Painting in a North-South Direction

The eight nearby tennis courts were resurfaced last week. Gone are the cracks and the humps and hillocks that gave them character, and instead an emerald green field is sparkling in the sunshine, waiting for the local Federer and Sharapova to start hitting again.

But in an unexpected twist, the repainting of the lines on the courts has taken longer than expected. Last week painter no 1 did all the north-south lines, so each court looked as shown left in this artist's impression.

And this week, the east-west specialist was called in to paint the other lines. So why not paint all the lines on half of the courts and let the people play a week earlier? Or does the line painting require specialised equipment that doesn't operate in all directions? Is it sensitively calibrated against the earth's magnetic field and cannot cross through lines of flux?