Polar Data
2-grid vector maps created with polar data require that one grid contains angle (direction) data and the other contains length (magnitude) data.
Negative Data and Polar Grids
Magnitude values cannot be negative. If you have chosen a gridding technique that displays trends in the data, such as Kriging, you may end up with negative values in your grid even if the original data are all positive.
When creating a grid, you can use the Grid Z Limits section of the Grid Data dialog to limit the grid Z values to a specific range. Set the Minimum option to Custom and the value to 0 to ensure negative values are not created in the gird. The Grid Z Limits options can be used with any Gridding Method.
Alternatively, regridding the magnitude data and decreasing the grid spacing is one way to make the grid honor the data better. If you regrid the magnitude data you must also regrid the angle data using the same output grid geometry. If you do not want to change the output grid geometry of the grids or if decreasing the grid nodes is not completely eliminating the negative data, then you can use Grid | Math to set all negative values equal to zero. Another alternative is to use a gridding method that does not trend the data as much (Triangulation with Linear Interpolation or Natural Neighbor).