[R-sig-Geo] What is the Coordinate Reference System?
Sergio Ibarra
zergioib@rr@ @ending from gm@il@com
Mon Jun 18 15:49:08 CEST 2018
Yes, thanks for the explanations.
Have a nice week! :)
Em seg, 18 de jun de 2018 às 10:29, Ben Tupper <btupper using bigelow.org>
escreveu:
> Hi,
>
> I will blissfully proceed as I have in the past. Thanks to each of you
> for the clear explanations.
>
> Cheers,
> Ben
>
> > On Jun 18, 2018, at 6:26 AM, Michael Sumner <mdsumner using gmail.com> wrote:
> >
> > Agree with Edzer this application of eqc is anomalous and after it
> confused
> > us initially we now ignore it and apply the unproblematic proj=longlat
> > interpretation to these products (after checking).
> >
> > I might explore the L3 products to find out why this occurred, but the
> > ocean colour forum is the usual place to find discussion.
> >
> > Cheers, Mike
> >
> > On Mon, 18 Jun 2018, 15:59 Edzer Pebesma, <edzer.pebesma using uni-muenster.de
> >
> > wrote:
> >
> >> Plate carree is essentially a linear mapping of long and lat to x and y,
> >> without changing aspect ratio (scale factor 1 at equator).
> >>
> >> projecting long/lat towards plate caree using the proj4 string you
> >> mention will however change the units from degrees to m. So while the
> >> plots of of data in linear long/lat (with aspect ration 1) looks
> >> identical to the one obtained in plate caree (with aspect ratio 1), the
> >> numerical values along their axes are very different.
> >>
> >> Many netcdf files carry a grid (raster) with the x and y coordinates of
> >> grid cells (centers?) in another coordinate system, but the ones you
> >> mention below don't, as far as I can tell. I think what they mean by
> >> mentioning plate carree is just saying that this is a regular long/lat
> >> degree grid; rather unlucky wording IMO.
> >>
> >> On 06/15/2018 08:14 PM, Ben Tupper wrote:
> >>> Hi,
> >>>
> >>> That's an interesting approach.
> >>>
> >>> On the other hand, sometimes I get bitten (well, muddled really) when I
> >> use raster to determine the crs of a ncdf resource. It all sort of
> depends.
> >>>
> >>> Below is an example for Aqua MODIS imagery from OBPG (
> >>
> https://oceandata.sci.gsfc.nasa.gov/MODIS-Aqua/Mapped/Daily/9km/sst/2017/
> >> <
> https://oceandata.sci.gsfc.nasa.gov/MODIS-Aqua/Mapped/Daily/9km/sst/2017/
> >).
> >> The OBPG docs (https://oceancolor.gsfc.nasa.gov/products/ <
> >> https://oceancolor.gsfc.nasa.gov/products/>) state that the data is
> >> served up as 'Plate Carrée' which I think has a 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 '
> >>>
> >>> ... as described here http://spatialreference.org/ref/epsg/32662/ <
> >> http://spatialreference.org/ref/epsg/32662/>
> >>>
> >>> You can see below that raster comes up with...
> >>>
> >>> '+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0'
> >>>
> >>> ... which is http://spatialreference.org/ref/epsg/4326/ <
> >> http://spatialreference.org/ref/epsg/4326/>
> >>>
> >>> Explicitly setting the crs while reading doesn't help. I have never
> >> resolved if it is an OBPG documentation bug, my confusion about
> >> projections, or some other mystery. So, my usual practice is to assume
> >> that raster makes the right call.
> >>>
> >>> #### start
> >>>
> >>> library(raster)
> >>> URI = '
> >>
> https://oceandata.sci.gsfc.nasa.gov/cgi/getfile/A2017175.L3m_DAY_SST_sst_9km.nc
> >> '
> >>>
> >>> # ~3.8MB
> >>> ok = download.file(URI, basename(URI))
> >>>
> >>> R = raster::raster(basename(URI), varname = 'sst')
> >>> R
> >>> # class : RasterLayer
> >>> # dimensions : 2160, 4320, 9331200 (nrow, ncol, ncell)
> >>> # resolution : 0.08333334, 0.08333334 (x, y)
> >>> # extent : -180, 180, -90, 90 (xmin, xmax, ymin, ymax)
> >>> # coord. ref. : +proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0
> >>> # data source : /Users/Shared/code/R/others/record/spnc/
> >> A2017175.L3m_DAY_SST_sst_9km.nc
> >>> # names : Sea.Surface.Temperature
> >>> # zvar : sst
> >>>
> >>> S = raster::raster(basename(URI), varname = "sst", 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')
> >>> # Warning message:
> >>> # In .getProj(prj, crs) :
> >>> # argument "crs" ignored because the file provides a crs
> >>>
> >>> #### end
> >>>
> >>>
> >>> Cheers,
> >>> Ben
> >>>
> >>>
> >>>> On Jun 14, 2018, at 6:42 PM, Michael Sumner <mdsumner using gmail.com>
> wrote:
> >>>>
> >>>> Try raster::projection(raster::raster(a)) first, then delve if
> necessary
> >>>> into the ncdump summary with print(raster::raster(a)))
> >>>>
> >>>> But, raster(a) may be enough here? To get that stuff more directly
> from
> >> the
> >>>> NetCDF API you have to use the inq and att functions of ncdf4 or
> RNetCDF
> >>>> and that's no fun.
> >>>>
> >>>> I'd also try raster(rgdal::readGDAL(a)) which is a totally different
> but
> >>>> equivalent read path.
