Aswan Nilometer is 7000 years old, these Nilometer were used to measure the level of the water on the Nile River, they were also used to predict the level of the water in the next season, these predictions also helped to calculate the local farmer taxes.
In Aswan on the Elephantine Island, there are two Nilometers that date back to the roman era, one at the temple of Khnum and one at the temple of Satet, owing to the fact that Elephantine Island was the first place where the onset of the annual flood was noticed.
Luckily, the Nilometer was invented to read and predict the level of the water and therefore the fate of the country. The ancient Egyptians almost relied on the Nile in various fields of life, during summer the water of the Nile would rise due to heavy rainfall, and then the Nilometer would be submerged with marked intervals indicating the depth of the water, if its level was too high, there would be a flood. If its level was too low, there would be a drought. Both meant disaster.
Therefore it is not surprising that this high tech invention was kept in temples and only priests and rulers were allowed to access it.
The Nilometers in the Pharaonic era took many different forms, at first it was just a rock located in a suitable place in the Nile, then it took the form of a wooden ladder to read the water level.