Hi, I'm using Idrisi Selva to do an unsupervised (ISOCLUST) classification of Landsat TM images. My study area is at the juncture of two Landsat scenes and it spans two UTM zones. I radiometrically corrected each band of each image in R, then mosaicked each band in ArcGIS v10, then clipped the mosaicked bands to the boundary of my study area. I exported the data to ASCII, then imported three bands (R, NIR, MIR) to Idrisi Selva. Using ISOCLUST, I specified that I wanted 10 classes with a minimum class size of 30 pixels. The problem is that every few minutes, I would get an error that said "Variance/covariance matrix is singular. Check for perfectly correlated variables or variables with zero variance." This message was immediately followed by: "Inverse matrix process failed." I found that if I sat there and clicked through the errors when they popped up every few minutes, the classification worked (at least from a quick visual assessment). My questions are: 1- Why is this error occurring? 2- How is the variance/covariance matrix generated? 3- Did the unsupervised classification work in spite of the error messages? Any help would be really appreciated.