Georeferencing tiff by creating tfw file
Ed McNierney
ed at TOPOZONE.COM
Thu Nov 17 04:40:43 PST 2005
Dejan -
Jacob is correct, but I wanted to remind you that "scale" is normally
reported as a ratio. The example you give is a scale of 1:25, not 25.
The ratio is unitless, but the resolution (as Jacob says) is not.
Digital images don't have a fundamental scale ratio - the ratio depends
on the *physical* size of the image, which can vary depending on how you
view or print it. The resolution, however, doesn't change, which is why
it's in the GeoTIFF tags and is a much more useful way of talking about
the image.
- 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: Gambin Dejan [mailto:Dejan.Gambin at pula.hr]
Sent: Thursday, November 17, 2005 2:39 AM
To: Ed McNierney
Cc: 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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