The axis order is specified by the authority (typically a national agency) defining the Coordinate Reference System (CRS). The order depends on the CRS type and the country defining the CRS. In the case of geographic CRS, the (latitude, longitude) axis order is widely used by geographers and pilots for centuries. However software developers tend to consistently use the (x, y) order for every kind of CRS. Those different practices resulted in contradictory definitions of axis order for almost every CRS of kind GeographicCRS, for some ProjectedCRS in the South hemisphere (South Africa, Australia, etc.) and for some polar projections among others.

Recent OGC standards mandate the use of axis order as defined by the authority. Oldest OGC standards used the (x, y) axis order instead, ignoring any authority specification. Many software products still use the old (x, y) axis order, maybe because such uniformization makes CRS implementation and usage apparently easier. Apache SIS supports both conventions with the following approach: by default, SIS creates CRS with axis order as defined by the authority. Those CRS are created by calls to the CRS.forCode(String) method and the actual axis order can be verified after the CRS creation with System.out.println(crs). But if (x, y) axis order is wanted for compatibility with older OGC specifications or other software products, then CRS forced to longitude first axis order can be created by a call to the following method:

CoordinateReferenceSystem crs = …;               // CRS obtained by any means.
crs = AbstractCRS.castOrCopy(crs).forConvention(AxesConvention.RIGHT_HANDED)

Among the legacy OGC standards that used the non-conform axis order, an influent one is version 1 of the Well Known Text (WKT) format specification. According that widely-used format, WKT 1 definitions without explicit AXIS[…] elements shall default to (longitude, latitude) or (x, y) axis order. In version 2 of WKT format, AXIS[…] elements are no longer optional and should contain an explicit ORDER[…] sub-element for making the intended order yet more obvious. But if AXIS[…] elements are nevertheless missing in a WKT 2 definition, Apache SIS defaults to (latitude, longitude) order. So in summary:

• Default WKT 1 axis order of geographic CRS is (longitude, latitude) as mandated by OGC 01-009 specification.
• Default WKT 2 axis order of geographic CRS is (latitude, longitude), but this is SIS-specific as ISO 19162 does not mention default axis order.

To avoid ambiguities, users are encouraged to always provide explicit AXIS[…] elements in their WKT.