interpp {akima} | R Documentation |
If ncp
is zero, linear
interpolation is used in the triangles bounded by data points.
Cubic interpolation is done if partial derivatives are used.
If extrap
is FALSE
, z-values for points outside the convex hull are
returned as NA
.
No extrapolation can be performed if ncp
is zero.
The interpp
function handles duplicate (x,y)
points in different ways. As default it will stop with an error message. But
it can give duplicate points an unique z
value according to the
parameter duplicate
(mean
,median
or any other user defined function).
The triangulation scheme used by interp
works well if x
and y
have
similar scales but will appear stretched if they have very different
scales. The spreads of x
and y
must be within four orders of magnitude
of each other for interpp
to work.
interpp(x, y, z, xo, yo, ncp=0, extrap=F)
x |
vector of x-coordinates of data points. Missing values are not accepted. |
y |
vector of y-coordinates of data points. Missing values are not accepted. |
z |
vector of z-coordinates of data points.
Missing values are not accepted.
|
xo |
vector of x-coordinates of points at which to evaluate the interpolating function. |
yo |
vector of y-coordinates of points at which to evaluate the interpolating function. |
ncp |
number of additional points to be used in computing partial
derivatives at each data point.
ncp must be either 0 (partial derivatives are not used, =
linear interpolation), or at
least 2 but smaller than the number of data points (and smaller than 25).
|
extrap |
logical flag: should extrapolation be used outside of the convex hull determined by the data points? |
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"
|
x |
vector of x-coordinates of output points, the same as the input
argument xo .
|
y |
vector of y-coordinates of output points, the same as the input
argument yo .
|
z |
fitted z-values. The value z[i] is computed
at the x,y point x[i], y[i] .
|
Use interp
if interpolation on a regular grid is wanted.
The two versions interpp.old
and interpp.new
refer to
Akimas Fortran code from 1978 and 1996 resp. At the moment
interpp.new
does not work porperly (it results in a segmentation fault), so it is
not used from the call wrapper interp
.
Akima, H. (1978). A Method of Bivariate Interpolation and Smooth Surface Fitting for Irregularly Distributed Data Points. ACM Transactions on Mathematical Software, 4, 148-164.
Akima, H. (1996). Algorithm 761: scattered-data surface fitting that has the accuracy of a cubic polynomial. ACM Transactions on Mathematical Software, 22, 362-371.
contour
, image
, approx
, spline
, outer
, expand.grid
,interp
.
data(akima) # linear interpolation at points (1,2), (5,6) and (10,12) akima.lip<-interpp(akima$x, akima$y, akima$z,c(1,5,10),c(2,6,12))