tfw v.s. tfwx

Question

I've read Esri documentation about tfwx and still don't get it.  When would an approximate Affine transformation be generated, or why is it an "approximate"?

I did this experiment:

(1) Load a tif image (a scanned map), load the control point file, run Rectify.  A new tif created with a tfw file.  There is no transformation stored in the asscciated aux.xml file.
(2) Load the same tif image, load the same control point file, run Update Georeferencing.  tfwx is created and there is transformation stored in the associated aux.xml file.

I expected the numbers in the tfw file and the tfwx file be the same, since they are generated from the same scanned image using the same control point file, but the numbers in the two files are slightly different.  Where is the discrepancy from?  Also, does it imply that Rectify creates an exact Affine transformation, and Update Georeferencing creates an approximate one?

Many thanks.

Wei-Hsin Fu

JohnSobetzer · Answer

You might look at this or similar explanations in other Help versions.  http://webhelp.esri.com/arcgisdesktop/9.2/index.cfm?TopicName=Georeferencing_a_raster_dataset

Wei-Hsin · Answer

I cannot vouch for this answer, but here's a link to one explanation: http://walt.therices.org/index.php/2009/09/georeferencing-dpi-quirks/

You might check out ESRI's knowledge based article 32868 if you haven't already done that.  Second and higher order transformations typically can't be expressed in world files.

One other thought.  Update georeferencing stores all the transformation information in external files while rectification permanently transforms the raster, hence it's external files won't carry the same transformation information.

Hi johns,

Thank you very much for helping me with this question.  Could you explain the last part, "Update georeferencing stores all the transformation information in external files while rectification permanently transforms the raster, hence it's external files won't carry the same transformation information", in more details?  Thanks again.

Wei-Hsin

TomasSchwandner_1-1713157635012.png

JohnSobetzer · Answer

I cannot vouch for this answer, but here's a link to one explanation: http://walt.therices.org/index.php/2009/09/georeferencing-dpi-quirks/

You might check out ESRI's knowledge based article 32868 if you haven't already done that.  Second and higher order transformations typically can't be expressed in world files.

One other thought.  Update georeferencing stores all the transformation information in external files while rectification permanently transforms the raster, hence it's external files won't carry the same transformation information.