When the Land Change Modeler for ArcGIS (LCM) Extension must call the Harmonize function, you may get an error that says:
"Spatial characteristics cannot be harmonized when the reference system is plane or incompatible with IDRISI reference system. Define a proper reference system in metadata for the land cover images."
Land Change Modeler for ArcGIS uses the IDRISI file format internally. When inserting land cover maps in other formats, the LCM Extension calls ArcGIS to convert these non-IDRISI format files to the IDRISI file format. In doing so, it must document a reference system name in the IDRISI file format metadata. By default, this name is given the name of the file.
This error occurs in Harmonize because in the metadata, the reference system is automatically filled in with the image name rather than a reference system name and is not recognized as a valid projection.
To resolve this issue, go to the folder in which your data is stored and open the two .rdc files (raster documentation files associated with the raster images) for your earlier and later land cover images in some type of text editing program. Then simply change the reference system field such that it matches between the two .rdc files. What you choose to type there is not important (often, it is easy to just type "plane" for both) - what is important is that it matches between the two files.
Once you have saved these edited .rdc files, you should be able to use LCM for Arc to harmonize your data without further issues. Although, this assumes that the spatial characteristics are the same between the two land cover maps. If they are not, e.g., rows and columns, this must be resolved in ArcGIS.