Merge two HGT files taking nodata (-32768) into account.

merge_hgt.ts

const base = Deno.args[0];

const data1 = await Deno.readFile(`in1/${base}.HGT`);
const data2 = await Deno.readFile(`in2/${base}.HGT`);

const buf1 = new DataView(data1.buffer);
const buf2 = new DataView(data2.buffer);

for (let i = 0; i < buf2.byteLength - 1; i += 2) {
  const val = buf2.getInt16(i, false);

  if (val !== -32768) {
    buf1.setInt16(i, val, false);
  }
}

await Deno.writeFile(`out/${base}.HGT`, data1);