ENVI uses the "no data" value in its calculations because the processing involved is kernel-based. To mitigate the effects of the "no data" value, you can try two approaches: 1) Build and apply a mask that will sets these values to something more reasonable. ENVI's masking tools can be found under the Basic Tools menu group. 2) Use the "Replace Bad Values" tool in the Topographic menu group to fill in these values with reasonable approximations based on surrounding "good" data. This is especially useful if you are working with data from missions like SRTM that tend to have voids that are arbitrarily filled with values of -32767.