The present invention relates to a computer-readable data storage medium comprising a graphic dataset in the form of a tiled mipmap 101, and to a method of extracting from said computer-readable data storage medium to a computer memory a subset of said mipmap 101 in the form of a clipmap 109. The present invention relates also to a computer memory containing such a clipmap 109, as well as to a method of rendering said clipmap 109 in a computer system. At each level of detail of the mipmap but the lowest, a tile block 105 formed by a discrete plurality of tiles 104 is coextensive with a whole single tile 104 at the next lower level of detail of the tiled mipmap 101.