R help - Jan 2009 - maptools, sunriset, POSIX timezones

Hi ...

I wonder if anyone can provide some insight into why the first three 
examples using the sunriset function (appended below, with results) give 
the correct answer, but the fourth generates and error.

The first two use ISOdatetime with and without a time zone attribute, 
and the sunriset function returns the correct sunset time.

The third and fourth adds 10 seconds to the ISOdatetime (with and 
without the time zone attribute) but the function only works when the 
time zone is specified (example 3).

When I look at the objects (print or str) they appear the same, and when 
I check to see if they are equivalent (e.g.)

 > time.1 <- ISOdatetime(1970, 1, 1, 10, 0, 0) + 10
 > time.2 <- ISOdatetime(1970, 1, 1, 10, 0, 0, tz="GMT") + 10
 > time.1 == time.2
[1] TRUE

they appear to be the same.

I wonder if I am either missing something important, doing something 
improperly, or if there is a small bug somewhere.

I'm using windows Vista and R 2.8.1

Thanks,

Phil Taylor
ptaylor at resalliance.org

 > require(maptools)
 > Sys.setenv(TZ = "GMT")
 > location <- matrix(c(-80.1,42.5), nrow=1)

 > sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0, tz="GMT"), 
direction="sunset", POSIXct.out=TRUE)
   day_frac                time
1 0.915226 1970-01-01 21:57:55

 > sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0), 
direction="sunset", POSIXct.out=TRUE)
   day_frac                time
1 0.915226 1970-01-01 21:57:55

 > sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0, tz="GMT") +
10,
direction="sunset", POSIXct.out=TRUE)
   day_frac                time
1 0.915226 1970-01-01 21:57:55

 > sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0) + 10, 
direction="sunset", POSIXct.out=TRUE)
Error in structure(.Internal(as.POSIXct(x, tz)), class = c("POSIXt", 
"POSIXct"),  :
         invalid 'tz' value

Phil Taylor <tabanid <at> gmail.com> writes:
> 
> Hi ...
> 
> I wonder if anyone can provide some insight into why the first three 
> examples using the sunriset function (appended below, with results) give 
> the correct answer, but the fourth generates and error.
> 
> The first two use ISOdatetime with and without a time zone attribute, 
> and the sunriset function returns the correct sunset time.
> 
> The third and fourth adds 10 seconds to the ISOdatetime (with and 
> without the time zone attribute) but the function only works when the 
> time zone is specified (example 3).
> 
> When I look at the objects (print or str) they appear the same, and when 
> I check to see if they are equivalent (e.g.)
> 
>  > time.1 <- ISOdatetime(1970, 1, 1, 10, 0, 0) + 10
>  > time.2 <- ISOdatetime(1970, 1, 1, 10, 0, 0, tz="GMT") +
10
>  > time.1 == time.2
> [1] TRUE
> 
> they appear to be the same.
> 
> I wonder if I am either missing something important, doing something 
> improperly, or if there is a small bug somewhere.
There may be a misunderstanding, but a small bug is also possible. In an
internal call to as.POSIXct(), a NULL value was being passed to the tz= 
argument when 10 seconds had been added and tz= not given in ISOdatetime().

In forthcoming maptools 0.7-18, the value being passed is tested, and the
tzargument dropped if the value is NULL. With this:
> library(maptools)
Loading required package: foreign
Loading required package: sp
> Sys.setenv(TZ = "GMT")
> location <- matrix(c(-80.1,42.5), nrow=1)
> sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0, tz="GMT"), 
direction="sunset", POSIXct.out=TRUE)
       day_frac                time
newlon 0.915226 1970-01-01 21:57:55
> sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0),
direction="sunset",
POSIXct.out=TRUE)
       day_frac                time
newlon 0.915226 1970-01-01 21:57:55
> sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0, tz="GMT") +
10,
direction="sunset", POSIXct.out=TRUE)
       day_frac                time
newlon 0.915226 1970-01-01 21:57:55
> sunriset(location, ISOdatetime(1970, 1, 1, 10, 0, 0) + 10, 
direction="sunset", POSIXct.out=TRUE)
       day_frac                time
newlon 0.915226 1970-01-01 21:57:55

Thanks for the report - it could well be that the time classes are not being 
used well here - in addition, the sunset etc. functions of course only look
at yearday, so adding 10 seconds isn't very meaningful, but I guess this is
a simple example of a real problem.

Roger Bivand

> 
> I'm using windows Vista and R 2.8.1
> 
> Thanks,
> 
> Phil Taylor
> ptaylor <at> resalliance.org
>