これは地図に経緯度が使われるようになってからの習慣であって、古い地図には南が上のものも多い

Question

これは地図に経緯度が使われるようになってからの習慣であって、古い地図には南が上のものも多い。 (source: line 8-9)

1. これは
   What does this これ represent? I guess it shall somehow pick up what was said before, but for that, I learnt that そ- is used. So I'd like to know whether it does the same thing as そ- here or something else.

2. から I guess this から has a temporal function and confers anteriority here? Since I have issues with the sentence in general, I'd like to confirm this, because I really don't know whether my interpretation of the sentence is reliable or not with so many uncertainties around...^^

3. であって
   て-form of である right?

4. 南が上のものも多い
   I really don't know what to make of this. First, もの. What is it? I guess its 物, but I'm not 100% sure. Second, as I said, I have no idea what to do with this phrase. I don't really know what to do with the particle も here, and while I do know that Japanese can work perfectly with nominal phrases only, I simply don't know what this part wants to tell me content-wise.

---

To give an overview how I interpreted these elements in context, here's my try at translating it as far as I could:

This since on maps longitude and latitude being used habit being there, on old maps the south has many upper things/things on top.

In my interpretation, I treated the parts in italics as one syntactical unit. Their correlate in the Japanese original would be これは...習慣であって.

Answer 1

1. これ can refer to what is previously mentioned, just like this in English. In conversations それ is usually used to refer to what the other person said (それは違います = "That's incorrect"), but in written documents これ is primarily used to refer to something just said in the previous sentence.

2. This から means since or after. Because it's used with の, this から modifies the following noun, which is 習慣. See: using の with と,で, から, まで

3. Yes であって is the te-form of である. Here this te-form is nothing more than and used to connect two sentences and forms a compound sentence.

4. 南が上のものも多い: This もの is a pronoun that is used instead of repeating 地図 again. The English equivalent is one as in "I don't have a map, do you have one?" This も is simply also, which can be used in place of the subject marker が. EDIT: 南が上の is a mini-sentence "南が上だ", used as a relative clause modifying もの. So a more literal translation is "ones where south is upper". だ is changed to either な or の depending on the noun before it. Since 上 is a noun also known as no-adjective, you have to use の. See this, too. Similar examples are 襟が緑色のドレス (a dress whose collar is green) and 漢字が大量の本 (a book full of kanji).

---

これは地図に経緯度が使われるようになってからの習慣であって、古い地図には南が上のものも多い。
(literally) This is a custom after longitude and latitude started to be used on maps, and, among old maps, ones with south at the top are also abundant.