Search code examples
rgisshapefilergdal

Unable to subset data in a shapefile

I would like to carry out a subsetting in my shapefile without specifying the name of the first column in the .dbf file. To be more precise I would like to select all the rows with value 1 in the first column of the .dbf, but I don't want to specify the name of this column.

For example this script works because I specify the name of the column (as columnName)

library(rgdal) # readOGR

shapeIn <- readOGR(nomeFile)
shapeOut <- subset(shapeIn, columnName == 1)

instead it doesn't works

shapeOut <- (shapeIn[,1] == 1)

and I get an error message: comparison (1) is possible only for atomic and list types shapeOut and shapeIn are ESRI vector files.

This is the header of my shapeIn

   coordinates mask_1000_
1 (54000, 1218000)          0
2 (55000, 1218000)          0
3 (56000, 1218000)          0

Can you help me? Thank you

Solution

  • This

    shapeOut <- (shapeIn[,1] == 1)

    doesn't work beacuse SpatialPolygonsDataFrames contain other info other than the data. So "common" data.frame subsetting doesn't work in the same way. To have it work, you must make the "logical check" for subsetting on the @data slot: this should work (either using subset or "direct" indexing):

    shapeOut <- subset(shapeIn, shapeIn@data[,1] == 1)

    OR 

    shapeOut <- shapeIn[shapeIn@data[,1] == 1,]

    (however, by recent experience, referencing to data by column number is seldom a good idea... ;-) )

    ciao Giacomo !!!