##### Pointwise interpolate irregular gridded data

This function implements bivariate interpolation onto a set of points for irregularly spaced input data.

This function is meant for backward compatibility to package
`akima`

, please use `interp`

with its `output`

argument set to `"points"`

now.

- Keywords
- spatial

##### Usage

```
interpp(x, y = NULL, z, xo, yo = NULL, linear = TRUE,
extrap = FALSE, duplicate = "error", dupfun = NULL,
deltri = "shull")
```

##### Arguments

- x
vector of x-coordinates of data points or a

`SpatialPointsDataFrame`

object. Missing values are not accepted.- y
vector of y-coordinates of data points. Missing values are not accepted.

If left as NULL indicates that

`x`

should be a`SpatialPointsDataFrame`

and`z`

names the variable of interest in this dataframe.- z
vector of z-coordinates of data points or a character variable naming the variable of interest in the

`SpatialPointsDataFrame`

`x`

.Missing values are not accepted.

`x`

,`y`

, and`z`

must be the same length (execpt if`x`

is a`SpatialPointsDataFrame`

) and may contain no fewer than four points. The points of`x`

and`y`

cannot be collinear, i.e, they cannot fall on the same line (two vectors`x`

and`y`

such that`y = ax + b`

for some`a`

,`b`

will not be accepted).- xo
vector of x-coordinates of points at which to evaluate the interpolating function. If

`x`

is a`SpatialPointsDataFrame`

this has also to be a`SpatialPointsDataFrame`

.- yo
vector of y-coordinates of points at which to evaluate the interpolating function.

If operating on

`SpatialPointsDataFrame`

s this is left as`NULL`

- linear
logical -- indicating wether linear or spline interpolation should be used.

- extrap
logical flag: should extrapolation be used outside of the convex hull determined by the data points? Not possible for linear interpolation.

- duplicate
indicates how to handle duplicate data points. Possible values are

`"error"`

- produces an error message,`"strip"`

- remove duplicate z values,`"mean"`

,`"median"`

,`"user"`

- calculate mean , median or user defined function of duplicate z values.- dupfun
this function is applied to duplicate points if

`duplicate="user"`

- deltri
triangulation method used, this argument will later be moved into a control set together with others related to the spline interpolation!

##### Value

a list with 3 components:

If `output="grid"`

:
vectors of \(x\)- and \(y\)-coordinates of output grid, the same
as the input
argument `xo`

, or `yo`

, if present. Otherwise, their
default, a vector 40 points evenly spaced over the range of the
input `x`

and `y`

.

If `output="points"`

: vectors of \(x\)- and \(y\)-coordinates of
output points as given by `xo`

and `yo`

.

If `output="grid"`

:
matrix of fitted \(z\)-values. The value `z[i,j]`

is computed
at the point \((xo[i], yo[j])\). `z`

has
dimensions `length(xo)`

times `length(yo)`

.

If `output="points"`

: a vector with the calculated z values for
the output points as given by `xo`

and `yo`

.

If the input was a `SpatialPointsDataFrame`

a
`SpatialPixelssDataFrame`

is returned for `output="grid"`

and a `SpatialPointsDataFrame`

for `output="points"`

.

##### Note

This is only a call wrapper meant for backward compatibility, see
`interp`

for more details!

##### References

Moebius, A. F. (1827) Der barymetrische Calcul. Verlag v. Johann Ambrosius Barth, Leipzig, https://books.google.at/books?id=eFPluv_UqFEC&hl=de&pg=PR1#v=onepage&q&f=false

Franke, R., (1979). A critical comparison of some methods for interpolation of scattered data. Tech. Rep. NPS-53-79-003, Dept. of Mathematics, Naval Postgraduate School, Monterey, Calif.

##### See Also

##### Examples

```
# NOT RUN {
### Use all datasets from Franke, 1979:
### calculate z at shifted original locations.
data(franke)
for(i in 1:5)
for(j in 1:3){
FR <- franke.data(i,j,franke)
IL <- with(FR, interpp(x,y,z,x+0.1,y+0.1,linear=TRUE))
str(IL)
}
# }
```

*Documentation reproduced from package interp, version 1.0-33, License: GPL (>= 2)*