTAG_YRESOLUTION=300
> > > > TIFFTAG_RESOLUTIONUNIT=2 (pixels/inch) Corner
> > Coordinates: Upper
> > > > Left ( 5455749.999, 5021000.001) Lower Left ( 5455749.999,
> > > > 5020987.300) Upper Right ( 5455769.049,
> > > > 5021000.001) Lower Right ( 5455769.049, 5020987.300)
> > > > Center ( 5455759.524, 5020993.651)
> > > > Band 1 Block=8857x5905 Type=Byte, ColorInterp=Palette
> > > > Color Table (RGB with 2 entries)
> > > > 0: 255,255,255,255
> > > > 1: 0,0,0,255
> > > >
> > > > I have xy coordinates (in meters): 5455750 and 5021000
> > and the scale
> >
> > > > of 25.40363554. I have tried using the simple formula like:
> > > >
> > > > pixel size in inches = scale/resolution, so 25.40363554/300 and
> > > > converting it to meters gives me 0.002150841. But this is
> > > not correct,
> > >
> > > > right?
> > > >
> > > > Wiht CAD overlay I have got 0.08467878514170, that is probably
> > > > right...
> > > >
> > > > What am I doing wrong? Is it the resolution that I am
> using? Do I
> > > > have to use screen resolution or what?
> > > >
> > > > thanks
> > > >
> > > > regards, dejan
> > > >
> > >
> > >
> >
>
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