NAME

v.surf.spline - GRASS module to interpolate vector contour data by fitting a cubic spline function to profiles in four directions. It has two advantages over other interpolation techniques: (1) The RMS Error can be calculated for EACH CELL in the DEM as well as an overall RMSE. Spatial variation in error can examined; (2) A cubic spline has the property of 2nd derivative being continuous therefore having implications for slope measurements. Terracing effects should be minimised.

Usage:

v.surf.spline[-rs] in=name out=name [interval=value]

Flags:

-r Rasterise contours using rooks case adjacency

-s Constrain interpolation using simple truncation

Parameters:

in vector contour map to be interpolated

out Resultant Digital Elevation Model

interval Contour interval (or 0 for no interval constraint)
default: 0

USAGE RECOMMENDATION:

r.contour input=elevation.dem output=contours.test step=20
v.support option=build map=contours.test
g.region vect=contours.test
v.surf.spline in=contours.test out=dem.test interval=20 -s > /dev/null

SPECIAL NOTE:

If you want to interpolate from self-digitized, avoid an "STEP 2-ERROR" in this way:
The region settings must satisfy the rule, that at least one vector line has to hit the border. Totally at least four vector lines have to hit each of the four regional borders. Check this by displaying your vector contour file and use the d.vect.zoom command eventually. Or you digitize more lines. 
+------+---+                   +------++
|     /    |                   |     / |
+----/     |                   +----/  |
|      -o  |  -> zoom it! ->   |      -+
+-----/    |                   +-----/ |
+----------+                   +-------+
  wrong                          right
The lower line does            This should work
not reach the border.          for v.surf.spline.

NOTE:

Method suggested by Yoeli(1986) and discussed in Wood and Fisher (1993). SEE ALSO r.surf.gauss, r.surf.random, r.surf.idw, r.surf.idw2, r.surf.contour 

REFERENCES

Yoeli, P. (1986) Computer executed production of regular grid of height points from digital contours, The American Cartographer, 13(3), pp.219-229.

Wood, J and Fisher, P (1993) Assessing interpolation accuracy in Elevation Models, IEEE Computer Graphics and Applications, 13(2), pp.48-56.

jwo@le.ac.uk

ASSIST's home