[GIS] Breaking out single polygon (feature) of SpatialPolygonDataframe

Question

I am trying to break out a single feature of a Spatialpolygon object.
That is e.g. plot a single polygon from a shapefile that contains multiple.
Any suggestions how to do this?

>#Import shapefile, generate spatialpolygondataframe
>vectormap<-readOGR(dsn="QGIS export", layer="section4") 
>
>OGR data source with driver: ESRI Shapefile 
>Source: "QGIS export", layer: "section4"
>with 4 features
>It has 6 fields
>
>
># Change vectormap CRS to Equirectangular Carree projection
>vectormap2 <- spTransform(vectormap, CRS("+proj=eqc +lat_ts=0 +lat_0=0 +lon_0=0 >+x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs"))

> #Summary of vectormap2
>Object of class SpatialPolygonsDataFrame
>Coordinates:
>       min      max
>x  1467755  5288689
>y -6018287 -3018986
>Is projected: TRUE 
>proj4string :........
>Data attributes:
>  Layer                       SubClasses ExtendedEn Linetype EntityHand Text  
> Layer 1:4   AcDbEntity:AcDb2dPolyline:3    NA's:4     NA's:4   41:1    NA's:4   
>             AcDbEntity:AcDbSpline    :1                        5B:1               
>                                                                7B:1               
>                                                                94:1               
> coordinates(vectormap2)
>     [,1]     [,2]
>0 4122892 -3791580
>1 4661889 -4500891
>2 4500413 -4079798
>3 3352295 -4392646

Happy for any input!

Side Note.
I can also get the shapefile in a format giving the features in separate layers.

> summary(vectormap)
>Object of class SpatialPolygonsDataFrame
>Coordinates:
>           min      max
>x  -0.01383442 457.4256
>y 125.27349310 484.3477
>Is projected: FALSE 
>proj4string :............
>Data attributes:
>         Layer                           SubClasses ExtendedEn Linetype >EntityHand   Text  
> big        :1   AcDbEntity:AcDbBlockReference:4    NA's:4     NA's:4   42:1       >NA's:4  
> fullsection:1                                                          48:1               
> medium     :1                                                          67:1               
> small      :1 
84:1

However when I do this, I cant change the projection.

vectormap2 <- spTransform(vectormap, CRS("+proj=eqc +lat_ts=0 +lat_0=0 +lon_0=0 +x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs"))
non finite transformation detected:
[1] 187.4585 468.4831      Inf      Inf
Error in .spTransform_Polygon(input[[i]], to_args = to_args, from_args = from_args,  : 
  failure in Polygons 1 Polygon 1 points 1
In addition: Warning message:
In .spTransform_Polygon(input[[i]], to_args = to_args, from_args = from_args,  :
  1808 projected point(s) not finite

Answer 1

You need to select which feature will be plotted. For this purpose, select the observation as the same way as you select a row or more from a data.frame object:

plot(shp)

plot(shp[1,])

plot(shp[2:3,])

plot(shp[shp$Nombre == 'Estanque',])

---

This also work to extract features to a new polygon:

shp2 <- shp[1,]

shp2

class       : SpatialPolygonsDataFrame 
features    : 1 
extent      : 317185.2, 317341.5, 6601234, 6601492  (xmin, xmax, ymin, ymax)
coord. ref. : +proj=utm +zone=19 +south +datum=WGS84 +units=m +no_defs +ellps=WGS84 +towgs84=0,0,0 
variables   : 3
names       : id,    Nombre,   Sup 
min values  :  1, Cuartel 1, 1.937 
max values  :  1, Cuartel 1, 1.937