# Read and write ESRI shapefiles This package includes functions to read and write ESRI shapefiles. ```r 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) ``` ## Arguments - `shape.name`: String of the shapefile file name without an extension - `shp.name`: String of the shp file name with an extension - `shx.name`: String of the shx file name with an extension - `dbf.name`: String of the dbf file name with an extension - `shapefile`: The shapefile object of lists created by read.shapefile - `out.name`: Filename to write the data to - `shp`: shp portion (list) of the shapefile object of lists - `shx`: shx portion (list) of the shapefile object of lists - `dbf`: dbf portion (list) of the shapefile object of lists - `scale.factor`: Number to divide the shapefile geography by - `arcgis`: Replace "." with "_" in column names for ArcGIS - `shpTable`: data.frame with columns in order Id, X, and Y - `attTable`: data.frame with first column names "Id" - polygon id (key) - `type`: ESRI Shape type 1=point, 3=polyLine, 5=polygon - `field`: A field name in the attTable - `newFieldAsVector`: A vector of Ids to replace to the Ids in the shpTable - `points`: A named list of two vectors (x and y) representing points - `tolerance`: A tolerance setting for the DP polyLine simplification algorithm - `header`: Should read.dbf return the header? ## 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 [https://www.esri.com/content/dam/esrisites/sitecore-archive/Files/Pdfs/library/whitepapers/pdfs/shapefile.pdf](https://www.esri.com/content/dam/esrisites/sitecore-archive/Files/Pdfs/library/whitepapers/pdfs/shapefile.pdf). A detailed description of DBF files can be found at [https://www.e-bachmann.dk/docs/xbase.htm](https://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 tools:::Rd_expr_doi("10.3138/FM57-6770-U75U-7727") for more information. ## Returns |||| |:--|:--|:--| |`read.shapefile`|list|shapefile list object| |`read.shp`|list|shp list object| |`read.shx`|list|shx list object| |`read.dbf`|list|DBF list object| |`write.shapefile`|NA|Nothing returned| |`write.shp`|NA|Nothing returned| |`write.shx`|NA|Nothing returned| |`write.dbf`|NA|Nothing returned| |`calc.header`|list|shapefile list object| |`add.xy`|list|shapefile list object| |`scaleXY`|list|shapefile list object| |`convert.to.shapefile`|list|shapefile list object| |`convert.to.simple`|list|data.frame list data.frame| |`change.id`|list|data.frame list data.frame| |`dp`|list|data.frame list data.frame| ## Examples ```r ## 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) ``` ## Author(s) Ben Stabler

shapefiles function