Best way to measure liquid level of a 10 meter high tank? I would like to measure the height at which the liquid inside a 10 meter high tank, what is the best way to achieve it?

What sensor should i use? I have seen ultrasound sensors but I have not found one that covers 10 meters. In context, I need to measure the volume of the liquid. 

I don't know whether to measure the distance between the bottom and the height of the liquid, or measure the vacuum. 

Example: If a 10 meter tank has 4 meters of liquid, you could calculate its volume by:

1) The maximum height at which the liquid reaches from the bottom to the top. Thus, it is estimated that from the bottom to the top there are 4 meters of liquid.

2) The distance at which the mouth of the tank touches the water. Thus, it is estimated that the tank is empty 6 meters and the rest is liquid. Could you instruct me a little? Thank you.

Edit: I add that the tank is fuel and that my budget does not exceed $ 20. Besides that I can not make modifications to the tank at my whim. Currently, to measure the height, a measuring tape attached to a lead is used so that it falls to the bottom of the tank, when the measuring tape is raised, the height at which the liquid wets the tape is seen. Hence the fuel level is obtained.
I don't know what is the “best”, and in fact I would be interested in
knowing the answer. So far the best option I found seems to be a
submersible level sensor (do a Web search for these words). These are
pressure sensors that you put at or near the bottom of the tank. They
give you a reading proportional to the height of water above the level
of the sensor.

Most of these sensors seems to give an output in the form of a
4&nbsp;–&nbsp;20&nbsp;mA current. They are have only two wires and
expect a supply somewhere in the range of 24&nbsp;V. You will then need
a suitable supply and some electronics to convert that to an analog
voltage that your Arduino can read. A simple resistor would be enough
but you probably want something that provides some safety in case the
sensor fails short. You can search this site for "4-20 mA", as the topic
of interfacing these kind of sensors has already been covered here
multiple times.