# [GRASS-dev] New module v.surf.trend but with external dependencies and ccmath problem

Jarek Jasiewicz jarekj at amu.edu.pl
Tue May 4 10:59:17 EDT 2010

```Hi all!

I wrote new GRASS module, v.surf.trend I think it could be useful.

v.surf.trend allow to fit set of points to trend surface at any degree
of polynomial (in fact if more than 6 has any  sense?)
Module v.surf.trend is ready to use (there remained some cosmetics) but
is based on external dependencies: GNU Scientific library. It uses
linear multifit with singular value matrix decomposition
(gsl_multifit_linear). As I understood above this is not especially
acceptable.

According some advice I decided to use function from ccmath and here is
the problem.

There are two approach which allow to use multifit in ccmath:

function qrlsq which uses qr decomposition to solve linear least squares
fitting and function lsqsv with singular value matrix decomposition

I assume that results shall be similar to that from gsl. In fact it is
absolutely not.

I prepared testing version of the module. Is someone can help it is
here: http://heretic.livenet.pl/heretic/v.surf.trend.tar.gz
(may be some problems with server)

To compile it gsl is necessary

This module performs calculating trend coefs with three methods:

1) gsl_multifit_linear gives perfect result up to 16 degree polynomial
(and probably more but I stopped testing here). These results are in
conicidence with result of trend from ArcGIS spatial analist

2) ccmath qrlsq gives perfect result to 3th degree of polynomial and
good result on 4th degree but above (5 and more) coef are completely
different from gsl and results are similar to good but with artifacts
(see output)

3) ccmath svdlsq gives completely wrong results if degree of polynominal
is greater than 2

if someone can help there only calc.c file is important: ccmath.c has
function from ccmath library, vector.c process input vector file there
is no help however  and there are some  issues like lack of 3D and WHERE
support

or mabye there is different way to solve that problem. In general it
would be nice to create that module without addational dependency

regards
Jarek

BTW:

there are two flags in current version -r and -s with no flag output is
from gsl with -r  output is from qr matrix reduction with -s flag svd
matrix reduction

```