Colleagues,
I am analyzing data collected during oceanographic cruises. We have
conducted many cruises over the last decade. On each cruise we visit ~50
stations. At each station (termed EventNum)we lower an instrument that
measures depth, temperature, salinity and oxygen every few seconds as it is
lowered through the water. Data from all EventNums on all cruises are
stacked, generating a data file that might look like
CruiseID Year Season EventNum Region Depth Salinity Temp Oxygen
1 CF0101 2001 Spring 849 Lower 0.056 21.1753 15.5626 8.12344
2 CF0101 2001 Spring 849 Lower 0.103 21.3264 15.5511 8.11873
3 CF0101 2001 Spring 849 Lower 0.131 21.4780 15.5492 8.10568
4 CF0101 2001 Spring 849 Lower 0.169 21.5296 15.5492 8.10633
....
15000 CF0103 2001 Fall 929 Mid 14.449 17.692 22.901 7.743
15001 CF0103 2001 Fall 929 Mid 14.478 17.691 22.900 7.743
15002 CF0103 2001 Fall 929 Mid 14.527 17.692 22.899 7.743
15003 CF0103 2001 Fall 929 Mid 14.595 17.692 22.900 7.743
15004 CF0103 2001 Fall 929 Mid 14.654 17.694 22.902 7.742
15005 CF0103 2001 Fall 929 Mid 14.722 17.695 22.903 7.742
15006 CF0103 2001 Fall 929 Mid 14.790 17.695 22.903 7.742
15007 CF0103 2001 Fall 929 Mid 14.839 17.694 22.903 7.742
15008 CF0103 2001 Fall 929 Mid 14.887 17.695 22.902 7.742
15009 CF0103 2001 Fall 929 Mid 14.946 17.694 22.902 7.742
15010 CF0103 2001 Fall 929 Mid 14.985 17.693 22.902 7.743
......
In total there are ~350k rows of data
I can calculate simple EventNum averages using
meanobs<-aggregate(ctd[,7:9],by=list(EventNum=EventNum),mean)
But, I want to calculate a depth-weighted mean for salinity, temp and oxygen
for each EventNum. The depth-weighted mean is calculated as
Xbar = Sum{depth_interval*temp}/Sum(depth_intervals)
Where the depth interval is the differenced depths (i.e., 0.103-0.056 for
the second row of data).
I cannot seem to get diff to work the way I want it to - advice appreciated
Tom
Thomas J. Miller, Ph.D.
Professor
Chesapeake Biological Laboratory
University of Maryland Center for Environmental Science
Solomons, MD 20688
Tel: (410) 326-7276
Fax: (410) 326-7318
WWW: hjort.cbl.umces.edu
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