# How to find crossing point using Spline1D in julia?

**URL:** <https://discourse.julialang.org/t/how-to-find-crossing-point-using-spline1d-in-julia/71364>\
**Category:** Numerics\
**Tags:** question, package, splines\
**Created:** [November 12, 2021, 7:55am UTC](https://discourse.julialang.org/t/how-to-find-crossing-point-using-spline1d-in-julia/71364 "2021-11-12T07:55:12Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Luigi\_Marongiu](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/luigi_marongiu/32/7909_2.png) [@Luigi\_Marongiu](https://discourse.julialang.org/u/Luigi_Marongiu)\
**Post date:** [November 12, 2021, 7:55am UTC](https://discourse.julialang.org/t/how-to-find-crossing-point-using-spline1d-in-julia/71364/1 "2021-11-12T07:55:12Z")

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I have fitted a spline function using Spline1D from the package Dierckx to the data:

```julia
x = [0.0, 0.25167322597331027, 1.6518531853618088, 4.032506892599284, 6.871304792339401, 10.090447889235595, 13.686254426579751, 17.610747596745075, 21.841405037619722, 26.366938232563164, 31.26706047764439, 37.49545103557282, 42.61189318926405, 48.0]
y = [50000.0, 53623.40794496114, 79163.0777300414, 153678.85449922792, 339502.75898037956, 835557.6724498648, 2.2886514929222167e6, 6.875522505010054e6, 2.239578715528581e7, 7.732681212840894e7, 2.691022277972607e8, 8.835297472095971e8, 1.4420590677965815e9, 1.7488564860223794e9]
spl = Spline1D(x, y, k=3)

```

I would like to know the abscissa value that gets an ordinate of let’s say 100 000. I tried with:

```julia
julia> fit_x = Spline1D(x, y)(100000)
1.7488564860223794e9

```

but it is obviously wrong. Is there an inverse function for spline and how to implement it in julia?

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<div class="post-metadata">

**Author:** ![rafael.guerra](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/rafael.guerra/32/216610_2.png) [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)\
**Post date:** [November 12, 2021, 8:37am UTC](https://discourse.julialang.org/t/how-to-find-crossing-point-using-spline1d-in-julia/71364/2 "2021-11-12T08:37:09Z")

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I think you were already provided with a solution [here](https://discourse.julialang.org/t/find-intersection-between-curves-in-julia/70494/4).  
But here it goes again:

```julia
y0 = 100_000
spl = Spline1D(x, y .- y0)
x0 = roots(spl) # 2.49089

```

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<div class="post-metadata">

**Author:** ![Luigi\_Marongiu](https://sea2.discourse-cdn.com/julialang/user_avatar/discourse.julialang.org/luigi_marongiu/32/7909_2.png) [@Luigi\_Marongiu](https://discourse.julialang.org/u/Luigi_Marongiu)\
**Post date:** [November 12, 2021, 9:38am UTC](https://discourse.julialang.org/t/how-to-find-crossing-point-using-spline1d-in-julia/71364/3 "2021-11-12T09:38:08Z")

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Yes, sorry, I overlooked the subtraction term… It worked!