Islands are defined as separate polygons. Remember that any geographical unit can have several polygons. True islands (surrounded by water) generally do not overlap with other geographic units and pose no particular problem. In some cases, however, you may have a geographical unit which is an "island" inside another unit, even on land. Examples would be Lesotho inside South Africa and the Vatican City inside Italy. This situation often arises with metropolitan areas. In these cases of overlap, the physical location of the two geographical units in question in the .MAP file will determine which one is on top. Make sure the unit which is to be on top appears AFTER the other unit in the .MAP file.
Lakes defined by the borders of more than one geographical unit do not need to be explicitly defined in the .MAP file. Map Viewer will show them as white (background color) and will treat them the same way as any other space on the screen which is outside the entire map. Lakes contained entirely within a geographical unit can be shown if the outline of the lake is defined as a "hole" in the bounding polygon as described in the Polygon section.