Thursday, October 13, 2016

Some differences when opening georeferenced imagery between QGIS and ArcGIS

Just did some minor testing after trying the Georeferencing toolbar in ArcGIS to create some world files (.jpgw, .pngw, .ecww, .tfw, etc.) and wanted to see how these two GIS suites treat the result. Normally I use MapWindow GIS to georeference aerial photos but it's very slow and has some strange bugs that might deserve another blog post.

The aerial photos were received from the provincial agency as MrSID files and were converted to ecw using IrfanView. Neither the original SID files or the ECW files had embedded projection information. In ArcGIS, I used the georeferencing toolbar and added five control points then chose Projective Transformation rather than the default Affine 1st Order Polynomial method to transform the raster. Upon clicking Update Georeferencing (Save the current warp to the dataset) from the toolbar, two files are created with the same name prefix as the .ecw file but with different extensions. They are photoname.ecw.aux.xml, and photoname.ewwx. The .ewwx file contains the image-to-world file transformation parameters using 1st Order Polynomial. The original ecw image file was untouched by this process.

I also made a copy of the photoname.ewwx file and renamed the extension to ecww so there was a file named photoname.ecww that QGIS and MapWindow can use.

It seems that although QGIS can read the ecw.aux.xml for the projection information (so it doesn't need a .prj file), it can not read this auxiliary file's contents for the higher order polynomial transformation. Thus there is a difference in the accuracy of the georeferenced raster when loaded, between ArcGIS and QGIS. In ArcGIS, if a more complex transform higher than 1st order polynomial was used to create the aux.xml and ewwx files, then it will show the raster with the higher order transformation method applied. If one decides to load the raster into QGIS, then QGIS will only use the world file (.ecww) and perform affine transformation on it.


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