# Trouble getting started with Dierckx 2D Spline
**URL:** https://discourse.julialang.org/t/trouble-getting-started-with-dierckx-2d-spline/54995
**Category:** General Usage
**Tags:** splines
**Created:** [February 10, 2021, 2:05pm UTC](https://discourse.julialang.org/t/trouble-getting-started-with-dierckx-2d-spline/54995 "2021-02-10T14:05:27Z")
**Posts on this page:** 2
**Page:** 1
### Author:  [@natemcintosh](https://discourse.julialang.org/u/natemcintosh)
#### Post date: [February 10, 2021, 2:05pm UTC](https://discourse.julialang.org/t/trouble-getting-started-with-dierckx-2d-spline/54995/1 "2021-02-10T14:05:27Z")
### Background
Looking at the documentation for creating [2D splines](https://github.com/kbarbary/Dierckx.jl#2-d-splines),
```julia
Spline2D(x, y, z; w=ones(length(x)), kx=3, ky=3, s=0.0)
Spline2D(x, y, z; kx=3, ky=3, s=0.0)
```
- Fit a 2-d spline to the input data. `x` and `y` must be 1-d arrays.
If `z` is also a 1-d array, the inputs are assumed to represent unstructured data, with `z[i]` being the function value at point `(x[i], y[i])` . In this case, the lengths of all inputs must match.
If `z` is a 2-d array, the data are assumed to be gridded: `z[i, j]` is the function value at `(x[i], y[j])` . In this case, it is required that `size(z) == (length(x), length(y))` . (Note that when interpreting `z` as a matrix, `x` gives the row coordinates and `y` gives the column coordinates.)
### Question starts here
So I am trying the first version where `z` is a 1-d array
```julia
julia> x = rand(5)*20; y = rand(5)*20; z = x .+ y;
julia> spl = Spline2D(x, y, z)
iopt,kx,ky,m= 0 3 3 5
nxest,nyest,nmax= 8 8 8
lwrk1,lwrk2,kwrk= 702 305 6
xb,xe,yb,ye= 0.71300324355841216 17.830219487302173 5.2433496228050380E-002 7.5073549961214914
eps,s 9.9999999999999998E-017 0.0000000000000000
ERROR: Error on entry, no approximation returned. The following conditions
must hold:
xb<=x[i]<=xe, yb<=y[i]<=ye, w[i]>0, i=0..m-1
If iopt==-1, then
xb0, i=0..m-1` and `iopt = 0`, so the if condition is not met. I thought maybe I need to sort my inputs, so I tried that as well.
```julia
julia> x = sort(rand(5))*20; y = sort(rand(5))*20; z = x .+ y;
julia> spl = Spline2D(x, y, z)
iopt,kx,ky,m= 0 3 3 5
nxest,nyest,nmax= 8 8 8
lwrk1,lwrk2,kwrk= 702 305 6
xb,xe,yb,ye= 6.3079216616346878 14.655827098859264 2.7957778783432774 16.638679432642004
eps,s 9.9999999999999998E-017 0.0000000000000000
ERROR: Error on entry, no approximation returned. The following conditions
must hold:
xb<=x[i]<=xe, yb<=y[i]<=ye, w[i]>0, i=0..m-1
If iopt==-1, then
xb x = [1,2,3]; y = [4,5,6]; z = x .+ y;
julia> spl = Spline2D(x, y, z)
iopt,kx,ky,m= 0 3 3 3
nxest,nyest,nmax= 8 8 8
lwrk1,lwrk2,kwrk= 686 305 4
xb,xe,yb,ye= 4.0000000000000000 6.0000000000000000 1.0000000000000000 3.0000000000000000
eps,s 9.9999999999999998E-017 0.0000000000000000
ERROR: Error on entry, no approximation returned. The following conditions
must hold:
xb<=x[i]<=xe, yb<=y[i]<=ye, w[i]>0, i=0..m-1
If iopt==-1, then
xb
### Author:  [@rafael.guerra](https://discourse.julialang.org/u/rafael.guerra)
#### Post date: [February 10, 2021, 10:02pm UTC](https://discourse.julialang.org/t/trouble-getting-started-with-dierckx-2d-spline/54995/2 "2021-02-10T22:02:42Z")
@natemcintosh, some possible issues in your two attempts. For the first: too few input points for bi-cubic (default) spline computations and possibly an inadequate error tolerance for the linear input data; for the second attempt: the sorted data defines a linear feature in the 2D plane, probably a ill-conditioned problem for 2D splines (plus the error tolerance issue).
Produced a working example below (_ps: used surface for a better display_)
```julia
using Dierckx, Plots
x = 2*rand(100); y = rand(100);
z = (x .- 1).^2 .+ y.^2
spl = Spline2D(x, y, z; kx=3, ky=3, s=1e-4)
xg = LinRange(0,2,200); yg = LinRange(0,1,200);
zspl = evalgrid(spl, xg, yg)
surface(xg, yg, zspl'; legend=:false, xl="x", yl="y", zl="z")
scatter!(x, y, z; ms=4, mc=:red, ma=0.6, title="Dierckx Spline2D",titlefont=12)
# xmesh = ones(200)'.*xg
# ymesh = yg'.*ones(200)
# scatter(xmesh, ymesh, zspl; ms=0, mc=:cyan, legend=:false)
```
