| Re: Interpolation of missing data [message #10749 is a reply to message #10746] |
Tue, 20 January 1998 00:00  |
Martin Schultz
Messages: 515
Registered: August 1997
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Senior Member |
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R. Kyle Justice wrote:
>
> I have a 2-D array with missing data. Is there an easy way to
> interpolate the missing values?
>
> I would like to replace a missing value with the average of
> its neighbors.
>
> Kyle J.
This may not be exactly what you want, but you could try to
"re-sample" your data as an array and use the TRI_SURF (or
MIN_CURVE_SURF) function. I have a piece of code that does something
like
; create x and y vectors that match with zz array
goodx = findgen(nx+3)/(nx+2)*(xrange(1)-xrange(0))+xrange(0)
goody = findgen(ny+3)/(ny+2)*(yrange(1)-yrange(0))+yrange(0)
; little trick to get the indices of valid zz's in goodx and goody
xind = (ind mod (nx+2))
yind = (ind/(nx+2)) ; integer division !!
newx = reform(goodx(xind),n_elements(ind))
newy = reform(goody(yind),n_elements(ind))
zzz = TRI_SURF(newz,newx,newy,gs=[dx,dy], $
bounds=[xrange(0),yrange(0),xrange(1),yrange(1)] )
(for regular readers: this turned out to be the best solution to my
contour problem that I described earlier - but I must warn of the use
of MIN_CURVE_SURF: it takes *forever* [i.e. I did not want to wait more
than 3 minutes for a data set of ~1000 points and interrupted])
Regards,
Martin
PS: another solution (which would involve a loop [nasty word ;-)])
would be to compute the averages of surrounding grid boxes like
ind = where(data eq MISSING) ; supply your code for missing data
if (ind(0) ge 0) then begin
for i=0,n_elements(ind)-1 do begin
x = (i mod (NX+2)) ; get indices in data array
y = (i/(NX+2)) ; integer division !!
; create index array for neighbouring points
xind = [ x-1>(-1), x, x+1<NX, x ]
yind = [ y, y-1>(-1), y, y+1<NY ]
; find out valid neighbours
ok = where(xind ne MISSING and yind ne MISSING)
if (ok(0) ge 0) then $
data(x,y) = total(data(xind(ok),yind(ok))/ $
float(n_elements(ok))
endfor
endif
This would of course only work if at least one neighbour is a valid
data point. In case you are not familiar with the < and > operators:
they are great to limit value ranges, I just recently understood them
and loved them immediately! Please NOTE: I did not test this code,
but it should give you something to start with at least.
------------------------------------------------------------ -------
Dr. Martin Schultz
Department for Earth&Planetary Sciences, Harvard University
186 Pierce Hall, 29 Oxford St., Cambridge, MA-02138, USA
phone: (617)-496-8318
fax : (617)-495-4551
e-mail: mgs@io.harvard.edu
IDL-homepage: http://www-as.harvard.edu/people/staff/mgs/idl/
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