| Find Closest Coincident Measurements In Time And Space Between Two Data Sets [message #62909] |
Mon, 13 October 2008 05:14  |
r083r7
Messages: 6
Registered: March 2007
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Junior Member |
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I was hoping that somebody could help and apply some magic to this
problem.
I have two sets of satellite measurement data and for dataset A want
to find the value in dataset B that are closest to it in time and
space.
If that's easy enough to speed it it'd also be useful to find ALL
values in databaset B that were within a certain spatial and temporal
distance of each point in dataset A (for example, 5deg lat/lon and 2
hours).
As both datasets are quite large I have a feeling that this problem is
quite similar to the one found here http://www.dfanning.com/code_tips/slowloops.html
but I don't think I understand it quite well enough to solve and
eliminate the FOR loops.
My (working but slow) code is below.
Any help would be great.
Cheers
;datasetA structure contains lat, long, time and values
;datasetB has lat, long, time and value arrays
count=0L
final_lat=fltarr(100000)
final_lon=fltarr(100000)
final_datasetA=fltarr(100000)
final_datasetB=fltarr(100000)
final_time=strarr(100000)
final_date=strarr(100000)
;value in hms of datasetB meas. either side of datasetA meas. time
where time is still considered coincident
time_margin=010000
;value in degrees of datasetB meas. either side of datasetA meas.
location where location is still considered coincident
space_margin=2. ;degrees lat/long
print, ‘Starting to loop through dataset A to find coincident dataset
B points'
FOR i=0L, n_elements(datasetA.lats)-1 DO BEGIN
;find measurement in datasetB that is within time and space margin of
datasetA
coin=where(datasetB_hms GE datasetA.time[i]-time_margin AND
datasetB_hms LE datasetA.time[i]+time_margin AND $
datasetB_lat GE datasetA.lats[i]-space_margin AND
datasetB_lat LE datasetA.lats[i]+space_margin AND $
datasetB_lon GE datasetA.lons[i]-space_margin AND
datasetB_lon LE datasetA.lons[i]+space_margin)
;account for no coincident matches between datasets
if coin[0] NE -1 THEN BEGIN
final_lat[count]=datasetA.lats[i]
final_lon[count]=datasetA.lons[i]
final_datasetA[count]=datasetA.value[i]
;store mean of datasetB values that are coincident to datasetA
measurments
final_datasetB[count]=mean(datasetB_values[coin])
final_time[count]=datasetA.time[i]
final_date[count]=datasetA.date
count=count+1L
END
ENDFOR
print, 'Finished looping through datasetA data to find coincident
datasetB data'
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