When to use "this" or "that"?

It depends on the context, but I'd go for "that", because the subject is not something physical located within proximity, but rather an explanation about a conversion regarding units of measurement. What is explained is a concept, a convention. That convention.

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.

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.

My guess as to why you want to use this is because as a demonstrative pronouns, this and that are sometimes opposed with this noting a closer relationship, and these sentences are so short and close together that it is difficult to imagine them having the sort of distant relationship which calls for that. However, that is not quite the semantic function wishes for it to serve.

That is sometimes used as what Noah Webster terms as a "pro-sentence or substitute", which means it can represent a prior sentence or a portion of a prior sentence. This lacks a corresponding definition in his dictionary. It also notes that this is opposed to that and American Heritage Dictionary 5th edition notes that both words are used to represent prior thought in the usage notes under the this entry, but the notes also claim that that "is sometimes considered the better option".

That is in particular is also a phrase used to use clarification, and Collins Cobuild gives the following example, under that is/that is to say

I am a disappointing, though generally dutiful, student. That is, I do as I'm told.

If I am forced to choose between one or the other, I would recommend leaving the author's version intact. He means to clarify that 10 centimeters represents 100 kilometers.

However, to be honest, I do not like either version very much. The given usage of that is so close to the sentence it is meant to encapsulates, and the encapsulated sentence is so short and closeby that it makes relatively little sense to use the word like this.

Since the latter sentence defines elements of the former sentence, it would be better to unify the two sentences into one with a relative pronoun. Both that and which are used this way according to English Grammar Today, but if this solution were to be taken in this context, I would prefer which, as in "It is 10 cm on the map[, which] is 100 km in real terms." I do not quite fully understand why I have that preference to be honest, but as of right now I suppose it has something to do with this portion of the usage note under the That entry in the American Heritage Dictionary 5th edition, which overall attempts to explain when to use which or that:

A related rule stipulates that which should be used with nonrestrictive (or nondefining) clauses, which give additional information about an entity that has already been identified in the context; in this use, which is always preceded by a comma. Thus, we say The students in Chemistry 101 have been complaining about the textbook, which (not that) is hard to follow. The clause which is hard to follow is nonrestrictive in that it does not indicate which text is being complained about; even if the clause were omitted, we would know that the phrase the textbook refers to the text in Chemistry 101.