how to stop interpolation for cyclical field ?

[Hinnerk Ries]

dear all,
I am plotting isolines of wind direction . This gets ugly if the wind
shifts over N, because grads interpolates all the contour lines from
0deg to 360deg inbetween. So it looks as if the wind was turning over S
instead just from 359deg to 1deg.
I tried to stop grads from interpolating by deviding the direction-field
into to parts and then plot them separately but without success. Has
anybody solved this problem (maybe by telling grads that the field is
cyclical)?

hoping for many answeres, Hinnerk

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Hinnerk Ries

Alfred-Wegener-Institut fuer Polar- und Meeresforschung
Postfach 120161
27515 Bremerhaven
Tel  ++49 (0)471/48311766
Fax  ++49 (0)471/48311797
Mail hries at awi-bremerhaven.de