Hi, This post gives more details on how to transform your lat/lon values to row/column indices. The question and answer are specifically about the ncdf package, but the workflow is identical when using the ncfd4 package.
https://stat.ethz.ch/pipermail/r-help/2011-March/272641.html Cheers, Ben > On Jan 9, 2016, at 7:14 AM, Anthoni, Peter (IMK) <peter.anth...@kit.edu> > wrote: > > Hi Peter, > > the start in nc_varget requires a latitude and longitude index, not the > latitude and longitude in double format. > So you need to figure out what index your latitude and longitude correspond > to, which will depends on what data are in your netCDF. > > it might have looked like that it worked for a positive latitude, but you got > the data from the latitude index 6 or 7, depends on how the double was > transformed into an integer. > > best regards > Peter > >> On 09 Jan 2016, at 12:28, Peter Tuju via R-help <r-help@r-project.org> wrote: >> >> I have data file in netcdf with three dimensions (x, y, t) and I want to >> extract a variable RAINC and RAINNC >> using longitude and latitude for a single point location with all the time, >> but no lucky. The syntax is as follows;; >> setwd( "/run/media/tuju/0767090047/extract_wrf_txt_file" ) >> rm( list = ls() ) >> library( ncdf4 ) >> inp_file <- nc_open( "wrfout_d01_2016-01-07.nc" )time <- ncvar_get( >> inp_file, "Times" ) # Reading the time variabledar_lon >> <- 39.2dar_lat <- -6.866667 >> RAINC <- ncvar_get( inp_file, varid = "RAINC", start = c( dar_lon, dar_lat, >> 1 ), count = c( 1, 1, -1 ) ) >> RAINNC <- ncvar_get( inp_file, varid = "RAINNC", start = c( dar_lon, >> dar_lat, 1 ), count = c( 1, 1, -1 ) ) >> RAIN <- RAINC + RAINNCRAIN_TABLE <- cbind( time, RAIN ) >> write.table( RAIN_TABLE, "Dar_es_Salaam.txt", row.names = FALSE, >> col.names = c( "Valid Forecast Time", "Rain (mm)", sep = "\t " ) >> >> # But no lucky with the red bolded syntax as I end up with the following >> error message> RAINC <- ncvar_get( inp_file, varid = "RAINC", start = c( >> Lon[2], Lat[2], 1 ), count = c( 1, 1, -1 ) ) >> Error in Rsx_nc4_get_vara_double: NetCDF: Index exceeds dimension bound >> Var: RAINC Ndims: 3 Start: 0,4294967289,38 Count: 17,1,1 >> Error in ncvar_get_inner(ncid2use, varid2use, nc$var[[li]]$missval, >> addOffset, : >> C function R_nc4_get_vara_double returned error >> However when I cahnge the latitude to postive it works fine. Note latitudes >> in the file data ranges from -16.71505 to 7.787529 as shown below; >> head(ncvar_get(inp_file, "XLAT")) >> [1] -16.71505 -16.71505 -16.71505 -16.71505 -16.71505 -16.71505 >>> tail(ncvar_get(inp_file, "XLAT")) >> [1] 7.787529 7.787529 7.787529 7.787529 7.787529 7.787529 >> ## So, how can I get the syntax correct? Please help _____________ >> Peter E. Tuju >> Dar es Salaam >> T A N Z A N I A >> ---------------------- >> >> >> >> >> >> [[alternative HTML version deleted]] >> >> ______________________________________________ >> R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see >> https://stat.ethz.ch/mailman/listinfo/r-help >> PLEASE do read the posting guide http://www.R-project.org/posting-guide.html >> and provide commented, minimal, self-contained, reproducible code. > > ______________________________________________ > R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see > https://stat.ethz.ch/mailman/listinfo/r-help > PLEASE do read the posting guide http://www.R-project.org/posting-guide.html > and provide commented, minimal, self-contained, reproducible code. Ben Tupper Bigelow Laboratory for Ocean Sciences 60 Bigelow Drive, P.O. Box 380 East Boothbay, Maine 04544 http://www.bigelow.org ______________________________________________ R-help@r-project.org mailing list -- To UNSUBSCRIBE and more, see https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
