Using a ecw file as a base map in Arcmap 10.2 desktop?

[DavidCummings]

HI I am new to GIS and still learning.  How can I use an ortho rectified ecw file as my base map.  Is it as simple as opening it in ArcMap, or not.   On attempting this I do not seem to be able to set the coordinate system and I get a warning that the data does not have a spatial reference information?

Thanks

David

[DanPatterson_Retired]

do you know what the coordinate system is?

​Are there any other files associated with the file? (use windows explorer to check for files with the same filename but a different extension)

[DavidCummings]

All files are in GDA94 Datum

I was also supplied

Mapinfo tab file with MGA coordinates ((GDA 94 Datum))

ER Viewer file with MGA coordinates (GDA 94 Datum)

[DanPatterson_Retired]

​If the coordinate information is not embedded in the file, did you try the Define Projection tool in Arctoolbox (Projections and Transformations) not sure it works with ecw files

[DavidCummings]

Ok I used the define projection tool to define it as WGS 1984 (which is what the rest of my features are in) and it seems to be working and matching both bing base map and the imagery I manually georectified.  So I think that simple solution has worked.  Thankyou.