There are many different kinds of rasters that can be used in ArcMap. To better describe and explain these different kinds of rasters, Esri has created help documentation on technical specifications
and supported raster formats
. Included in the documentation is the ESRI GRID format. This is a very flexible format that many users are comfortable using to process and display data. There is a unique aspect to this format that many users are unaware of that is mentioned in the help documentation
A section in the help documentation on bit depth capacity
A grid dataset is always stored as 32 bit (either signed, unsigned, or floating point), but ArcGIS shows it above as being the most appropriate bit depth with regard to the cell values it contains.
So while the bit depth of the ESRI GRID may say 8 bit unsigned or 16 bit signed, the storage is still 32 bit. However, the properties in the source tab of the particular ESRI GRID raster may say something different. This is important when considering the behavior of the GRID compared to other formats like .imgs or .tifs.
This does not affect the average user of ESRI GRIDs, but just something to keep in mind and understand about the format. So fear not if you create a new ESRI GRID and the bit depth does not match your defined bit depth; it is still okay. Once the data is added,it should report the most appropriate bit depth. ESRI GRIDs are always 32 bit rasters regardless of what the source properties report.Jeff S. - Raster Support Analyst