I had the same kind of problem: when I build a histogram from the Spatial Analyst Toolbar, if I sum the frequency of the data which produce the histogram, the result doesn't match the total number of the cell of the raster. It happens for large rasters (in my case over 3 millions cells, it is a Digital Terrain Model of a watershed). If I use the Zonal Histogram tool (available at ArcToolBox > Spatial Analayst Tools > Zonal) the count matches perfectly. So this problem is only of the Spatial Analyst Toolbar Histogram tool. It seems that the computation depends on some kind of sampling related to the map scale and the screen resolution, as is possible to notice from the image below: [ATTACH=CONFIG]14102[/ATTACH] The histogram on the left is obtained at a 1:100,000 scale, the one on center at a 1:25,000 scale, the one on the right with the Zonal Histogram tool; of the course they are all obtained from the same dataset. This seem confirmed from what we can read from In the What's new in ArcGIS 10.1 document, at page 120: �??The interactive tools from the Spatial Analyst toolbar have been updated to operate on the data within the extent of your ArcMap window and at the screen resolution�?�. I think anyway ESRI should address this problem in ArcGIS 10 or, at least, declare this limit on help or documentation. I hope in the Service Pack 5.
