Hello,

I am trying to use sea ice concentrations data (spatial resolution=25 
km) from the NSIDC database: 
http://nsidc.org/data/docs/daac/nsidc0051_gsfc_seaice.gd.html#spatialcover
but data are projected in a modified version of the "polar stereographic 
projection" (projection plane or grid tangent to the Earth's surface at 
70 degrees + Hughes ellipsoid).
The parameters of this projection are available in their website and I 
found an epsg code on "spatial reference" website 
_http://spatialreference.org/ref/epsg/3412/_  ("epsg=3412), however I 
could not figure out how to project these data in WGS84.

The data are in binary format :download : 
_ftp://sidads.colorado.edu/pub/DATASETS/seaice/polar-stereo/nasateam/final-gsfc/south/monthly/_

sic.data <- matrix(data=readBin("C:\\nt_200712_f13_v01_s.bin", 
"integer", n=316*332, size=1, signed=FALSE), ncol=316, byrow=TRUE)

The sea ice data are projected in the particular version of the "polar 
stereographic projection"
image(sic.data)
but I'm not sure whether this image is "correctly " projected or need to 
be modified with the provided lats and longs files :
_ftp://sidads.colorado.edu/pub/DATASETS/seaice/polar-stereo/tools/_

sic.lat <-  matrix(data=readBin("C:\\Sea Ice NSIDC 
reso_25km\\polar-stereo\\nasateam\\pss25lats_v2.dat", "integer", 
n=316*332, size=4,           signed=FALSE, endian="little"), ncol=316, 
byrow=TRUE)
sic.lon <-  matrix(data=readBin("C:\\Sea Ice NSIDC 
reso_25km\\polar-stereo\\nasateam\\pss25lons_v2.dat", "integer", 
n=316*332, size=4, signed=FALSE, endian="little"), ncol=316, byrow=TRUE)

that we need to divide by 100000 :
sic.lat <- sic.lat/100000
sic.lon <- sic.lon/100000

But these coordinates are not those of the special version of the "polar 
stereographic projection" but look like WGS84 coordinates...
I am looking for a way to match these 3 matrix so that I can use the sea 
ice concentrations data with WGS84 coordinates...
I tried :
lon <- as.vector(sic.lon)
lat <- as.vector(sic.lat)
sic <- as.vector(sic.data)
SIC <- SpatialPoints(data.frame(sic = sic.data*0.4, lon = sic.lon, lat = 
sic.lat), proj4string=CRS("+init=epsg:3412"))
SIC2 <- spTransform(SIC, CRS=CRS("+proj=latlong +ellps=WGS84")) 
but it does look like it should...

Has anybody already used these kind of data ?

Many thanks for your reply

Clara Péron


************************************************
Clara Péron PhD Student
Spatial distribution of seabirds in the Indian Austral Ocean
Centre d'Etudes Biologiques de Chizé - CNRS UPR1934
79360 Villiers-en-Bois, France
http://www.cebc.cnrs.fr/Fidentite/peron/peron.htm
************************************************



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