Yes, sorting longitude and latitude correctly solves the problem! However, this
only works near the Canaries. When selecting the January data (probable
location: South African waters) sunrise time (but not sunset time) for the
given coordinates is about 4 hours later than the local sunrise.
Sys.setenv( TZ="GMT" ) # set to the local timezone for column "time"
dta
<-read.table("eg.txt",sep=',',col.names=c("ok","time","secs","lig"),stringsAsFactors=FALSE)
dta$dtm <- as.POSIXct(dta$time, format="%d/%m/%y %T" ) # depends on TZ
site <- SpatialPoints( matrix(c(-15.77, 27.82),
nrow=1),proj4string=CRS("+proj=longlat +datum=WGS84")) # Canary Is
dta$rise <- sunriset(site, dta$dtm, direction="sunrise",POSIXct.out=TRUE)$time
dta$set <- sunriset( site, dta$dtm, direction="sunset", POSIXct.out=TRUE )$time
dta$isnight <- with( dta, !( rise < dtm & dtm < set ) )
A=cbind(dta,colsplit(dta$time, split = "\\s", names = c("date", "clock")))
B=cbind(A,colsplit(A$date, split = "/", names = c("d", "m", "y")))
C<-subset(B,B$m==1) # January data; the bird was in South Atlantic waters
write.table(C)
Thank you very much for your help,
Santi
>________________________________
> From: Jeff Newmiller <jdnew...@dcn.davis.ca.us>
>To: Santiago Guallar <sgual...@yahoo.com>
>Cc: "r-help@r-project.org" <r-help@r-project.org>
>Sent: Tuesday, May 1, 2012 1:14 AM
>Subject: Re: [R] Subtract days to dates in POSIXct format
>
>The sunrise and sunset are calculated for each time value on the input vector,
>but all times are treated in the TZ timezone. I can see that the Canary
>Islands are in the GMT timezone, so the example TZ is right.
>
>I can see that you entered the latitude and longitude backward (longitude is
>"x"), but I am not able to look at your data at the moment so I don't know
>whether that fixes your problem. Maptools assumes one location Depending on
>where the bird went in the South Atlantic, the assumed latitude could be
>several hours wrong, though. Also, there is dependence of length of day on
>latitude (made more noticeable by crossing the equator).
>---------------------------------------------------------------------------
>Jeff Newmiller The ..... ..... Go Live...
>DCN:<jdnew...@dcn.davis.ca.us> Basics: ##.#. ##.#. Live Go...
> Live: OO#.. Dead: OO#.. Playing
>Research Engineer (Solar/Batteries O.O#. #.O#. with
>/Software/Embedded Controllers) .OO#. .OO#. rocks...1k
>---------------------------------------------------------------------------
>Sent from my phone. Please excuse my brevity.
>
>Santiago Guallar <sgual...@yahoo.com> wrote:
>
>>Thanks a lot, Jeff
>>
>>Yes, I applied colsplit in package reshape. Sorry, I forgot to state it
>>in the piece of code I attached.
>>Actually, these are archival data from a geolocator attached to a
>>seabird. The origin lies on Gran Canaria and data were gathered all
>>along its migratory journey for one full year, from the Canary Islands
>>all the way to the South Atlantic Ocean and back.
>>Variable lig, which stands for light, indicates daylight (aprox lig>2)
>>or its absence (lig<2), so I know when it is nighttime or daytime for
>>sure. However, I'd like to compare it with the output of the maptools
>>functions.
>>If I understood well maptools functions use astronomical algorithms to
>>calculate sunrise and sunset based on one location and one POSIXct
>>time. If I changed your invented coordinates and time for the real ones
>>in my dataset would do the job correctly? When I put the aproximate
>>coordinates it yields an incorrect output:
>>site <- SpatialPoints( matrix(c(27.82, -15.77),
>>nrow=1),proj4string=CRS("+proj=longlat +datum=WGS84")) # didn't touch
>>the rest of your code
>>
>>But I'm probably doing something wrong.
