1a. Zoom to area and add there the bounding box visually.
You can zoom to the corners of the box and do correction. Search for place names is available as well.
1b. Or type coordinates of the box.
Notation 16 38 50 is equal to 16°38´50".
Prime meridian:
Note: When you are using coordinates from the frame of an old map be aware of the used geodetic datums. Shift of the prime meridian is just first aproximation. Use database SpatialReference.org. Here is WGS84 Datum (EPSG:4326).
2. Paste result to your metadata record.
Result format: MARC | DublinCore | Plain Content of this box is under IE automatically copied into Clipboard, so you can just paste with Ctrl-V.
