Dan Tien confusion

Upper Dantian is the head space.
Middle Dantian is the heart space.
Lower Dantian is the navel space.

“The” Dantian is the same as the Lower Dantian. No difference.

The exact external correspondence for “the dantian” can be 1.5 inches below the navel as you describe–or even 2-3 inches below–but it is also said to vary from person to person slightly. For some people, their dantian can be at the navel or even slightly higher. This is typically said to occur in women, if they are big breasted (the breasts raise the lower dantian slightly). In general if a person thinks about the navel or slightly below, then that is about right.

However, it is important to recognize that this “location” is external, and the dantian is actually internal. More appropriately, the dantian is inside your belly, occupying the space between the navel and kidneys in the lower back. It is typically more associated with the external “marker” of the navel than with the kidneys, because the dantian is thought to be the place where you give “birth” to your vitality (the navel being the remnant of the umbilical cord from which you were originally fed your vitality for the first 9 months of life). However, as far as a geographic location goes, it is in the internal space between the navel and the kidneys.

However, the dantian is not simply a geographical location. More precisely, the dantian is a multi-dimensional energetic space within you from which the multi-dimensional “you” gives birth to your post-natal [this realm] vitality. Unlike the geographical location which is bounded by your navel in the front and the kidneys in the back, the actual dantian is boundless . . . a sea of infinite depth, to which no matter how deeply you penetrate, there is never any bound.

In Chinese, dantian is written in pinyin with tonal marks as: dāntián
This is composed of two words: dān and tián
Tián means field
Dān means red or cinnabar

Now what does “red” or “cinnabar” mean?

They are code words.

Red represents the energy of vitality, your life-force in the physical world.
Cinnabar is a red mineral containing mercury (known to be poisonous), but was originally associated with many external alchemical potions that people tried making for physical immortality. Thus the true meaning of cinnabar here is that of “elixir of immortality”, not the actual mineral of cinnabar. This latter bit is also true because Dān is the root word of Xiāndān, which is the Chinese word for elixir.

Thus the literal translation of dāntián is “red field”, “cinnabar field”, or even “elixir field”.

IN SHORT:
In other words, it is “the energetic field from which your vitality arises, and in which the elixir of immortality can be cultivated”. The location of this energetic space is as described above in the second large paragraph.