Dear List,
I need some help in coming up with a function that will take two data sets,
determine if a value is missing in one, find a value in the second that was
taken at about the same time, and substitute the second value in for where the
first should have been. My problem is from a fish tracking study. We put
acoustic tags in fish and track them for several days. Location data is
supposed to be automatically recorded every time we detect a "ping" from the
fish. Unfortunately the GPS had some problems and sometimes the fishes depth
was recorded but not its location. I fortunately had a back-up GPS that was
taking location data every five minutes. I would like to merge the two files,
replacing the missing value in the vscan (automatic) file with the location
from the garmin file. Since we were getting vscan records every 1-2 seconds
and garmin records every 5 minutes, I need to find the right place in the vscan
file to place the garmin record - i.e. the
closest in time, but not greater than 5 minutes. I have written a function
that does this. However, it works with my test data but locks up my computer
with my real data. I have several million vscan records and several thousand
garmin records. Is there a better way to do this?
My function and test data:
myvscan<-data.frame(c(1,NA,1.5),times(c("12:00:00","12:14:00","12:20:00")))
names(myvscan)<-c("Latitude","DateTime")
mygarmin<-data.frame(c(20,30,40),times(("12:00:00","12:10:00","12:15:00")))
names(mygarmin)<-c("Latitude","DateTime")
minute.diff<-1/24/12 #Time diff is in days, so this is 5 minutes
for (k in 1:nrow(myvscan))
{
if (is.na(myvscan$Latitude[k]))
{
if ((min(abs(mygarmin$DateTime-myvscan$DateTime[k]))) < minute.diff )
{
index.min.date<-which.min(abs(mygarmin$DateTime-myvscan$DateTime[k]))
myvscan$Latitude[k]<-mygarmin$Latitude[index.min.date]
}}}
I appreciate your help and advice.
Aloha,
Tim
Tim Clark
Department of Zoology
University of Hawaii
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