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The camera appears to be directly over the center line and presumably you know the horizontal and vertical scale (or use distace between the other lines) so think of a triangle 3m high and with a base of the horizontal distance then use trig functions to work out the angle between them and hence their spacing.
A general solution that doesn't depend on a specific camera position and lens properties would be a calibration grid placed on the playground. Then determine a mapping function (e.g. 2-D look-up table) that calculates x-y coordinates for pixel positions.