Hi, Well, you have to extrapolate from that post that wherenearest is a function you must create on your own. Maybe something like this?
#' Find the index into a dataset that is 'closest' to a specified point. #' #' Adpated from https://stat.ethz.ch/pipermail/r-help/2011-March/272641.html #' @param myPoint numeric, one point #' @param allpoints numeric, one or more points #' @return index into allPoints that myPoint falls closest to wherenearest <- function(myPoint, allPoints){ d <- abs(allPoints-myPoint[1]) index <- which.min(d) return( index ) } I haven't tried the above. You haven't provided much detail on what's in the file, but if it is a grid then perhaps you would find it easier to use the raster package. It has functions to read gridded data stored in ncdf files. It comes with a very good tutorial and makes working with gridded data a breeze. Using raster, you can bypass the nitty-gritty of getting data out of a ncdf file and just get to work on your data. https://cran.r-project.org/web/packages/raster/index.html https://cran.r-project.org/web/packages/raster/vignettes/Raster.pdf Bon chance! Ben P.S. Do yourself (and everyone else on the list) a favor by making your email client use plain text rather than html or rich text when sending messages to the list. The html/rich text scrambles your code making it hard to read. > On Jan 9, 2016, at 9:01 AM, Peter Tuju <petere...@ymail.com> wrote: > > Thank you Be for the good guide, however no luck with the syntax used namely; > > ix0 = wherenearest( lower_left_lon_lat[1], lon ) > ix1 = wherenearest( upper_right_lon_lat[1], lon ) > iy0 = wherenearest( lower_left_lon_lat[2], lat ) > iy1 = wherenearest( upper_right_lon_lat[2], lat ) > > # I end up with this error, "Error: could not find function "wherenearest" > > Is there any other way I can get the index corresponding/or rearing to the > longitude and latitude of interests? > > _____________ > Peter E. Tuju > Dar es Salaam > T A N Z A N I A > ---------------------- > > > > From: Ben Tupper <btup...@bigelow.org> > To: "r-help@r-project.org" <r-help@r-project.org> > Cc: Peter Tuju <petere...@ymail.com> > Sent: Saturday, January 9, 2016 4:01 PM > Subject: Re: [R] Extracting point data using longitude and latitude from > netcdf file using R > > 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 > > > 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.
