The following table summarizes the coordinate system aspects that may be modified by each enum value, with an example of change applied by the enum. Blank cells mean that the property is not changed by the enum value.
|Axis order||(longitude, latitude)||✔||✔||✔|
|Axis direction||(east, north)||✔||✔|
|Unit of measurement||Angular degrees & metres||✔|
|Range of values||[0 … 360]° of longitude||✔|
Note on axis orderThe 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 pilotes 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
ProjectedCRSin 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, because it is easier to implement. Apache SIS supports both conventions.
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. The actual axis order can be verified after the CRS
System.out.println(crs). If (x,y) axis order is wanted for compatibility
with older OGC specifications or other software products, CRS forced to "longitude first" axis order can be created
NORMALIZED enumeration value.
Note on range of longitude valuesMost geographic CRS have a longitude axis defined in the [-180 … +180]° range. All map projections in Apache SIS are designed to work in that range. This is also the range of
Mathtrigonometric functions like
atan2(y,x). However some data use the [0 … 360]° range instead. A geographic CRS can be shifted to that range of longitude values using the
POSITIVE_RANGEenumeration value. The choice of longitude range will impact not only some coordinate conversions, but also the methods that verify the domain of validity (e.g.
Nested Class Summary
Enum Constant SummaryEnum ConstantDescriptionAxes are reordered and oriented toward directions commonly used for displaying purpose.Axes order, direction and units of measurement are forced to commonly used predefined values.Axes having a wraparound range meaning are shifted to their ranges of positive values.Axes are ordered for a right-handed coordinate system.
Methods inherited from class Enum
clone, compareTo, describeConstable, equals, finalize, getDeclaringClass, hashCode, name, ordinal, toString, valueOf
Enum Constant Details
NORMALIZEDpublic static final AxesConvention NORMALIZEDAxes order, direction and units of measurement are forced to commonly used predefined values. This convention implies the following changes on a coordinate system:
Rational: The reason why we do not normalize the range and the prime meridian is because doing so would cause the conversion between old and new coordinate systems to be non-affine for axes having
- Axes are oriented and ordered as defined for
- Known units are normalized (this list may be expanded in future SIS versions):
RangeMeaning.WRAPAROUND. Furthermore changing the prime meridian would be a datum change rather than a coordinate system change, and datum changes are more difficult to handle by coordinate operation factories.
- Axes are oriented and ordered as defined for
DISPLAY_ORIENTEDpublic static final AxesConvention DISPLAY_ORIENTEDAxes are reordered and oriented toward directions commonly used for displaying purpose. Units of measurement are unchanged. This convention can be used for deriving a coordinate system with the (longitude, latitude) or (x,y) axis order. A similar concept appears in the Web Map Services (WMS) 1.3 specification, quoted here:6.7.2 Map CS — The usual orientation of the Map CS shall be such that the i axis is parallel to the East-to-West axis of the Layer CRS and increases Eastward, and the j axis is parallel to the North-to-South axis of the Layer CRS and increases Southward. This orientation will not be possible in some cases, as (for example) in an orthographic projection over the South Pole. The convention to be followed is that, wherever possible, East shall be to the right edge and North shall be toward the upper edge of the Map CS.The above-cited (i,j) axes are mapped to display right and display down directions respectively. Other kinds of axis are mapped to east and north directions when possible. More specifically, Apache SIS tries to setup the following directions (replacing a direction by its "forward" counterpart when necessary, e.g.
NORTH) in the order shown below:
Axis directions and order Preferred axis sequences Purpose
Commonly used (x, y, z, t) directions for coordinates.
Commonly used (x, y) directions for screen devices.
Indices in grids or matrices.Notes: we do not provide a "longitude or x axis first" enumeration value because such criterion is hard to apply to inter-cardinal directions and has no meaning for map projections over a pole. The display oriented enumeration name applies to a wider range of cases, but still have a loosely definition which may be adjusted in future Apache SIS versions. If a more stable definition is needed, consider using
RIGHT_HANDEDinstead since right-handed coordinate systems have a more precise meaning in Apache SIS.
RIGHT_HANDEDpublic static final AxesConvention RIGHT_HANDEDAxes are ordered for a right-handed coordinate system. Axis directions, ranges or coordinate values and units of measurement are unchanged. In the two-dimensional case, the handedness is defined from the point of view of an observer above the plane of the system.
Note that a right-handed coordinate system does not guarantee that longitude or x axis will be first in every cases. The most notable exception is the case of (West, North) orientations. The following table lists that case, together with other common axis orientations. The axes orientations implied by this
RIGHT_HANDEDenum is shown, together with
DISPLAY_ORIENTEDaxes for reference:
Examples of left-handed and right-handed coordinate systems Left-handed Right-handed Display oriented Remarks North, East East, North East, North This is the most common case. West, North North, West East, North This right-handed system has latitude first. South, West West, South East, North Used for the mapping of southern Africa. South along 0°,
South along 90° West
South along 90° West,
South along 0°
(Same as right-handed) Can be used for the mapping of North pole.
POSITIVE_RANGEpublic static final AxesConvention POSITIVE_RANGEAxes having a wraparound range meaning are shifted to their ranges of positive values. The units of measurement and range period (difference between maximum and minimum value) are unchanged.
UsageThe most frequent usage of this enum is for shifting longitude values from the [-180 … +180]° range to the [0 … 360]° range. However this enum could also be used with climatological calendars if their time axis has a wrapround range meaning.
Note that conversions from an coordinate system using the [-180 … +180]° range to a coordinate system using the [0 … 360]° range may not be affine. For example the data in the West hemisphere ([-180 … 0]°) may need to move on the right side of the East hemisphere ([180 … 360]°). Some geometries may need to be separated in two parts, and others may need to be merged.
- See Also:
valuespublic static AxesConvention values()Returns an array containing the constants of this enum type, in the order they are declared.
- an array containing the constants of this enum type, in the order they are declared
valueOfReturns the enum constant of this type with the specified name. The string must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.)