1. DiGiuseppe , J. L. A Survey of Pictorial Data Compression Techniques Technical Report No. 16, Contract N. DA-49-083 OSA-3050, University of Michigan, Ann Arbor, University of Michigan 1969
2. “Smoothing and Filtering of Time Series and Space Fields,”;Holloway;Advances in Geophysics,1958
3. “Generalization in Statistical Mapping,”;Jenks;Annals, Association of American Geographers,1963