Each sector is centered on a coordinate intersection.
The sector just north of the man is centered on the intersection of x = 1200 and y = 1527. The corners of that sector are: 1199.5,1527.5 (NW), 1200.5,1527.5 (NE), 1200.5,1526.5 (SE), and 1199.5,1526.5 (SW).
Coordinate intersections can be located.
We can't see the decimal value of our location. We see the coordinates x = 1200.00 to x = 1200.99 merely as 1200. But we can locate the intersections by finding a spot (like where the man is standing) where a small movement will change the apparent coordinates.
All compounds styles have a special "cornerstone" sector.
The cornerstone is important because (a) it is located at the builder's location when the compound is initially constructed, and (b) expansion costs are based on the distance from the cornerstone.
Builder's location determines cornerstone placement.
Because the cornerstone sector is placed on a coordinate intersection, it might not be precisely centered on the builder. In fact, it can be as much as 0.5 coordinates away, both in the x and y directions, depending how far the builder is from the closest intersection.
To accurately place the cornerstone, the builder should get as close as possible to the desired intersection. In the example compound, the builder was located where the chest placed now, and the cornerstone was placed as expected.