MAPC2MAPC Help pages :TFW

Overview How to .... Grids and Datums Map Images Uses System Issues

Most calibration files define a series of points matching pixels on the screen to coordinates. The TFW is different, being a series of numbers making a mathematical formula called an affine transformation. Conventionally, the six numbers are calld TF1,TF2,TF3,TF4,X0 and Y0 and the formulae for the transformation are:   X' = X0 + TF1*x + TF3*y and Y' = Y0 + TF2*x + TF4*y TFW files are created alongside TIF files when GDAL is used to process various raster formats such as ECW and SID. The problem is that the file does not describe the units : TFW files are used for Latitude/Longitude, UTM and other grids. Nor does it define the datum. If the X0 and Y0 are valid as longitude and latitude that is what they are assumed to be; otherwise, this screen asks for a choice of UTM zone or one of the Grids or ESG projections and to choose a datum. If the image file is a TIFF with text information, this is displayed. There is also the option to input the grid parameters for Transverse Mecator, Lambert Conformal Conic and Albers projections. The box in the centre of the window contains any text from with a TIF file - sometimes it is helpful, for example "UTMzone28|NAD27", sometimes there is nothing. It depends on who or what created the TIF file. A small area of the map is displayed in case the image contains useful information - use the scroll bars to move around. The default unit is metres, but you can also choose US Feet ( US survey feet = 0.3048006096 metres) or International feet(= 0.3048 metres) From version 513, the program offers both Spherical Mercator and Mercator. Most maps coming from the Web (Open Street Map and others) are Spherical Mercator; but Mercator (based on an ellipsoid rather than a sphere) may be used in some ECW and similar maps.