protected String getFileName(double lat, double lon) {
        lon = 1 + (180 + lon) / LAT_DEGREE;
        int lonInt = (int) lon;
        lat = 1 + (60 - lat) / LAT_DEGREE;
        int latInt = (int) lat;

        if (Math.abs(latInt - lat) < invPrecision / LAT_DEGREE)
            latInt--;

        // replace String.format as it seems to be slow
        // String.format("srtm_%02d_%02d", lonInt, latInt);
        String str = "srtm_";
        str += lonInt < 10 ? "0" : "";
        str += lonInt;
        str += latInt < 10 ? "_0" : "_";
        str += latInt;

        return str;
    }
@Disabled
    @Test
    public void testGetEleVerticalBorder() {
        // Border between the tiles srtm_39_04 and srtm_39_03
        assertEquals("srtm_39_04", instance.getFileName(44.999999, 11.5));
        assertEquals(5, instance.getEle(44.999999, 11.5), precision);
        assertEquals("srtm_39_03", instance.getFileName(45.000001, 11.5));
        assertEquals(6, instance.getEle(45.000001, 11.5), precision);
    }