the CRAN supplies a package, Shapfiles, to implement the reading and writing of shapefile. and meanwhile, this package could create simple shapefiles by converting from dataframes. According to the dp algorithm, R could simplify the polylines  with ease.

ps:Function dp is an implementation of the Douglas-Peucker polyLine simplification algorithm. Douglas, D. and Peucker, T. (1973). “Algorithms for the reduction of the number of points required to represent a digitized line or its caricature.” The Canadian Cartographer 10(2). 112-122. dp currently uses the line, not the line segment to determine the distance of the points from the line. This can result in the omission of extreme “outlier-like” points. See http://www.lgc.com/resources/Doug_Peucker.pdf for more information.

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