How to define spatial domains?

[ShaningYu]

In old verion of ArcGIS Desktop, I could manually define Spatial Domains (or X/Y Domains) when I created a Dataset using ArcCatalog.  Now I am using V10.  When I created a dataset, I chosen WGS_1984_Web_Mercator_Auxiliary_Sphere.  Then the X/Y domain is automatically defined.  For example,
Min X = -20037700
Max X = 900699887774.099
Min Y = -30241100
Max Y = 900689684374.099
The unit is meter.  It looks that the values of Max X and Max Y do not make sense.  Max X =  900,699,887 km, which is 225 times longer than the earth equator's perimeter (about 40,000 km).
Can anyone explain this issue?  Besides, is there a way to manually define the X/Y domain values using V10?  Thanks.

[MelitaKennedy]

Hello,

When using 'high precision' geodatabases, ArcGIS looks at the coordinate system and sets the lower left (minimum x and y) xy domain values based on what's mathematically supported for the coordinate system. The xy resolution is based on the coordinate system's unit. If the unit is meter, 1/10 mm (0.0001 m) is used. If the unit is foot, for instance, the resolution is still 0.0001 m, but converted to feet.

Only the minimum xy values and the resolution (and tolerance!) are actually stored. The maximum xy values are calculated from the stored values plus the maximum integer range supported, 2e53 - 1 or 9007199254740991. That's why the maximum values are so large.

When using basic precision geodatabases (9.1 or earlier), they supported a max range of 2e31 - 1 or 2147483647. This more limited range means that it's only possible to map the entire world at around a few centimeters of precision and much more difficult to pick a domain for non-world areas that would cover the entire valid area of a projected coordinate system.

For ArcGIS 9.2 and up, the only way to set your own domain values is to not set the coordinate system (leave it unknown) and do not take the defaults, then define the coordinate system afterwards. Do not set the minimum and maximum domain values without also setting a reasonable resolution value. If you set a small domain (that matches your data), the resolution value will be very tiny. That affects storage and processing.

There's a white paper you may want to look at:

Understanding Coordinate Management in the Geodatabase

Melita