function packpos(x, y, z)
    x = 819200+math.floor(x*100) -- -8192 - 8192   3 byte
    y = 819200+math.floor(y*100)
    z = math.floor(1300*65535/(z+100)) -- -100 - 1200   2 byte
    return string.byte(
        x, BitExtract(x, 8, 8), BitExtract(x, 16, 8)
        y, BitExtract(y, 8, 8), BitExtract(x, 16, 8),
        z, BitExtract(z, 8, 8))
end

function unpackpos(string, startindex)
    local x1, x2, x3, y1, y2, y3, z1, z2 = string:byte(startindex, startindex + 7)
    return  ((x1 + x2*256 + x3*65536) - 819200)/100,
            ((y1 + y2*256 + y3*65536) - 819200)/100,
            (z1 + z2*256)/65535*1300 - 100
end