Hi @jthi! This is very nearly what I wanted, thank you so much! I just had to create a concatenate column with Wetland Code and Date (called Sampling Code) to pick up when the site or date changed but the other variable stayed the same and it worked!
Col Cumulative Sum( If( Row() == Col Min( Row(), :Date, :Latitude, :Longitude ), 1, 0 ), :Sampling Code )
Thanks so much again for the help