Intergraph Raster (INGR) is a raster graphics format associated with geospatial products from Intergraph.
Some sources mention a bi-level format named Intergraph File Format, Intergraph, IntergraphRLE, or ITG. It's not clear whether this refers to a particular type of INGR file, or to an entirely different file format.
One source gives the following "partial listing" of INGR file extensions: .cot, .ctc, .rgb, .ctb, .grd, .crl, .tpe, .lsr, .rle, .cit, .g3, .g4, .tg4, .cmp, .jpg.
XnView also gives .itg.
LEADTOOLS also gives .t27, .t29.
PRONOM gives .ing.
The documentation implies that all INGR files begin with
- Intergraph Raster File Format (from archive.org)
- Intergraph Raster File Format Reference (formerly at ftp://ftp.intergraph.com/pub/bbs/scan/note/rffrgps.zip)