[Freegis-list] TFW format

[Bill Thoen]

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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