The problem of presenting a scalar field on the surface of the earth or sea can be solved by drawing the contour lines or isopleths of the field. If the values of the scalar are measured at points of a grid, it is necessary to interpolate between the points in constructing the contours. The resulting curves are therefore somewhat subjective; however, if the data are reasonably accurate and closely spaced, experienced workers can reduce this subjectiveness to a minimum.
Pritchard, Donald W.. 1948. "Streamlines from a discrete vector field: With application to ocean currents." Journal of Marine Research 7, (3). https://elischolar.library.yale.edu/journal_of_marine_research/668