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I have a .ecw aerial image and one of the associated files is .prj, which i assume defines the Coordinate Reference System of the image. The .prj is as follows:

PROJCS["PNG94_PNGMG94_zone_55",GEOGCS["GCS_PAPUA_NEW_GUINEA_GEODETIC_DATUM_1994",DATUM["D_PAPUA_NEW_GUINEA_GEODETIC_DATUM_1994",SPHEROID["GRS_1980",6378137,298.257222101]],PRIMEM["Greenwich",0],UNIT["Degree",0.017453292519943295]],PROJECTION["Transverse_Mercator"],PARAMETER["latitude_of_origin",0],PARAMETER["central_meridian",147],PARAMETER["scale_factor",0.9996],PARAMETER["false_easting",500000],PARAMETER["false_northing",10000000],UNIT["Meter",1]]

Which both here and here is described as EPSG:5551. Based on this I chose EPSG 5551 as the CRS of my QGIS project and made sure in settings that "When a new layer is created, or when a layer is loaded that has no CRS" QGIS "Use project CRS".

However, when i import my .ecw file, the project CRS is changed to EPSG 7855 (GDA2020 / MGA zone 55) and when i do right click on the layer and go to properties, i see that the layer's CRS is EPSG 7855. In other cases, another image's CRS changed to EPSG 28355.

I know that EPSG 5551, 7855 and 28355 are related, and all cover Zone 55 between Australia and Papua New Guinea. I want to understand why the CRS of my project is changing? Why the layer appears to have another CRS when i check its properties in QGIS? and if this may have a consequence in my project?

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    Do you have the ECW header editor? I have experienced similar issues when the CRS in the ECW header is defined, perhaps you could change the header to unknown/unknown/metres/downward. The ECW header editor was available as a freebie from Hexagon Geospatial. It's possible the .prj file is being ignored; I've not had an issue with .aux files generated with Esri. Although 7855 is related to 58355 the NTv2 transformation does not extend to much outside mainland Australia and Tasmania, there is no transformation for the whole of New Guinea. Jun 17 '19 at 6:06
  • Thanks a lot for your reply. I do not have a .ecw header editor and had not heard from them. I did a fast search on the web and saw that you may be able to do this in global mapper, but this is not a free tool. Are you aware of a free ECW header editor?
    – Juan Ossa
    Jun 19 '19 at 3:49
  • download.hexagongeospatial.com/en/downloads/apollo/… register to download. Contains the ECW Header Editor, OTDF builder and ECW Opacity Bulder. Another handy tool is the ER Viewer which is a lightweight georaster viewer. Jun 19 '19 at 4:34
  • Thanks a lot! This seems to work. Feel free to include this as an answer and will mark it as answered.
    – Juan Ossa
    Jun 21 '19 at 7:42
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I have experienced similar issues when the CRS in the ECW header is defined as different to the .prj CRS, perhaps you could change the header to unknown/unknown/metres/downward which should force the implementation of the .prj CRS:

enter image description here

The ECW header editor was available as a freebie from Hexagon Geospatial. It's possible the .prj file is being ignored; I've not had an issue with .aux files generated with Esri.

Although 7855 is related to 28355 the NTv2 transformation does not extend to much outside mainland Australia and Tasmania, there is no transformation for the whole of New Guinea:

Extent of GDA2020 NTv2 transformation

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