[R-sig-Geo] reproject coordinates from meters to decimal degrees

[Roger Bivand]

On Thu, 5 Jan 2017, Nikolioudakis Nikolaos wrote:

> Thank you for the prompt response Mike. It would be great if you could 
> do so and I can later post the solution on the list in case someone 
> needs it in the future

This topic is also a clear candidate for a vignette for the rgdal package; 
it needs documenting where users can find it.

Roger

>
> Cheers,
> Nikos
>
> From: Michael Sumner [mailto:mdsumner at gmail.com]
> Sent: Thursday, January 05, 2017 9:01 AM
> To: Nikolioudakis Nikolaos <nikolaos.nikolioudakis at imr.no>; r-sig-geo at r-project.org
> Subject: Re: [R-sig-Geo] reproject coordinates from meters to decimal degrees
>
>
> This data is in polar stereographic so it's best to ignore the lonlats and apply the right affine transform (extent and resolution) directly. I.e. no transformation or hacky workarounds required. We do this routinely from similar files, happy to explain off-list.
>
> For annoying reasons this is poorly understood on both the GIS and NetCDF (modelling) sides.
>
> Cheers, Mike
>
> On Thu, 5 Jan 2017, 18:43 Nikolioudakis Nikolaos <nikolaos.nikolioudakis at imr.no<mailto:nikolaos.nikolioudakis at imr.no>> wrote:
> Dear members of the list,
> My question might be trivial but please bear with me as I am quite new in GIS and R so my terminology and/or expressions might not be the right ones. I have a set of coordinates in lat/lon for which I need to extract values from a netCDF file. The file is downloaded from http://marine.copernicus.eu/ (needs registration to reach data from there). For this reason you can find in the following link a .nc subset (single raster) from a multiband .nc I am using.
> .nc file:
>
> https://dl.dropboxusercontent.com/u/2156982/subset.nc
>
> The coordinates of my locations (in lon/lat) are in the coords.csv file can be found here:
>
> https://dl.dropboxusercontent.com/u/2156982/coords.csv
>
> The raster's projection is (according to the data source EPSG: 3411 (NSIDC Polar Stereographic North)) so the obvious thing to do is reproject my points to the same projection as the raster's in order to overplot them (and subsequently extract values from the raster based on the points). Reprojecting the rasters would also be an option, but in my specific case I have multiband (many dates) netCDFs and also many different variables in each netCDF so reprojecting the coordinates if possible and performing the extraction from the epsg 3411 rasters would be ideal for me. My problem is that when I reproject my coordinates I get them in meters and not in decimal degrees as the raster is.
>
>
>
> So the question is how can I obtain my reprojected coordinates in decimal degrees instead of meters in order to be able to perform the extraction of values from the raster (I have done this for another dataset that both the locations and the rasters were in the same projection and units). I understand that my problem is probably related with how to declare that the output units of my reprojected coordinates should be in dd instead of meters but I am not sure on the syntax of the proj4 string and on the web the inverse procedure is usually described (dd to meters). The r code I am using is the following:
>
> library(raster)
>
> library(rgdal)
>
>
>
> #setwd to the location of the files
>
> subset.r = raster('subset.nc<http://subset.nc>', varname='chla')
>
> #print(subset.r)
>
>
>
> locs <- read.table('coords.csv',header = T,sep = ';')
>
> coordinates(locs) <- ~ LON+LAT
>
> # par(mfrow=c(1,2))
>
> # plot(subset.r)
>
> # plot(locs at coords)
>
>
>
> #set the projection to EPSG: 4326 for the locs
>
> wgs84 <- CRS('+init=epsg:4326')
>
> proj4string(locs) <- wgs84
>
> #reproject locs to polar stereographic (EPSG 3411)
>
> locs.repr <- spTransform(locs, crs(subset.r))
>
> #compareCRS(locs.repr, subset.r)
>
> plot(subset.r)
>
> plot(locs.repr at coords<mailto:locs.repr at coords<mailto:locs.repr at coords>>)
>
> # extent(locs.repr)
>
> # extent(subset.r)
>
>
>
> Any help would be greatly appreciated as well as any comments on things I might be forgetting to provide so as to make my problem better understood.
>
> Best regards,
> Nikos
> __________________________
> Nikos Nikolioudakis
> PostDoc Researcher
> Pelagic Fish Research Group
> Institute of Marine Research (IMR)
> P.O. Box 1870 - Nordnes
> 5817 Bergen, Norway
> E-mail:nikolaos.nikolioudakis at imr.no<mailto:E-mail%3Anikolaos.nikolioudakis at imr.no>
> Tel:0047489981
>
> [cid:image001.jpg at 01D0B00F.4BA30FC0]
> __________________________
>
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>
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> --
> Dr. Michael Sumner
> Software and Database Engineer
> Australian Antarctic Division
> 203 Channel Highway
> Kingston Tasmania 7050 Australia
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-- 
Roger Bivand
Department of Economics, Norwegian School of Economics,
Helleveien 30, N-5045 Bergen, Norway.
voice: +47 55 95 93 55; e-mail: Roger.Bivand at nhh.no
http://orcid.org/0000-0003-2392-6140
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