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*Subject*: Re: Reverse interpolation?*From*: Pavel Romashkin <promashkin(at)cmdl.noaa.gov>*Date*: Wed, 16 Aug 2000 10:18:06 -0600*Newsgroups*: comp.lang.idl-pvwave*Organization*: NOAA-CMDL-CIRES*References*: <399AAB76.2E547046@mathstat.dal.ca> <399AB853.4291ED25@mathstat.dal.ca>*Reply-To*: promashkin(at)cmdl.noaa.gov*Xref*: news.doit.wisc.edu comp.lang.idl-pvwave:20894

I'd recommend checking out VALUE_LOCATE function. Overall, solving the proportion in a triangle will be the simpliest approach (unless you want to do a more sophisticated spline as suggested by Martin), but VALUE_LOCATE eliminates the need for subtracting and finding near-zero elements of the array. Also, once you have X, there is a library function INTERPOL that will take X for input and locate the interpolated value for you. Cheers, Pavel Simon de Vet wrote: > > Simon de Vet wrote: > > > I understand how to use a subscript to find an interpolated value. How > > would I go in the opposite direction, using a value to find a subscript? > > I have an idea (I used this for a similar problem with Matlab) that may > work. It's ackward and bulky, but I can understand it. > > First, I subtract the known altitude from the altitude list. Some of the > values will be positive, and some negative. I use this to find the points > on either side of known altitude. I can use their subscripts as x-values > and their altitudes as y-values to find the equation of a line connecting > the two points. I can then substitute my known altitude (y value) into the > equation, and solve for x (the required subscript). > > I hope it works. > > Simon

**References**:**Reverse interpolation?***From:*Simon de Vet

**Re: Reverse interpolation?***From:*Simon de Vet

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