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:[EMAIL PROTECTED] On Behalf Of Gambin Dejan > Sent: 17 November 2005 15:39 > To: [email protected] > 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:[EMAIL PROTECTED] On Behalf Of Ed McNierney > > Sent: Thursday, November 17, 2005 12:43 AM > > To: [email protected] > > 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 > > [EMAIL PROTECTED] > > > > -----Original Message----- > > From: Dejan Gambin [mailto:[EMAIL PROTECTED] > > Sent: Wednesday, November 16, 2005 6:07 PM > > To: Ed McNierney > > Cc: [email protected] > > 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:[EMAIL PROTECTED] > > > Sent: Wednesday, November 16, 2005 10:54 PM > > > To: Dejan Gambin; [email protected] > > > 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 > > > [EMAIL PROTECTED] > > > (978) 251-4242 > > > > > > -----Original Message----- > > > From: Dejan Gambin [mailto:[EMAIL PROTECTED] > > > 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:[EMAIL PROTECTED] > > > > On Behalf Of Ed McNierney > > > > Sent: Wednesday, November 16, 2005 4:51 PM > > > > To: [email protected] > > > > 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 > > > > [EMAIL PROTECTED] > > > > (978) 251-4242 > > > > > > > > -----Original Message----- > > > > From: UMN MapServer Users List > > > [mailto:[EMAIL PROTECTED] > > > > On Behalf Of Gambin Dejan > > > > Sent: Wednesday, November 16, 2005 8:59 AM > > > > To: [email protected] > > > > 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 > > > > > > > > > > > > >
