When to use ところ / て+いるところ / た+ところ


In this sentence:

> 「もしもし、今、駅に向かって（_______）ところです。 あと５、６分でつきます。」

What should be the correct tense of the verb 歩く? Should I base my answer on
the word 今 or the てform of 向かう?

If there is 今 does that mean that た+ところ is not appropriate?  
If I see a Verbてform, all my V should be in てform?

Thank you




>   1. 今、駅に向かって歩いているところです。  
>  I am walking toward the station. (i.e. I am on the way.)
>   2. 今、駅に向かって歩くところです。  
>  I am just about to walk toward the station. (i.e., I have not departed
> yet.)
>   3. 今、駅に向かって歩いたところです。  
>  I have just walked toward the station. (i.e., I have finished walking.)
>

These three sentences are all at least grammatical, but Sentence 1 is the most
natural. Sentence 2 is not entirely wrong, but people usually say something
like 今から出発します if they have not departed yet. Sentence 3 doesn't make sense at
all in this context.

The existence of 今 is not really important because these sentences describe
his _current_ situation even without saying 今 (Note all the sentences end with
です, which is the present tense). Even in sentence 3, it describes his
_current_ situation. It's similar to [ている describing the continuation of
state](https://japanese.stackexchange.com/q/3122/5010), except that the
original action happened shortly before _now_ (hence "just").

Related:

  * [Can ところで be ambiguous (time vs. location)?](https://japanese.stackexchange.com/q/39699/5010)
  * [A bunch of questions about ところ](https://japanese.stackexchange.com/q/27541/5010)

