Extract field's names in R without reading the file

Question

I was wondering if it's possible to use the R package sf for extracting the names of the fields in a file without reading it. For example, I would like to replicate the following behaviour:
system(paste0("ogrinfo -so ", system.file("shape/nc.shp", package = "sf"), " nc"))
#> INFO: Open of `/home/andrea/R/x86_64-pc-linux-gnu-library/3.6/sf/shape/nc.shp'
#>       using driver `ESRI Shapefile' successful.

#> Layer name: nc
#> Metadata:
#>   DBF_DATE_LAST_UPDATE=2016-10-26
#> Geometry: Polygon
#> Feature Count: 100
#> Extent: (-84.323853, 33.881992) - (-75.456978, 36.589649)
#> Layer SRS WKT:
#> GEOGCRS["NAD27",
#>     DATUM["North American Datum 1927",
#>         ELLIPSOID["Clarke 1866",6378206.4,294.978698213898,
#>             LENGTHUNIT["metre",1]]],
#>     PRIMEM["Greenwich",0,
#>         ANGLEUNIT["degree",0.0174532925199433]],
#>     CS[ellipsoidal,2],
#>         AXIS["latitude",north,
#>             ORDER[1],
#>             ANGLEUNIT["degree",0.0174532925199433]],
#>         AXIS["longitude",east,
#>             ORDER[2],
#>             ANGLEUNIT["degree",0.0174532925199433]],
#>     ID["EPSG",4267]]
#> Data axis to CRS axis mapping: 2,1
#> AREA: Real (24.15)
#> PERIMETER: Real (24.15)
#> CNTY_: Real (24.15)
#> CNTY_ID: Real (24.15)
#> NAME: String (80.0)
#> FIPS: String (80.0)
#> FIPSNO: Real (24.15)
#> CRESS_ID: Integer (9.0)
#> BIR74: Real (24.15)
#> SID74: Real (24.15)
#> NWBIR74: Real (24.15)
#> BIR79: Real (24.15)
#> SID79: Real (24.15)
#> NWBIR79: Real (24.15)

without directly calling system or ogrinfo.

Spacedman · Accepted Answer

If you use st_read with a query that selects zero rows you get back an empty dataframe with all the columns intact:
> nc_sql = st_read(system.file("shape/nc.shp", package="sf"), query="select * from "nc" limit 0")

> dim(nc_sql)
[1]  0 15

> names(nc_sql)
 [1] "AREA"           "PERIMETER"      "CNTY_"          "CNTY_ID"       
 [5] "NAME"           "FIPS"           "FIPSNO"         "CRESS_ID"      
 [9] "BIR74"          "SID74"          "NWBIR74"        "BIR79"         
[13] "SID79"          "NWBIR79"        "_ogr_geometry_"

If you are also interested in the column types you can get those even though the data frame has no rows:
> class(nc_sql$FIPS)
[1] "factor"
> class(nc_sql$FIPSNO)
[1] "numeric"

and the projection:
> st_crs(nc_sql)
Coordinate Reference System:
  User input: 4267 
  wkt:
GEOGCS["GCS_North_American_1927",
    DATUM["North_American_Datum_1927",
        SPHEROID["Clarke_1866",6378206.4,294.9786982]],
    PRIMEM["Greenwich",0],
    UNIT["Degree",0.017453292519943295],
    AUTHORITY["EPSG","4267"]]

I'm not sure if this counts as "without reading it", since at some level the code has to read the file. I don't think this reads the whole object and then drops all the rows, I think it is probably smart enough to stop after zero rows have been read. Would need to be tested to see how long it takes to read zero rows from a huge shapefile.
Another way which is basically wrapped around calling ogrinfo is to use the gdalUtils package:
> gdalUtils::ogrinfo(system.file("shape/nc.shp", package="sf"),al=TRUE,so=TRUE)
 [1] "INFO: Open of `/home/rowlings/R/x86_64-pc-linux-gnu-library/3.6/sf/shape/nc.shp'"
 [2] "      using driver `ESRI Shapefile' successful."                                 
 [3] ""                                                                                
 [4] "Layer name: nc"                                                                  
 [5] "Metadata:"                                                                       
 [6] "  DBF_DATE_LAST_UPDATE=2016-10-26"                                               
 [7] "Geometry: Polygon"                                                               
 [8] "Feature Count: 100"                                                              
 [9] "Extent: (-84.323853, 33.881992) - (-75.456978, 36.589649)"                       
[10] "Layer SRS WKT:"                                                                  
[11] "GEOGCS["GCS_North_American_1927","                                             
[12] "    DATUM["North_American_Datum_1927","                                        
[13] "        SPHEROID["Clarke_1866",6378206.4,294.9786982]],"                       
[14] "    PRIMEM["Greenwich",0],"                                                    
[15] "    UNIT["Degree",0.017453292519943295],"                                      
[16] "    AUTHORITY["EPSG","4267"]]"                                               
[17] "AREA: Real (24.15)"                                                              
[18] "PERIMETER: Real (24.15)"                                                         
[19] "CNTY_: Real (24.15)"                                                             
[20] "CNTY_ID: Real (24.15)"                                                           
[21] "NAME: String (80.0)"                                                             
[22] "FIPS: String (80.0)"                                                             
[23] "FIPSNO: Real (24.15)"                                                            
[24] "CRESS_ID: Integer (9.0)"                                                         
[25] "BIR74: Real (24.15)"                                                             
[26] "SID74: Real (24.15)"                                                             
[27] "NWBIR74: Real (24.15)"                                                           
[28] "BIR79: Real (24.15)"                                                             
[29] "SID79: Real (24.15)"                                                             
[30] "NWBIR79: Real (24.15)"

but you'd then have to analyse the output carefully to find the lines of the field names.

Extract field's names in R without reading the file

One Answer

Add your own answers!

Ask a Question