Change "altitude" to "elevation" in manual

Vanessa finds "altitude" to carry too much of an aviation connotation.
Zefram finds "elevation" to carry insufficient connotation of being
relative to sea level, so also adds a clarifying note.  Beware of changing
this terminology without negotiation, as it aroused surprisingly strong
opinions.

manual.md

@@ -29,58 +29,65 @@ ore
 The technic mod makes extensive use of not just the default ores but also
 some that are added by mods.  You will need to mine for all the ore types
 in the course of the game.  Each ore type is found at a specific range of
-altitudes, and while the ranges mostly overlap, some have non-overlapping
-ranges, so you will ultimately need to mine at more than one altitude
-to find all the ores.  Also, because one of the best altitudes to mine
+elevations, and while the ranges mostly overlap, some have non-overlapping
+ranges, so you will ultimately need to mine at more than one elevation
+to find all the ores.  Also, because one of the best elevations to mine
 at is very deep, you will be unable to mine there early in the game.
 
+Elevation is measured in metres, relative to a reference plane that
+is not quite sea level.  (The standard sea level is at an elevation
+of about +1.4.)  Positive elevations are above the reference plane and
+negative elevations below.  Because elevations are always described this
+way round, greater numbers when higher, we avoid the word "depth".
+
 The ores that matter in technic are coal, iron, copper, tin, zinc,
 chromium, uranium, silver, gold, mithril, mese, and diamond.
 
-Coal is part of the basic Minetest game.  It is found from altitude
-+64 downwards, so is available right on the surface at the start of the
-game, but it is far less abundant above altitude 0 than below.  It is
-initially used as a fuel, driving important machines in the early part
-of the game.  It becomes less important as a fuel once most of your
+Coal is part of the basic Minetest game.  It is found from elevation
++64 downwards, so is available right on the surface at the start of
+the game, but it is far less abundant above elevation 0 than below.
+It is initially used as a fuel, driving important machines in the early
+part of the game.  It becomes less important as a fuel once most of your
 machines are electrically powered, but burning fuel remains a way to
 generate electrical power.  Coal is also used, usually in dust form, as
 an ingredient in alloying recipes, wherever elemental carbon is required.
 
-Iron is part of the basic Minetest game.  It is found from altitude
+Iron is part of the basic Minetest game.  It is found from elevation
 +2 downwards, and its abundance increases in stages as one descends,
-reaching its maximum from altitude -64 downwards.  It is a common metal,
+reaching its maximum from elevation -64 downwards.  It is a common metal,
 used frequently as a structural component.  In technic, unlike the basic
 game, iron is used in multiple forms, mainly alloys based on iron and
 including carbon (coal).
 
 Copper is part of the basic Minetest game (having migrated there from
-moreores).  It is found from altitude -16 downwards, but is more abundant
-from altitude -64 downwards.  It is a common metal, used either on its
+moreores).  It is found from elevation -16 downwards, but is more abundant
+from elevation -64 downwards.  It is a common metal, used either on its
 own for its electrical conductivity, or as the base component of alloys.
 Although common, it is very heavily used, and most of the time it will
 be the material that most limits your activity.
 
-Tin is supplied by the moreores mod.  It is found from altitude +8
-downwards, with no altitude-dependent variations in abundance beyond
+Tin is supplied by the moreores mod.  It is found from elevation +8
+downwards, with no elevation-dependent variations in abundance beyond
 that point.  It is a common metal.  Its main use in pure form is as a
 component of electrical batteries.  Apart from that its main purpose is
 as the secondary ingredient in bronze (the base being copper), but bronze
 is itself little used.  Its abundance is well in excess of its usage,
 so you will usually have a surplus of it.
 
-Zinc is supplied by technic.  It is found from altitude +2 downwards, with
-no altitude-dependent variations in abundance beyond that point.  It is
-a common metal.  Its main use is as the secondary ingredient in brass
-(the base being copper), but brass is itself little used.  Its abundance
-is well in excess of its usage, so you will usually have a surplus of it.
+Zinc is supplied by technic.  It is found from elevation +2 downwards,
+with no elevation-dependent variations in abundance beyond that point.
+It is a common metal.  Its main use is as the secondary ingredient
+in brass (the base being copper), but brass is itself little used.
+Its abundance is well in excess of its usage, so you will usually have
+a surplus of it.
 
-Chromium is supplied by technic.  It is found from altitude -100
-downwards, with no altitude-dependent variations in abundance beyond
+Chromium is supplied by technic.  It is found from elevation -100
+downwards, with no elevation-dependent variations in abundance beyond
 that point.  It is a moderately common metal.  Its main use is as the
 secondary ingredient in stainless steel (the base being iron).
 
