Which instrument is required to measure longitude / latitude?

Which instrument is required to measure longitude / latitude?

Well, since you want to measure both latitude and longitude with “some handy instrument” , a G.P.S receiver would be your best bet. While you could do it with a sextant and a very good clock (just a sextant or theodolite alone is not enough for longitude) , the portable GPS receiver will be both less expensive and far more “handy”.