GPS Logging On/Off

I'm kind of new to Arduino and I'm interested in using a GPS to log the location of where I spray weeds using a backpack sprayer. The basic idea is to use a flow sensor (on/off type) to turn on logging the location then when I stop spraying it stops logging. I am not sure if its in the code for the GPS or the Logger and what that would look like. I'm a GIS professional that sprays weeds I can use the data that's logged to create a map of exactly where I sprayed this would very useful in record keeping.

Any help would be awesome thanks

Ken