When to use "this" or "that"?

It is 10 cm on the map. That is 100 km in real terms.

I am proofreading and it seems odd that the writer always uses that in these cases. I would have said this.

Who is right and who is wrong?

In writing fiction, 'this' and 'that' can be used very effectively to connote different things. They are functionally equivalent, but the closeness of 'this' and the distance of 'that' can be used as tools.

'This' can be used to connote how close a thought is to a character, while 'that' can be used to connote how far away, or unimportant a thought might be to a character. Ex:

"I remember, because that was the day before Grandpa died."

"I remember, because this was the day before Grandpa died."

In the first case, the subtext is that this event might have meant something to the speaker, but that he holds it somewhat at arm's length. In the second case, the subtext is that this event might have meant something much more emotional and personal, and that it is an event that still moves him.

All because the author used 'this' in place of 'that'. It is subtle, but powerful. Readers pick up on this, even if unconsciously, and it can make the story better for them.

Conversely, if a character is telling an anecdote about something that was troubling, using 'that' rather than 'this' can connote how much the character wishes to not feel close to that experience.

It can also be used to make the time frame seem more immediate. If a narrator is speaking about past events, 'this' often puts the reader into the scene directly, while 'that' often makes it feel like something more from the past.

It all really depends on whether context the sentence is set on. I would say that using "that" would sound more correctly than using "this", as using "this" would be put in the context of someone talking about an object or place.

This is up for debate...

Nearness and farness is only one use of this and that. Or the use of this when it is in relationship to "my" and of "that" when it is in relationship to "your".

But in a lot of cases there is no distinction between two things at different places. "that" is the stronger of the two demontratives.

• That silly friend of yours. - Deprecating use. I think here "that" is preferred to "this".

In explanations that refer to something said or written before as in

• one cm on this map - that means 1 kilometre in reality

I think the stronger that is preferred.

Grammars, especially school grammars , treat this grammar point on a low school level. In reality the idiomatic use of this and that is a complicated thing and a study of this problem would be a whole chapter. There are cases where native speakers prefer one of the two demonstratives, and there are cases where either of the two is possible.