# The actual size of each grid cell with considerable distortion in area of map projection. This function helps to calculate size of pixels in the unit of area (squared km) for zonal statistics with taking into account distortion in area for classes of projections. ```r pixelsize(obj, verbose = FALSE) ``` ## Arguments - `obj`: Either `ursaRaster` object or `ursaGrid` object or `NULL` or `missing`. - `verbose`: Logical. Value `TRUE` may provide some additional information on console. Default is `FALSE`. ## Details `pixelsize()` is applied to coordinate reference system (grid) of `ursaRaster` object or to raster grid directly. If argument `obj` is missed , then session grid is used. Currently, only **Stereographic** (`"+stere"` in PROJ.4 notation), **Mercator** (`"+merc"`), and **Lambert Azimuthal Equal Area** (`"+laea"`) classes of map projections are implemented, though the last one (LAEA) has no distortion in area. ## Returns Object of class `ursaRaster`, single-band. If size of cell is more than `10e5` square meters, then the unit is squared kilometers (band name is "Pixel Size (sq.km)") else squared meters (band name is "Pixel Size (sq.m)"). ## Author(s) Nikita Platonov platonov@sevin.ru ## Examples ```r session_grid(NULL) pixelsize() ## internet connection is required; access was tested on 2018-06-04 invisible({ dpath <- file.path("ftp://sidads.colorado.edu/pub/DATASETS" ,"nsidc0081_nrt_nasateam_seaice/north") dst <- tempfile(fileext=".bin") isOK <- FALSE d3 <- Sys.Date() for (i in seq(5)) { src <- file.path(dpath,format(d3,"nt_%Y%m%d_f18_nrt_n.bin")) a <- try(download.file(src,dst,mode="wb")) if ((is.integer(a))&&(a==0)) { isOK <- TRUE break } d3 <- d3-1 } if (isOK) { g1 <- regrid(bbox=c(-385,-535,375,585)*1e4,res=25*1e3 ,crs=paste("+proj=stere +lat_0=90 +lat_ts=70 +lon_0=-45" ,"+k=1 +x_0=0 +y_0=0 +a=6378273 +b=6356889.449" ,"+units=m +no_defs")) session_grid(g1) b <- readBin(dst,integer(),size=1L,n=136492L,signed=FALSE) ice <- ursa_new(value=tail(b,-300)) ice[ice>251] <- NA ## keep Pole ice[ice==251] <- 250 ## consider 100% ice at Pole ice <- ice/2.5 ## uncategorize ice[ice<15] <- 0 ## not ice, if less 15% ice[ice>0] <- 100 extent1 <- band_sum(ice*1e-2*ursa(ice,"cell")^2*1e-6)*1e-6 extent2 <- band_sum(ice*1e-2*pixelsize(ice))*1e-6 message(paste("Near real-time Arctic sea ice extent (NASA Team algorithm, NSIDC)")) message(sprintf(" Direct area calculation: %5.2f*1e6 km^2.",extent1)) message(sprintf(" Distortion in area is corrected: %5.2f*1e6 km^2.",extent2)) } else message("It is failed to get sea ice concentration data.") }) ```

pixelsize function