There are many answers for this, but looking at the dictionary we get:

at: In or near the area occupied by; in or near the location of

 

in: Within the limits, bounds, or area of

People are usually using in to note a general location and at for a more specific location.

I'm in the building, at the front desk

 

I'm in New York, at the conference

 

I'm in New York, at the Empire State Building

EDIT: But note also the difference when in is used to indicate inside

I'm in the elevator = I'm inside the elevator

 

I'm at the elevator = I'm near the elevator

There are many answers for this, but looking at the dictionary we get:

at: In or near the area occupied by; in or near the location of

in: Within the limits, bounds, or area of

People are usually using in to note a general location and at for a more specific location.

I'm in the building, at the front desk

I'm in New York, at the conference

I'm in New York, at the Empire State Building

EDIT: But note also the difference when in is used to indicate inside

I'm in the elevator = I'm inside the elevator

I'm at the elevator = I'm near the elevator

There are many answers for this, but looking at the dictionary we get:

at: In or near the area occupied by; in or near the location of

in: Within the limits, bounds, or area of

People are usually using in to note a general location and at for a more specific location.

I'm in the building, at the front desk

I'm in New York, at the conference

I'm in New York, at the Empire State Building

There are many answers for this, but looking at the dictionary we get:

at: In or near the area occupied by; in or near the location of

in: Within the limits, bounds, or area of

People are usually using in to note a general location and at for a more specific location.

I'm in the building, at the front desk

I'm in New York, at the conference

I'm in New York, at the Empire State Building

EDIT: But note also the difference when in is used to indicate inside

I'm in the elevator = I'm inside the elevator

I'm at the elevator = I'm near the elevator