How to measure the voltage drop between 2 points separated by a long distance?
Unfortunately, there is no way to do it without a wire.

You obviously can't use ground to bring your reference point. Because you are measuring voltage differences between the ground points, you know that ground voltage is not uniform.

Air can be used to transmit waves (conveying wireless data), but it can't be used to transmit an absolute voltage either.

So you can't use air, you can't use ground. You have to use a wire.

And since you seem to measure very low voltages, be careful regarding the wire section of this cable. Because it is very long, the current that the probe creates through this wire (although very low) may create a significant voltage drop that may affect the measurement (due to the wire resistance):


Check the AWG of the cable
Look at this table to get the resistance/1000m
Calculate the total resistance depending on the wire length
Get the probe leakage current through the wire (given in probe datasheet, or measure it using an ammmeter)
Use ohm's law to check the resulting voltage drop is well below the voltages you actually measure.