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The GCV is an estimate of the interpolation error obtained by removing each data point in turn and fitting a surface to the remaining data to see how well each omitted point can be predicted . The surface hardness of the composite would determine whether there is a tendency to pick up dust , in which case a surface coating might be needed . When a surface plot is being done without the removal of hidden lines , the value of samples specifies the number of samples that are to be evaluated for the isolines . Such low level flow is common when a surface low moves south or east of this region , which occurred in all three tornado cases .