I'm trying to figure out where exactly the Field of the Lark was, both in terms of the original landscape and in terms of modern streets. (Yep, it's for my web site.) I've found a few resources--Hugo's description, old maps, a fairly detailed history of the street names thanks to the Nomenclature des Voies database, a handful of webpages about the Bièvre river which passed through it, and a handful of pages about the 1826 scandal over a murder that took place there. And this page, about a famous (infamous?) tavern in the vicinity which Hugo apparently frequented. The page doesn't actually talk about the Champ de l'Alouette, but it does have a map--the map looks like it predates the 1830s by a few decades, but is close enough to be useful.
This is what Hugo has to say:
Quand on a monté la rue Saint-Jacques, laissé de côté la barrière et suivi quelque temps à gauche l’ancien boulevard intérieur, on atteint la rue de la Santé, puis la Glacière, et, un peu avant d’arriver à la petite rivière des Gobelins, on rencontre une espèce de champ, qui est, dans toute la longue et monotone ceinture des boulevards de Paris, le seul endroit où Ruisdael serait tenté de s’asseoir.
So basically, it's on the boulevard, between the Rue de la Glacière and the Bièvre river (which ran next to the still-existing Rue Croulebarbe). Approximately where Le Monde's headquarters are right now. All the other sources I've found back me up on this--the southern end of the Field of the Lark was delimited by the boulevard, the Rue de la Glacière, and the rue Croulebarbe. Fine and dandy, but how far did it extend northwards?
We have two streets whose names have or had something to do with the field--the modern day Rue du Champ de l'Alouette, formerly the Rue du Petit-Champ, and the Rue Corvisart, formerly the Rue du Champ de l'Alouette. Conceivably, their intersection could be the northern edge of the field.
However, in the map linked to above, the field-y looking space extends far north of that, along the Bièvre into the backyard of the Gobelins factory. That space is currently occupied by the Square René-le-Gall, which is not a square but a park, and I would love to imagine that there is still green space to wander around in where Marius brooded himself into a Romantic stupor over Cosette. But I have no idea whether that was actually the Champ de l'Alouette--the park doesn't extend to the space delimited by the streets mentioned above.
I suppose there is a possible compromise--at the very southern end of the park, there is a spot called the Place de la Bergère d'Ivry, named after aforementioned scandalous murder. It's fairly well established that the murder took place in the Champ de l'Alouette, both by the sources I've dug up and by Hugo (who has a passerby mention that "it was here that Ulbach killed the Ivry shepherdess"). So that one place, at least, is definitely both a park (playground actually) and the site of the Field of the Lark.
So, to cut short the tl;dr: could one get away with saying that the Square René-le-Gall stands on the site of the former Field of the Lark? Or was the field proper limited to the place where the Le Monde building is now?