@article{Tobler1979,
author = {Tobler, Waldo R.},
title= {Smooth pycnophylactic interpolation for geographical regions},
year = {1979},
URL = {10.1080/01621459.1979.10481647},
journal = {Journal of the American Statistical Association},
volume = {74},
number = {367},
pages = {519--530},
}