// Otherwise, for a group of markers with the same Link-URI (e.g. an
// audio file) calculate the offset relative to the first marker of
// that group. This way the user can jump to the corresponding
// playback positions in a long audio track.
Extensions exts = (Extensions) wpt.get(GpxConstants.META_EXTENSIONS);
if (exts != null && exts.containsKey("offset")) {
try {
offset = Double.parseDouble(exts.get("offset"));
} catch (NumberFormatException nfe) {
Main.warn(nfe);
}