With an iso-rectangular map, global operations are difficult using map coordinates. Imagine a simple iso-rectangular map. Its "natural" representation is

A B C
 D E F
G H I

(0,0)   (2,0)   (4,0)
    (1,1)   (3,1)   (5,1)
(0,2)   (2,2)   (4,2)

while its representation in map coordinates would be

  CF
 BEI
ADH
 G

            (2,0) (3,0)
      (1,1) (2,1) (3,1)
(0,2) (1,2) (2,2)
      (1,3)

Neither is particularly good for a global map operation such as whole_map_iterate. Something better is needed.

Native coordinates compress the map into a continuous rectangle; the dimensions are defined as map.xsize × map.ysize. For instance the above iso-rectangular map is represented in native coordinates by compressing the natural representation in the X axis to get the 3 × 3 iso-rectangle of

ABC
DEF
GHI

(0,0) (1,0) (2,0)
(0,1) (1,1) (2,1)
(0,2) (1,2) (2,2)

The resulting coordinate system is much easier to use than map coordinates for some operations. These include most internal topology operations (e.g., normalize_map_pos, whole_map_iterate) as well as storage (in map.tiles and savegames, for instance).

In general, native coordinates can be defined based on this property: the basic map becomes a continuous (gap-free) cardinally-oriented rectangle when expressed in native coordinates.