shapefiles

Read and write ESRI shapefiles

This package includes functions to read and write ESRI shapefiles.

Usage

read.shapefile(shape.name)
read.shp(shp.name)
read.shx(shx.name) 
read.dbf(dbf.name, header=FALSE) 
write.shapefile(shapefile, out.name, arcgis=FALSE)
write.shp(shp, out.name)
write.shx(shx, out.name)
write.dbf(dbf, out.name, arcgis=FALSE)
calc.header(shapefile)
add.xy(shapefile)
scaleXY(shapefile, scale.factor)
convert.to.shapefile(shpTable, attTable, field, type)
convert.to.simple(shp)
change.id(shpTable, newFieldAsVector)
dp(points, tolerance)

Details

ESRI shapefiles consist of three files. The first file (*.shp) contains the geography of each shape. The second file (*.shx) is an index file which contains record offsets. The third file (*.dbf) contains feature attributes with one record per feature. read.shapefile calls read.shp, read.shx, and read.dbf to read in an entire shapefile. The result of read.shapefile is a list of many more lists. The sublists are shp, shx, and dbf. Each sublist contains a header list and some sort of data list. The shp list is a list of $shp$header and $shp$shp. The shx list is a list of $shx$header and $shx$index. The dbf list is a list of $dbf$header and $dbf$dbf. The write functions write out a shp, shx, and dbf file from the shapefile list structure. To write out a shapefile from simple R data, you need to run convert.to.shapefile. The inputs to this function are a simple data frame of points (for points, polyLines, or polygons) and a data frame representing the dbf file. Examples are below. The package reads shape types 1 (point), 3 (polyLine), 5 (polygon), 13 (polyLineZ), and 15 (polygonZ). Reading of shape type 13 and 15 from Don MacQueen, macq@llnl.gov The package writes shape types 1 (point), 3 (polyLine), 5 (polygon), 13 (polyLineZ), and 15 (polygonZ). Conversion of simple polygons to shapefile format from Manuel Chirouze, Manuel.Chirouze@benfieldgroup.com For simple features, the only difference between polyLines and polygons is that the first and last point is the same for a polygon. The convert.to.simple function can be used to simplify the shp file to a simple data.frame. The change.id function can then be used to change the Id field for the simple shp data.frame to a field from a data.frame (dbf). For details about the ESRI shapefile structure refer to http://www.esri.com/library/whitepapers/pdfs/shapefile.pdf. A detailed description of DBF files can be found at http://www.e-bachmann.dk/docs/xbase.htm. The arcgis argument to write.dbf replaces "." with "\_" in field names since ArcGIS does not allow the former. Note that the read.dbf and write.dbf functions in the foreign package are now used for reading and writing dbfs, which greatly improves the speed of reading/writing dbfs. 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.

Aliases

• read.shapefile
• read.shp
• read.shx
• read.dbf
• write.shapefile
• write.shp
• write.shx
• write.dbf
• calc.header
• add.xy
• scaleXY
• convert.to.shapefile
• convert.to.simple
• change.id
• dp

Examples

## Not run: 
# #Read entire shapefile
# shapefile <- read.shapefile("links")
# 
# #Write entire shapefile
# write.shapefile(shapefile, "temp", T)
# 
# #Read shp, shx, or dbf file
# dbf <- read.dbf("links.dbf")
# 
# #Write shp, shx, or dbf file
# write.dbf(dbf, "links.dbf", T)
# 
# #Calculate header (to clean up GeoMedia shapefile exports)
# shapefile <- calc.header(shapefile)
# 
# #Add the X and Y coordinates to the dbf list of the shapefile list object
# shapefile <- add.xy(shapefile)
# 
# #Scale the shapefile by scale.factor
# shapefile <- scaleXY(shapefile, scale.factor)
# 
# #Samples of using the convert.to.shapefile function to write out simple shapefiles
# #from basic R data.frames
# 
# #Point
# dd <- data.frame(Id=c(1,2),X=c(3,5),Y=c(9,6))
# ddTable <- data.frame(Id=c(1,2),Name=c("Item1","Item2"))
# ddShapefile <- convert.to.shapefile(dd, ddTable, "Id", 1)
# write.shapefile(ddShapefile, "c:/test", arcgis=T)
# 
# #PolyLine
# dd <- data.frame(Id=c(1,1,1,2,2,2),X=c(3,5,8,6,7,8),Y=c(9,8,3,6,7,4))
# ddTable <- data.frame(Id=c(1,2),Name=c("Item1","Item2"))
# ddShapefile <- convert.to.shapefile(dd, ddTable, "Id", 3)
# write.shapefile(ddShapefile, "c:/test", arcgis=T)
# 
# #Polygon
# dd <- data.frame(Id=c(1,1,1,1,2,2,2,2),X=c(3,5,8,3,6,7,8,6),Y=c(9,8,3,9,6,7,4,6))
# ddTable <- data.frame(Id=c(1,2),Name=c("Item1","Item2"))
# ddShapefile <- convert.to.shapefile(dd, ddTable, "Id", 5)
# write.shapefile(ddShapefile, "c:/test", arcgis=T)
# 
# #Convert to list of shapes
# ddAsList <- by(dd,dd$Id, function(x) x)
# 
# #Convert to data.frame
# dd <- do.call(rbind, ddAsList)
# 
# #Read in shp file and convert to simple format
# shpTest <- read.shp("c:/test.shp")
# simpleShpFormat <- convert.to.simple(shpTest)
# simpleShpFormat <- change.id(simpleShpFormat, c("a","b"))
# simpleAsList <- by(simpleShpFormat, simpleShpFormat[,1], function(x) x)
# backToShape <- convert.to.shapefile(simpleShpFormat, 
# 	data.frame(index=c("a","b")), "index", 5)
# write.shapefile(backToShape, "c:/test", arcgis=T)
# 
# #Polyline simplification with dp algorithm
# x <- c(5,3,4,1,8,9,10,11)
# y <- c(6,4,2,1,1,5,2,3)
# points <- list(x=x,y=y)
# plot(points, type="l")
# simpleLine <- dp(points, 2)
# lines(simpleLine, type="l", col="blue")
# 
# ## End(Not run)