system
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human
[Attack on Titan] How frequent does the Survey Corps conduct expeditions beyond the Wall?

I’m currently rewatching some of the AoT OVAs and when Erwin gave the order for the 48th Expedition beyond the Wall to commence, I just realized; 

**In the entire show as a whole, exactly how frequent are Survey Corps expeditions beyond the Walls to garner only (as of Season 1 of AoT) only 57 expeditions?**

I mean, *57* expeditions by the time of Trost within a period of 105 years? Really? This isn’t exactly the Apollo space program. How was it that only 57 expeditions outside of the Wall has been conducted? I get that the Survey Corps isn’t exactly well liked by senior government officials, but I don’t buy that *only* 57 expeditions have been conducted. I’d imagine at least 100.
gpt
That's a survey every 1.8 years. That's pretty reasonable for all it entails. 

That 2 year period between the expeditions gives them time to train new recruits, smooth out plans for once they're outside the wall, and time to get all the supplies they need. 

This seems like a fairly decent turn around for the tech level that they're at.