Thank you for your answer.. They structure of my project is as follows: Project - Wind farm - - Map - - WTG - - Turbine group (Mast1) - - - - - - Wind atlas - - - - Met station - - - - - Observed wind climate (mast1) - - - - - Map 2010 - - Turbine group (mast2) - - - - - - Wind atlas - - - - Met station - - - - - Observed wind climate (mast2) - - - - - Map 2010 etc... In my project I have 9 masts, and therefore 9 mast groups.. My point is, that for every mast i have the 2010 map added.. This allows me to change the map in the parent Wind Farm, to let simulate the flow seen by the turbines with other future maps (other forestry) This results in that I have 10 maps in the file.. (1 for every mast and 1 in the parent wind farm).. My suggestion is, as all mast uses the same 2010 map, to make a function that will allow a mast-map, so that all nine masts uses the same map, so that it is only necessary to save 2 maps to the file.. Regarding AEP calculations, the AEP for the turbines in "Turbine Group (mast1)" will be calculated from mast1 and so on.. The program works as it's is supposed to, the only thing I'm talking about is, that when WAsP has to save 10 maps to a file it takes quite a while and the .wwh will be quite big.. So the idea is, that if you only need to save 2 maps to the .wwh, the save time could be reduced dramatically.. Yours Mark Herskind