NAG Library Function Document

nag_2d_triangulate (e01eac)

nag_2d_triangulate (e01eac) creates a Thiessen triangulation with a given set of two-dimensional data points as nodes. This triangulation will be as equiangular as possible (Cline and Renka (1984)). See Renka and Cline (1984) for more detailed information on the algorithm, a development of that by Lawson (1977). The code is derived from Renka (1984).

The computed triangulation is returned in a form suitable for passing to nag_2d_triang_bary_eval (e01ebc) which, for a set of nodal function values, computes interpolated values at a set of points.

4 References

Cline A K and Renka R L (1984) A storage-efficient method for construction of a Thiessen triangulation Rocky Mountain J. Math. 14 119–139

Lawson C L (1977) Software for

C^{1}

surface interpolation Mathematical Software III (ed J R Rice) 161–194 Academic Press

Renka R L (1984) Algorithm 624: triangulation and interpolation of arbitrarily distributed points in the plane ACM Trans. Math. Software 10 440–442

Renka R L and Cline A K (1984) A triangle-based

C^{1}

interpolation method Rocky Mountain J. Math. 14 223–237

5 Arguments

1: $n$ – IntegerInput: On entry: $n$ , the number of data points.

Constraint: $n \geq 3$ .
2: $x [n]$ – const doubleInput: On entry: the $x$ coordinates of the $n$ data points.
3: $y [n]$ – const doubleInput: On entry: the $y$ coordinates of the $n$ data points.
4: $triang [7 \times n]$ – IntegerOutput: On exit: a data structure defining the computed triangulation, in a form suitable for passing to nag_2d_triang_bary_eval (e01ebc). Details of how the triangulation is encoded in triang are given in Section 9. These details are most likely to be of use when plotting the computed triangulation which is demonstrated in Section 10.
5: $fail$ – NagError *Input/Output: The NAG error argument (see Section 3.6 in the Essential Introduction).

6 Error Indicators and Warnings

NE_ALL_DATA_COLLINEAR: On entry, all the $(x, y)$ pairs are collinear.
NE_ALLOC_FAIL: Dynamic memory allocation failed.
See Section 3.2.1.2 in the Essential Introduction for further information.
NE_BAD_PARAM: On entry, argument $〈value〉$ had an illegal value.
NE_INT: On entry, $n = 〈value〉$ .
Constraint: $n \geq 3$ .
NE_INTERNAL_ERROR: An internal error has occurred in this function. Check the function call and any array sizes. If the call is correct then please contact NAG for assistance.

An unexpected error has been triggered by this function. Please contact NAG.
See Section 3.6.6 in the Essential Introduction for further information.
NE_NO_LICENCE: Your licence key may have expired or may not have been installed correctly.
See Section 3.6.5 in the Essential Introduction for further information.

7 Accuracy

Not applicable.

8 Parallelism and Performance

Not applicable.

9 Further Comments

The time taken for a call of nag_2d_triangulate (e01eac) is approximately proportional to the number of data points,

n

. The function is more efficient if, before entry, the

(x, y)

pairs are arranged in x and y such that the

x

values are in ascending order.

The triangulation is encoded in triang as follows:

set j0=0; for each node, k=1,2,…,n, (using the ordering inferred from x and y)
- $i_{k} = j_{k - 1} + 1$
- $j_{k} = triang [6 \times n + k - 1]$
- $triang [j - 1]$ , for $j = i_{k}, \dots, j_{k}$ , contains the list of nodes to which node $k$ is connected. If $triang [j_{k} - 1] = 0$ then node $k$ is on the boundary of the mesh.

10 Example

In this example, nag_2d_triangulate (e01eac) creates a triangulation from a set of data points. nag_2d_triang_bary_eval (e01ebc) then evaluates the interpolant at a sample of points using this triangulation. Note that this example is not typical of a realistic problem: the number of data points would normally be larger, so that interpolants can be more accurately evaluated at the fine triangulated grid.

NAG Library Function Documentnag_2d_triangulate (e01eac)

▸▿ Contents

1 Purpose

2 Specification

3 Description