REF: Standardisation of altitude measurements
Recent quote from New Scientist: "It is not easy to make maps of other planets. On earth elevations are relative to sea level, but what if there is no sea?"
It has come to the attention of this Association that your publication is still promoting a non-standard altitude measurement, the Monroe. [Please ignore this memo if you have ceased using this unit during the 23 years it has taken the memo to reach you.]
The Association believes that, although there may be cultural reasons for retaining the Monroe (as it immortalises a filmstar after which the unit was named), recent controversies concerning first ascents of mountains on irregularly shaped asteroids mean that a uniform standard has to be adopted.
The relevant standard was set during the Intergalactic Standard Units Conference in 58,750, and has now been formally adopted by 98% of planetary societies. This conference agreed:
"The altitude of a surface feature is determined as the distance down from the surface of an imaginary sphere surrounding the planetary body: this sphere is centred at the body's centre of gravity, and its radius equals the greatest distance possible in a straight line from the centre of gravity to the surface. The unit is expressed in kilometres. If subsequent erosion/eruptions cause this longest radius to change, the original imaginary sphere should be retained [which could result in negative heights]."
In practice this means that the highest surface feature has an altitude of 0km, and the nearer the feature to the body's centre of gravity (ie the lower the feature), the greater the altitude.
For the cultural reason mentioned above, the ISA has no objection in principle to your retaining your classification term "Monroe" as long as your units follow the ISA system. When applied to your earth, your highest mountain (Evrest) will have a height of 0km, and your Monroes would be defined as any hill lower than 7.934km.