Why is this load resistor connected this way and what could be its value?
My guess:

The potentiometer is not a full 360&deg; to prevent the wiper short-circuiting X to Y. This means that when the wind is from the west the wiper will not be in contact with the resistance track and so will be floating. This could lead to false readings so the designers have a pull-down resistor so that the wiper will appear to be at Y while in the dead zone.

Channel 3 is measuring the voltage at Y and this will be slightly higher than GND (> 0 V). If RL were to be connected to GND then that would pull Channel 2's input below the Channel 3 zero reference and the subtraction would result in a negative value for the position (which is always positive with the arrangement shown).


  And what value for RL can be reasonable?


RL is going to pull the wiper voltage away from the potentiometer's divider voltage. This effect will be lower at either end of the wiper because the low impedance of that end of the track will win over the value of RL. The maximum loading effect will be at mid position. You can run some calculations in a spreadsheet for this to see the effect. 50 &times; the pot resistance seems good enough to me (without doing any calculations) to get you within a couple of percent. With wind variations I doubt that accuracy is all that important but I'd be inclined to put the dead-zone opposing the prevailing wind.