Q:

How Dakuten works with Romaji?

I was reading different hiragana charts and saw that "ち(chi)", applying dakuten it can be read as "dzi" or "ji". 
Other hiragana words in dakuten also have different ways of pronunciation depending on the charts.
How does romaji works on dakuten? 
As an example, I'm writing the word "ぢめん" on romaji, how do I write it? Dzimen? Jimen? Djimen?
Thanks!

A:

This is not a problem of dakuten in general, but is a problem specific to じ, ぢ, ず and づ. There is a bit complicated history regarding the pronunciations of these four characters, and they even have a special name, yotsugana. English Wikipedia has a dedicated article about this topic.
In short, the standard Japanese accent no longer retains the distinction between the sounds of じ vs ぢ, or ず vs づ. You can pronounce じ and ぢ either like dzi or like ji, and native Japanese speakers using the standard accent cannot even hear the difference. Please see this answer and this question for details.
Even though じ and ぢ share the same pronunciation, sometimes you want to distinguish the spelling of these two characters using Latin alphabet. Some romanization systems including 日本式 distinguish the romaji spelling of じ (zi) and ぢ (di). Hepburn system, which tries to approximate the sound as much as possible, does not distinguish the spelling of じ/ぢ (both ji), or ず/づ (both zu). If you want to write 地面 using the Hepburn system, it's "jimen".

