You will not be able to successfully use Band Math to return the maximum value from a layer stack of NDVI images. This is due to how Band Math retrieves data from an image and calculates values (in a block rather than pixel by pixel). There is a programmatic alternative, though. Below is an example program that should do what you want, but you will have to place it into a .pro file in IDL and then place that file in the SAVE_ADD directory of your ENVI installation. It requires that you have ENVI+IDL, not just ENVI. ==================== ;+ ;create_composite.pro ; ;This program is designed to create a new "composite" image based on multiple band input. ;The entire spectrum for each pixel is examined to determine the new value to place in ;the corresponding pixel of the composite image. The examination methods are minimum value, ;maximum value, and mean value. ; ;This program is not officially supported by ITT-VIS Technical Support and is not guaranteed to work with versions of ENVI ;beyond 4.3. ; ;Author: Devin Alan White, ITT-VIS Technical Support ;Email: dwhite@ittvis.com ;Date: 09/22/2006 ;- pro create_composite_define_buttons, buttonInfo compile_opt idl2 ;Create button so that the program can be called from within ENVI. ;Button will appear under File->Basic Tools->Create Composite. envi_define_menu_button, buttonInfo, value = 'Create Composite', $ event_pro='create_composite', POSITION='last', ref_value = 'Basic Tools', $ uvalue='Create Composite' end pro composite_functions, fid, pos, max_val=max_val, min_val=min_val, mean_val=mean_val compile_opt idl2 envi_file_query, fid, ns=ns, nl=nl, nb=nb, data_type=dt, dims=dims inherit = envi_set_inheritance(fid, dims, /spatial) ;Create output file output_file = dialog_pickfile(title='Select Output File Name and Location') if output_file eq '' then return openw, lun, output_file, /get_lun if keyword_set(max_val) then begin envi_report_init, title='Maximum Value Composite Image Creation', $ ['Accessing data from original file...','Creating Composite Image...'], $ base=base, /interrupt envi_report_inc, base, nl-1 ;Set up for loop to grab data slices in BIP interleave for i=0, nl-1 do begin envi_report_stat, base, i, nl-1, cancel=cancel if cancel eq 1 then begin envi_report_init, base=base, /finish free_lun, lun file_delete, output_file return endif slice = envi_get_slice(fid=fid, line=i, pos=pos, $ xs=0, xe=ns-1, /bip) ;Set up for loop to grab maximum value for each pixel and place ;it in a new file at the same spatial location for j=0, ns-1 do begin max_value = max(slice[*,j]) writeu, lun, max_value endfor endfor envi_report_init, base=base, /finish envi_setup_head, inherit=inherit, nb=1, data_type=dt, fname=output_file, $ interleave=0, ns=ns, nl=nl, offset=0, /write endif if keyword_set(min_val) then begin envi_report_init, title='Minimum Value Composite Image Creation', $ ['Accessing data from original file...','Creating Composite Image...'], $ base=base, /interrupt envi_report_inc, base, nl-1 ;Set up for loop to grab data slices in BIP interleave for i=0, nl-1 do begin envi_report_stat, base, i, nl-1, cancel=cancel if cancel eq 1 then begin envi_report_init, base=base, /finish free_lun, lun file_delete, output_file return endif slice = envi_get_slice(fid=fid, line=i, pos=pos, $ xs=0, xe=ns-1, /bip) ;Set up for loop to grab minimum value for each pixel and place ;it in a new file at the same spatial location for j=0, ns-1 do begin min_value = min(slice[*,j]) writeu, lun, min_value endfor endfor envi_report_init, base=base, /finish envi_setup_head, inherit=inherit, nb=1, data_type=dt, fname=output_file, $ interleave=0, ns=ns, nl=nl, offset=0, /write endif if keyword_set(mean_val) then begin envi_report_init, title='Mean Value Composite Image Creation', $ ['Accessing data from original file...','Creating Composite Image...'], $ base=base, /interrupt envi_report_inc, base, nl-1 ;Set up for loop to grab data slices in BIP interleave for i=0, nl-1 do begin envi_report_stat, base, i, nl-1, cancel=cancel if cancel eq 1 then begin envi_report_init, base=base, /finish free_lun, lun file_delete, output_file return endif slice = envi_get_slice(fid=fid, line=i, pos=pos, $ xs=0, xe=ns-1, /bip) slice = float(slice) ;Set up for loop to grab mean value for each pixel and place ;it in a new file at the same spatial location for j=0, ns-1 do begin mean_value = mean(slice[*,j]) writeu, lun, mean_value endfor endfor envi_report_init, base=base, /finish envi_setup_head, inherit=inherit, nb=1, data_type=4, fname=output_file, $ interleave=0, ns=ns, nl=nl, offset=0, /write endif free_lun, lun envi_open_file, output_file end pro create_composite, event compile_opt idl2 ;Prompt user to select input file envi_select, fid=input_fid, /file_only, pos=pos, $ title='Select Input File for Composite Generation', $ /no_dims if (input_fid[0] eq -1) then return ;Create widget to facilitate method selection base = widget_auto_base(title='Type?') list = ['Minimum Value', 'Maximum Value', 'Mean Value'] choice = widget_menu(base, list=list, /excl, /auto, uvalue='choice', rows=3) result = auto_wid_mng(base) if (result.accept eq 0) then return ;Call composite program based on user choice if result.choice eq 0 then begin composite_functions, input_fid, pos, /min_val endif if result.choice eq 1 then begin composite_functions, input_fid, pos, /max_val endif if result.choice eq 2 then begin composite_functions, input_fid, pos, /mean_val endif end