> >>>>
> >>>> Cheers, Mike
> >>>>
> >>>> On Thu, 14 Jun 2018, 11:03 Sergio Ibarra, <zergioibarra using gmail.com>
> >> wrote:
> >>>>
> >>>>> Greetings,
> >>>>>
> >>>>> I'm want to read NetCDF spatial data from a meteorological model (WRF
> >>>>> model). The NetCDF can have one of these three projections:
> >>>>>
> >>>>> Polar stereographic
> >>>>> Lambert conformal
> >>>>> Mercator projection
> >>>>>
> >>>>> (See figure
> >>>>>
> >>>>>
> >>
> https://user-images.githubusercontent.com/27447280/41389756-ed23f91a-6f68-11e8-961e-6ba901913c54.png
> >>>>> )
> >>>>>
> >>>>> What is the Coordinate Reference System for each case?
> >>>>>
> >>>>> Thanks!
> >>>>>
> >>>>> Here an example with ncdf4 and raster:
> >>>>>
> >>>>> library(ncdf4)
> >>>>> library(raster)
> >>>>> a <- tempfile()
> >>>>> download.file(url = "
> >>>>>
> >>
> http://www.unidata.ucar.edu/software/netcdf/examples/wrfout_v2_Lambert.nc
> >>>>> ",
> >>>>> destfile = a) #78 Mb
> >>>>> wrf <- nc_open(a)
> >>>>> paste("The file has",wrf$nvars,"variables")
> >>>>> paste("The file has",names(wrf$var))
> >>>>> lat <- ncvar_get( wrf, "XLAT" )
> >>>>> lon <- ncvar_get( wrf, "XLONG" )
> >>>>> t2 <- ncvar_get( wrf, "T2" ) - 273.15
> >>>>> dim(t2) #73 60 13
> >>>>> #image
> >>>>> image(t2[, , 12])
> >>>>> # raster
> >>>>> rt2 <- raster(t(t2[1:dim(t2)[1],
> >>>>> dim(t2)[2]:1,
> >>>>> 12]),
> >>>>> xmn = min(lon),
> >>>>> xmx = max(lon),
> >>>>> ymn = min(lat),
> >>>>> ymx = max(lat),
> >>>>> * crs="+init=epsg:4326") # ???*
> >>>>> spplot(rt2)
> >>>>>
> >>>>>
> >>>>>
> >>>>> --
> >>>>> Sergio Ibarra Espinosa
> >>>>> Post Doc
> >>>>> PhD in Atmospheric Sciences
> >>>>> Instituto de Astronomia, Geofísica e Ciências Atmosféricas
> >>>>> Universidade de São Paulo
> >>>>> Rua do Matão, 1226
> >>>>> São Paulo-SP - Brasil -
> >>>>> 05508-090
> >>>>> +55-11-97425-3791 <(11)%2097425-3791>
> >>>>> Skype: sergio_ibarra1
> >>>>>
> >>>>> [[alternative HTML version deleted]]
> >>>>>
> >>>>> _______________________________________________
> >>>>> R-sig-Geo mailing list
> >>>>> R-sig-Geo using r-project.org
> >>>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >>>>>
> >>>> --
> >>>> Dr. Michael Sumner
> >>>> Software and Database Engineer
> >>>> Australian Antarctic Division
> >>>> 203 Channel Highway
> >>>> Kingston Tasmania 7050 Australia
> >>>>
> >>>> [[alternative HTML version deleted]]
> >>>>
> >>>> _______________________________________________
> >>>> R-sig-Geo mailing list
> >>>> R-sig-Geo using r-project.org
> >>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >>>>
> >>>
> >>> Ben Tupper
> >>> Bigelow Laboratory for Ocean Scien
> <https://maps.google.com/?q=for+Ocean+Scien&entry=gmail&source=g>ces
> >>> 60 Bigelow Drive, P.O. Box 380
> >>> East Boothbay, Maine 04544
> >>> http://www.bigelow.org
> >>>
> >>> Ecological Forecasting: https://eco.bigelow.org/
> >>>
> >>>
> >>>
> >>>
> >>>
> >>>
> >>> [[alternative HTML version deleted]]
> >>>
> >>> _______________________________________________
> >>> R-sig-Geo mailing list
> >>> R-sig-Geo using r-project.org
> >>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >>>
> >>
> >> --
> >> Edzer Pebesma
> >> Institute for Geoinformatics
> >> Heisenbergstrasse 2, 48151 Muenster, Germany
> >> Phone: +49 251 8333081 <+49%20251%208333081>
> >>
> >> _______________________________________________
> >> R-sig-Geo mailing list
> >> R-sig-Geo using r-project.org
> >> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >>
> > --
> > Dr. Michael Sumner
> > Software and Database Engineer
> > Australian Antarctic Division
> > 203 Channel Highway
> > Kingston Tasmania 7050 Australia
> >
> > [[alternative HTML version deleted]]
> >
> > _______________________________________________
> > R-sig-Geo mailing list
> > R-sig-Geo using r-project.org
> > https://stat.ethz.ch/mailman/listinfo/r-sig-geo
> >
>
> Ben Tupper
> Bigelow Laboratory for Ocean Sciences
> 60 Bigelow Drive, P.O. Box 380
> East Boothbay, Maine 04544
> http://www.bigelow.org
>
> Ecological Forecasting: https://eco.bigelow.org/
>
>
>
>
>
>
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>
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>
--
Sergio Ibarra Espinosa
Post Doc
PhD in Atmospheric Sciences
Instituto de Astronomia, Geofísica e Ciências Atmosféricas
Universidade de São Paulo
Rua do Matão, 1226
São Paulo-SP - Brasil -
05508-090
+55-11-97425-3791
Skype: sergio_ibarra1
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