| Re: Matching Lats and Lons from two arrays [message #62140 is a reply to message #62139] |
Tue, 26 August 2008 11:42   |
Conor
Messages: 138
Registered: February 2007
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Senior Member |
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On Aug 26, 2:41 pm, Conor <cmanc...@gmail.com> wrote:
> On Aug 26, 11:47 am, wilsona <awils...@bigred.unl.edu> wrote:
>
>
>
>> I have 2 seperate arrays of Latittudes and Longitudes.
>> CS_LATLON(0,4607) is one latitude array and dlat(192,139) is the
>> other
>> CS_LATLON(1,4607) is one longitude array and dlon(192,139) is the
>> other.
>> I want to index through each element in both CS_LATLON arrays and
>> find
>> which point(s) in the dlat and dlong arrays are closest.
>> I tried using nested loops but that gave me 12 million+ loops which
>> is
>> too many for my liking. I now am trying search2d
>> NUM_PNTS = N_ELEMENTS(CS_LATLON(0, *)) - 1
>
>> FOR J = 0, NUM_PNTS DO BEGIN
>> CLOSE_LATS = SEARCH2D(dlat, 0, 0, CS_LATLON(0,J),
>> CS_LATLON(0,J), INCREASE=0.5, $
>> DECREASE=0.5)
>> lat1 = CS_LATLON(0,J) * PI / 180.0
>> FOR K = 0, CLOSE_LATS DO BEGIN
>> lat2 = dlat(K) * PI / 180.0
>> d_long = CS_LATLON(1,J) - dlon(K)) * PI / 180.0
>> DISTANCE = 10800.0 / PI * acos(sin(lat1) * sin(lat2)
>> +
>> cos(lat1) * $
>> cos(lat2) * cos(d_long))
>> ENDFOR ; K
>> ENDFOR ; J
>> This is not working they way I would like. Any suggestions on this
>> would be greatly appreciated.
>
> I often have to match up two star fields, which is pretty much the
> same thing. You can download the routine I use here:
>
> astro.ufl.edu/~cmancone/pros/qfind.pro
>
> Here's the source. It basically just uses histogram to bin everyhing
> and then searches one bin left and right:
>
> function qfind,x1,y1,x2,y2,posshift=posshift
>
> if n_elements(posshift) eq 0 then posshift = 1
>
> n1 = n_elements(x1)
> n2 = n_elements(x2)
>
> ; this is where I'll store the result
> res = lonarr(2,n1)
>
> ; this mask list will tell us if an entry is from list one or list two
> allinds = lindgen(n2)
>
> ; the histogram will tell us how many stars left and right we have to
> check
> hist = histogram(x2,binsize=posshift,omin=minx,reverse_indices=ri)
>
> ; calculate which bin each x went into
> bins = floor((x1-minx)/posshift)>0
> nbins=n_elements(hist)
>
> f = 0L
> for i=0L,n1-1 do begin
> ; figure out which bin this star is in
> bin = bins[i]
>
> ; adjust the indexes accordingly
> inds = ri[ri[(bin-1)>0]:ri[((bin+2)<nbins)]-1]
>
> ; calculate the distance from this star to its neighbors
> dist = sqrt( (x2[inds]-x1[i])^2 + (y2[inds]-y1[i])^2 )
>
> ; get the closest star within posshift that is from the second list
> mindist = min( dist, wm )
>
> ; no stars found
> if mindist gt posshift then continue
>
> ; record result
> res[*,f] = [i,inds[wm]]
> ++f
> endfor
>
> if f eq 0 then return,-1
>
> ; get rid of any blank entries
> res = res[*,0:f-1]
>
> return,res
>
> end
I asked this same question before. You might find the discussion
useful.
http://groups.google.com/group/comp.lang.idl-pvwave/browse_t hread/thread/629cbb2a852c5371/b568d74f6d539b79?hl=en&lnk =gst&q=That+works+well+enough%2C+but+is+certainly+not+op timal.++It+uses+the#b568d74f6d539b79
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