-Uranium is supplied by technic.  It is found only from altitude -80 down
-to -300; using it therefore requires one to mine above altitude -300 even
+Uranium is supplied by technic.  It is found only from elevation -80 down
+to -300; using it therefore requires one to mine above elevation -300 even
 though deeper mining is otherwise more productive.  It is a moderately
 common metal, useful only for reasons related to radioactivity: it forms
 the fuel for nuclear reactors, and is also one of the best radiation
@@ -89,37 +96,37 @@ ore to satisfy these uses.  Beware that the ore is slightly radioactive:
 it will slightly harm you if you stand as close as possible to it.
 It is safe when more than a metre away or when mined.
 
-Silver is supplied by the moreores mod.  It is found from altitude -2
-downwards, with no altitude-dependent variations in abundance beyond
+Silver is supplied by the moreores mod.  It is found from elevation -2
+downwards, with no elevation-dependent variations in abundance beyond
 that point.  It is a semi-precious metal.  It is little used, being most
 notably used in electrical items due to its conductivity, being the best
 conductor of all the pure elements.
 
 Gold is part of the basic Minetest game (having migrated there from
-moreores).  It is found from altitude -64 downwards, but is more abundant
-from altitude -256 downwards.  It is a precious metal.  It is little used,
-being most notably used in electrical items due to its combination of
-good conductivity (third best of all the pure elements) and corrosion
-resistance.
+moreores).  It is found from elevation -64 downwards, but is more
+abundant from elevation -256 downwards.  It is a precious metal.  It is
+little used, being most notably used in electrical items due to its
+combination of good conductivity (third best of all the pure elements)
+and corrosion resistance.
 
-Mithril is supplied by the moreores mod.  It is found from altitude
--512 downwards, the deepest ceiling of any minable substance, with no
-altitude-dependent variations in abundance beyond that point.  It is a
-rare precious metal, and unlike all the other metals described here it
-is entirely fictional, being derived from J. R. R. Tolkien's Middle-Earth
-setting.  It is little used.
+Mithril is supplied by the moreores mod.  It is found from elevation
+-512 downwards, the deepest ceiling of any minable substance, with
+no elevation-dependent variations in abundance beyond that point.
+It is a rare precious metal, and unlike all the other metals described
+here it is entirely fictional, being derived from J. R. R. Tolkien's
+Middle-Earth setting.  It is little used.
 
-Mese is part of the basic Minetest game.  It is found from altitude
--64 downwards.  The ore is more abundant from altitude -256 downwards,
-and from altitude -1024 downwards there are also occasional blocks of
+Mese is part of the basic Minetest game.  It is found from elevation
+-64 downwards.  The ore is more abundant from elevation -256 downwards,
+and from elevation -1024 downwards there are also occasional blocks of
 solid mese (each yielding as much mese as nine blocks of ore).  It is a
 precious gemstone, and unlike diamond it is entirely fictional.  It is
 used in many recipes, though mainly not in large quantities, wherever
 some magical quality needs to be imparted.
 
 Diamond is part of the basic Minetest game (having migrated there from
-technic).  It is found from altitude -128 downwards, but is more abundant
-from altitude -256 downwards.  It is a precious gemstone.  It is used
+technic).  It is found from elevation -128 downwards, but is more abundant
+from elevation -256 downwards.  It is a precious gemstone.  It is used
 moderately, mainly for reasons connected to its extreme hardness.
 
 rock
@@ -140,16 +147,16 @@ cobblestone generator, can be an easier way to acquire sand than
 collecting it where it occurs naturally.
 
 Desert stone is part of the basic Minetest game.  It is found specifically
-in desert biomes, and only from altitude +2 upwards.  Although it is
+in desert biomes, and only from elevation +2 upwards.  Although it is
 easily accessible, therefore, its quantity is ultimately quite limited.
 It is used in a few recipes.
 
 Marble is supplied by technic.  It is found in dense clusters from
-altitude -50 downwards.  It has mainly decorative use, but also appears
+elevation -50 downwards.  It has mainly decorative use, but also appears
 in one machine recipe.
 
 Granite is supplied by technic.  It is found in dense clusters from
-altitude -150 downwards.  It is much harder to dig than standard stone,
+elevation -150 downwards.  It is much harder to dig than standard stone,
 so impedes mining when it is encountered.  It has mainly decorative use,
 but also appears in a couple of machine recipes.