I am planning on conducting a Multi-Criteria-Analysis with a number of different source datasets. A number of the datasets are found in different coordinate systems and projections. Do I need to ***Project*** these datasets into the same coordinate system before conducting this analysis? My data currently covers more than one UTM Zone, usually they fall within one. Or should all of the data be in the same coordinate system, but unprojected?

Since there is such a large amount of data I would like it to be as efficient as possible.