>>
>>Santi
>>
>>
>>
>>
>>>________________________________
>>> From: Jeff Newmiller <jdnew...@dcn.davis.ca.us>
>>>To: Santiago Guallar <sgual...@yahoo.com>
>>>Cc: "r-help@r-project.org" <r-help@r-project.org>
>>>Sent: Monday, April 30, 2012 5:51 PM
>>>Subject: Re: [R] Subtract days to dates in POSIXct format
>>>
>>>On Mon, 30 Apr 2012, Santiago Guallar wrote:
>>>
>>>> Hello,
>>>>
>>>> I'm having problems working with date values in POSIXct format.
>>>
>>>Indeed you are.
>>>
>>>> Here is what I got (eg.lig attached):
>>>>
>>>> x <- read.table("eg.txt", sep = ',',
>>col.names=c("ok","time","secs","lig")) # it gives time as factor
>>>> z <- cbind(x,colsplit(x$time, split="\\s", names=c("date",
>>"clock")))
>>>
>>>colsplit is not defined in base R, and it is redefined in a couple of
>>packages. I am guessing you mean the one that is in the reshape
>>package.
>>>
>>>> zh<-cbind(z,colsplit(z$clock,split=":",names=c("h","m","s")))
>>>> zn <- subset(zh, zh$lig<= 5 & zh$h<8 | zh$h>=22) # nighttime
>>>> zn$timepos<-as.POSIXct(zn$time,tz="GMT",format="%d/%m/%y %H:%M:%S")
>>>
>>>> I want to assign timepos to its ?natural? night, that is, the night
>>that goes from sunset of day x to sunrise of day x+1 corresponds to day
>>x:
>>>
>>>You are working here with GMT, but there are many other possible
>>timezones that this data could correspond to, and if the input time
>>really is in GMT, then the interval of night could be anytime.
>>>
>>>>
>>>> zn$night<-ifelse(zn$h>=0 & zn$h<9,zn$timepos
>>??oneday?,zn$timepos) #doesn?t work
>>>>
>>>> How can I subtract one day (24 hours) to zn$timepos and obtain a
>>POSIXct output?
>>>
>>>Well, a literal answer to your question is:
>>>
>>>zn$timepos - as.difftime( 1, units="days" )
>>>
>>>but in response to your stated goal perhaps you should study the
>>following:
>>>
>>>library(maptools)
>>>Sys.setenv( TZ="GMT" ) # set to the local timezone for column "time"
>>>dta <- read.table( "eg.txt"
>>> , sep=','
>>> , col.names=c("ok","time","secs","lig")
>>> , stringsAsFactors=FALSE)
>>>dta$dtm <- as.POSIXct( dta$time, format="%d/%m/%y %T" ) # depends on
>>TZ
>>># site is invented in GMT timezone for illustration
>>>site <- SpatialPoints( matrix( 0, 51.5, nrow=1 )
>>> , proj4string=CRS("+proj=longlat +datum=WGS84" )
>>)
>>>dta$rise <- sunriset( site
>>> , dta$dtm
>>> , direction="sunrise"
>>> , POSIXct.out=TRUE )$time
>>>dta$set <- sunriset( site
>>> , dta$dtm
>>> , direction="sunset"
>>> , POSIXct.out=TRUE )$time
>>>dta$isnight <- with( dta, !( rise < dtm & dtm < set ) )
>>>
>>>
>>>---------------------------------------------------------------------------
>>>Jeff Newmiller The ..... ..... Go
>>Live...
>>>DCN:<jdnew...@dcn.davis.ca.us> Basics: ##.#. ##.#. Live
>>Go...
>>> Live: OO#.. Dead: OO#..
>>Playing
>>>Research Engineer (Solar/Batteries O.O#. #.O#. with
>>>/Software/Embedded Controllers) .OO#. .OO#.
>>rocks...1k
>>>---------------------------------------------------------------------------
>>>
>>>
>
>
>
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