Plotting irregularly-spaced vectors on a map

I want to plot 40 irregularly-spaced vectors on a map of N America.

I would be happy to plot them one at a time if I could find a routine to
do that but all the routines I have found (eg gsn_csm_vector_map) want
data on a lat-lon array with a minimum length of 3 in each direction, so
I tried setting up a 2-D array of data, with zero magnitude except where
I want the arrows.
I split it up into sets of 4 and after much trial, I got 5 sets to plot
ie 20 of the vectors.

It keeps giving me errors
" warning:_NhlCreateSplineCoordApprox: Attempt to create spline
approximation for X axis failed: consider adjusting trXTensionF value
warning:_NhlCreateSplineCoordApprox: Attempt to create spline
approximation for Y axis failed: consider adjusting trYTensionF value
warning:IrTransInitialize: error creating spline approximation for
trXCoordPoints; defaulting to linear
warning:IrTransInitialize: error creating spline approximation for
trYCoordPoints; defaulting to linear"

I have seen that other people have asked the same sort of question on
ncl-talk but apart from a suggestion to try changing the values of
trXTensionF and trYTensionF (I tried both large and small values) or
plotting subsets of the data (which I tried as I mentioned above), I
have not found a helpful answer.

Any suggestions appreciated!
Rosemarie Drummond

-- 
Dr Rosemarie Drummond
rmarie_at_atmosp.physics.utoronto.ca
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