[Freegis-list] TFW format

[Bill Thoen]

Petr,

Since this list is based in Germany, somebody here should know the details,
but System 42/83 apppears to be a datum. From
http://www.killetsoft.de/transdpe.htm I read,  "The former German Democratic
Republic used the System 42/83 based on a Krassowskij ellipsoid and
projection into the Gauss-Krueger coordinate system with six degrees
meridian strips..." The projection sounds like it's essentially UTM but I
don't know enough of the details to tell you any more than that.

Then to calculate the pixel position from your TFW parameters:
200.00000000
0.00000000
0.00000000
-200.00000000
3408200.0000
5635500.0000

you subtract the last two parameters from your coordinates (x-3408200,
y-5635500) and divide the x result by 200 and the y result by -200, and
that should be the pixel you seek (note: the pixel at 0,0 is at the upper
left corner of the image.) I'm also assuming that the coordinates are in
meters. The coordinate listed in your TFW file is the coordinate at the 0,0
pixel.


On Fri, Oct 07, 2005 at 10:46:04AM +0200, Petr wrote:
> S42 coordinate system is described on the web, but i don't know how can i 
> use it in PROJ.4 or other program.
> In projections list isn't anything like S42 ;( Maybe it has other name then 
> S42.
> 
> If i convert my coordinate to S42 or get coordinate in S42 system from GPS 
> what else i need to compute pixel position?
> What units and begin have values in my TFW file on line 5 and 6? Meters?
> 
> Thank you
> 
> Peter
> 
> Bill Thoen wrote:
> >Petr wrote:
> >
> >>ok, i know that my map is in S-42 coordinate system. what else i need 
> >>to know to convert GPS coordinate to position in bitmap?
> >
> >
> >You first need to convert your coordinates from degrees, minutes, 
> >seconds to decimal degrees, and then run them through some sort of 
> >coordinate conversion software (Try GEO at http://opensourcegis.org/). 
> >To convert 50d04m55sN 14d26m03sE to decimal degrees, you use the 
> >following formula: d + m/60 + s/3600 where d is your degrees, m is your 
> >minutes and s is the seconds. Then if the longitude is west, make your 
> >longitude negative, and if your latitude is south, make your latitude 
> >negative. To convert the coordinate you list above to decimal degrees, 
> >it's longitude is 14 + 26/60 + 3/3600, or 14.4341667, and latitude is 50 
> >+ 4/60 + 55/3600, or 50.0819444. Then you just need to convert 
> >(14.4341667, 50.0819444) to S-42 coordinates (and I don't recognize that 
> >coordinate system, so I can't help you there) with some coordinate  
> >conversion tool.
> >
> >HTH,
> >- Bill Thoen
> >
> >>Thnak you
> >>
> >>Bill Thoen wrote:
> >>
> >>>On Thu, Oct 06, 2005 at 04:12:01PM +0200, Petr wrote:
> >>>
> >>>>i need help with tfw format. I have tiff image and tfw file with 
> >>>>description:
> >>>>200.00000000
> >>>>0.00000000
> >>>>0.00000000
> >>>>-200.00000000
> >>>>3408200.0000
> >>>>5635500.0000
> >>>>
> >>>>I searched internet and not found any information about using of 
> >>>>this map.
> >>>
> >>>
> >>>
> >>>
> >>>Googling on "tfw format" I found the following on
> >>>http://www.asmail.be/msg0055111818.html:
> >>>---
> >>>A sample TFW file opened up would look like:
> >>>
> >>>                +6.00
> >>>                -0.00
> >>>                -0.00
> >>>                -6.00
> >>>                1709053.00
> >>>                807714.00
> >>>
> >>>        Line 1 - Cell size in the "X" direction
> >>>        Line 2 - Insertion point in the "X" direction
> >>>        Line 3 - Insertion point in the "Y" direction
> >>>        Line 4 - Cell size in the "Y" direction
> >>>        Line 5 - Easting value of insertion point "X"
> >>>        Line 6 - Northing value of insertion point "Y"
> >>>
> >>>Note that sign on line 4 will tell you if the insertion point is the 
> >>>upper
> >>>left or lower left corner.  A positive means the "Y" values are 
> >>>increasing
> >>>upwards and therefore, the registration must be starting at the 
> >>>bottom or
> >>>lower corner. Vice-versa for a negative sign.
> >>>---
> >>>
> >>>Also, the GeoTIFF format spec can be found at:
> >>>http://www.remotesensing.org/geotiff/spec/geotiffhome.html
> >>>
> >>>
> >>>
> >>>>I need transform standard GPS coordinate (eg.: 50d04m55sN 
> >>>>14d26m03sE) to pixel coordinate in my map (eg.: 256px,414px 
> >>>>aproximately) and backwards.
> >>>
> >>>
> >>>
> >>>
> >>>Unfortunately, knowing the TFW format won't help you with converting 
> >>>your
> >>>GPS coordinate. The TFW file doesn't contain coordinate system 
> >>>information,
> >>>so you don't know the projection method, datum, etc.  But once you 
> >>>convert
> >>>it into the same system used in your TIFF file, the tfw parameters 
> >>>can be
> >>>used to locate it on your image accurately.  - Bill Thoen
> >>
> >>
> >>
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