UTM zone off by one shifts Xi'an 551 km east

• Xi'an Bell Tower 108.940174, 34.341568 projected as EPSG:32649 gives x=308600, y=3804900; as EPSG:32650 it gives x=-241300.
• Rendering the zone-49 x=308600 on a layer declared EPSG:32650 puts the point near Luliang, Shanxi, 551 km east.
• Negative x values written to an integer column are truncated to 0, collapsing many points onto the central meridian.

1. Check x: the UTM false easting is 500000, so normal values sit in 160000~840000; negative or out-of-range x means the point crossed a zone.
2. Check y to confirm the latitude is still right, which isolates the problem to the zone number.
3. Compute the zone from longitude with floor(lon/6)+31 and compare it with the layer's declared EPSG code.
4. Project the same point with the neighbouring zone and keep the one whose x is closest to 500000.

Declare the CRS the data was actually projected in (EPSG:32649 for 108.94 E), and split multi-zone datasets per zone or store them as longitude/latitude before ingestion.

Math.floor(108.940174 / 6) + 31;   // 49  <- 正确
Math.floor(108.940174 / 6);        // 18  <- 少了 +31，EPSG:32618 是大西洋
const zone = 49;
const cm = zone * 6 - 183;         // 111  (度)

for (let z = 47; z <= 51; z++) {
  const epsg = 'EPSG:326' + z;
  const [x, y] = proj4('EPSG:4326', epsg, [108.940174, 34.341568]);
  console.log(z, x.toFixed(0), y.toFixed(0));
}
// 48:  -360400  3805200
// 50:  -241300  3804900

• Computing the zone as floor(lon/6) and forgetting the +31, which shifts every point by one zone.
• Picking the EPSG code by province or project name instead of by the data's actual central meridian.