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.

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.

Although I am utilizing Esri tools for this problem my question is not specific to those. I am working with a dataset which covers 650,000 square kilometres of the NWT and NU in Canada. I've decided to use spatialreference.org/ref/epsg/nad83-nwt-lambert for all of the data. I was wondering if it would be better to leave the data unprojected or pick a projection for all of the data to be in.