Converting table UTM coordinates to decimal lat-long in Attribute table using expression

Question

I would like to formulate an equation in the attribute table calculator tool to calculate the decimal latitude and longitude values from the existing UTM NAD3 coordinates in the table.

The UTM NAD83 coordinate fields are "UTM_X" and "UTM_Y", and the floating-point fields that I want to receive the calculation results are floating-point fields "Lat_DD" and "Long_DD". The "UTM_X" and "UTM_Y" fields have already been populated with the expression $x and $y respectively. My project CRS is UTM NAD83 zone 16.

I have an intuition that this is probably a routine calculation that is processed by a standard function but I'm new enough to QGIS that my browsing through the "Geometry" and "Function" calculator categories has not yielded the correct combo.

Answer 1

You can this for "Long_DD":

x(transform(make_point("UTM_X","UTM_Y"),'EPSG:26916','EPSG:4326'))

and this for "Lat_DD":

y(transform(make_point("UTM_X","UTM_Y"),'EPSG:26916','EPSG:4326'))

Replace EPSG:26916 with your actual source-UTM-EPSG-Code in case I picked the wrong one.