I am planning to plot all the observations made by a satellite in two respective grid boxes. One grid located at the equator and one grid located at the mid latitudes... Each grid box will be a 10° x 10° box. However, due to the shape of the earth the area of each box will indeed not be the same size. Is there some formula to convert latitudinal and longitundal degrees dependent on location of earth? To some useful unit (km). Thanks!