Docs: restore Erdas Imagine .ige format documentation

doc/source/drivers/raster/erdas_imagine_ige.rst

@@ -0,0 +1,80 @@
+Erdas Imagine (.ige)  Large Raster Spill File
+=============================================
+
+Overview
+--------
+
+The Erdas Imagine ``.ige`` format (Large Raster Spill File) is used by Erdas
+Imagine for raster datasets that exceed the 4 GB file size limit.
+
+In such cases, the dataset is split into two files:
+
+- ``.img``: Contains the traditional Imagine metadata and dataset structure
+- ``.ige``: Stores the actual raster pixel data in a separate file
+
+This design works around 32-bit file offset limitations of the classic
+Imagine (HFA) format.
+
+GDAL Support
+------------
+
+GDAL provides **read support** for raster datasets using the ``.ige`` spill
+file mechanism.
+
+The ``.ige`` format itself is not publicly documented by Erdas. GDAL support
+is based on reverse-engineering and analysis of sample datasets.
+
+Driver Characteristics
+----------------------
+
+- **Format type**: Raster
+- **Data storage**: External tiled raster data
+- **Compression**: None (tiles are stored uncompressed)
+- **Tile indexing**: Implicit (no explicit tile index structure)
+
+The raster data is organized into tiles, typically 64×64 pixels in size,
+stored sequentially without an index.
+
+Internal Structure (Historical Notes)
+-------------------------------------
+
+The ``.img`` file references the external ``.ige`` file through an
+``ExternalRasterDMS`` structure instead of the usual ``RasterDMS``.
+
+The ``.ige`` file begins with a fixed ASCII magic header:
+
+::
+
+  ERDAS_IMG_EXTERNAL_RASTER\0
+
+Following the header, the file contains:
+
+- A layer stack prefix describing raster dimensions and tiling
+- A valid-flags section indicating which tiles are present
+- Raw, uncompressed raster tile data
+
+Tiles are stored in row-major order. The valid-flags section is a packed bit
+array where each bit corresponds to a tile’s availability.
+
+Limitations
+-----------
+
+- The ``.ige`` format is **not officially documented**
+- Write support is not guaranteed
+- No compression or overviews are stored in the ``.ige`` file
+- Tile indexing is implicit, which may impact random access performance
+
+Background
+----------
+
+This documentation is based on historical GDAL documentation originally
+authored by Frank Warmerdam and preserved via the Internet Archive.
+
+The content has been restored to improve visibility of legacy format support
+and to assist users encountering existing Erdas Imagine datasets.
+
+Acknowledgements
+----------------
+
+Thanks to Mark Audin (Keyhole / EarthViewer) for providing sample datasets and
+to Erdas for enabling analysis using Imagine software.

doc/source/drivers/raster/index.rst

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    ehdr
    eir
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+   erdas_imagine_ige
    esat
    esric
    ers