Package org.apache.sis.referencing
Class GeodeticCalculator
Object
GeodeticCalculator
Performs geodetic calculations on a sphere or an ellipsoid. This class computes the distance between two points,
or conversely the point located at a given distance from another point when navigating in a given direction.
The distance depends on the path (or track) on Earth surface connecting the two points.
The track can be great circles (shortest path between two points) or rhumb lines (path with constant heading).
This class uses the following information:
 The start point, which is always considered valid after the first call
to
setStartPoint(…)
. Its value can only be changed by another call tosetStartPoint(…)
.  One of the followings (the latest specified properties override other properties and determines what will be calculated):
 the end point, or
 the azimuth at start point together with the geodesic distance from that point.
Algorithms
GeodeticCalculator
uses two set of formulas, depending if the figure of the Earth
is a sphere or an ellipsoid.
Publications relevant to this class are:
 Wikipedia, Greatcircle navigation for spherical formulas.
 Wikipedia, Rhumb line for spherical formulas.
 Charles F. F. Karney (2013), Algorithms for geodesics for ellipsoidal formulas.
 G.G. Bennett, 1996. Practical Rhumb Line Calculations on the Spheroid for ellipsoidal formulas.
 Charles F. F. Karney (2010), Test set for geodesics
for
GeodeticCalculator
tests.  Charles F. F. Karney, GeographicLib for the reference implementation.
Accuracy
GeodeticCalculator
aims for a positional accuracy of one centimetre.
The accuracy is often better (about one millimetre), but not everywhere.
Azimuthal accuracy corresponds to an error of one centimetre at a distance of one kilometer,
except for nearly antipodal points (less than 1° of longitude and latitude from antipode)
and points close to the poles where the azimuthal errors are larger.
Karney's GeographicLib should be used if better accuracy is desired.
Apache SIS accuracy does not go as far as GeographicLib because the rest of Apache SIS
library (map projections, etc.) aims for an one centimetre accuracy anyway.
Limitations
Current implementation can not compute the geodesics in some cases. In particular, calculation may fail for antipodal points on an ellipsoid. Karney's algorithm should cover those cases, but thisGeodeticCalculator
implementation may not be sufficiently tuned.
See SIS467 for more information.
Thread safety
This class is not threadsafe. If geodetic calculations are needed in a multithreads environment, then a distinct instance ofGeodeticCalculator
needs to be created for each thread. Since:
 1.0
Defined in the sisreferencing
module

Method Summary
Modifier and TypeMethodDescriptionstatic GeodeticCalculator
Constructs a new geodetic calculator expecting coordinates in the supplied CRS.createGeodesicCircle2D
(double tolerance) Creates an approximation of the curve at a constant geodesic distance around the start point.createGeodesicPath2D
(double tolerance) Creates an approximation of the geodesic track from start point to end point as a Java2D object.Creates an Azimuthal Equidistant projection centered on current starting point.double
Computes the angular heading of a rhumb line path.Returns the unit of measurement of all distance measurements.double
Computes the angular heading at the ending point of a geodesic path.Returns or computes the destination in the CRS specified at construction time.double
Returns or computes the shortest distance from start point to end point.Returns the coordinate reference system for all methods expecting (φ,λ) asdouble
values.Returns the Coordinate Reference System (CRS) in whichPosition
s are represented, unless otherwise specified.double
Returns or computes the length of rhumb line (part of constant heading) from start point to end point.double
Returns or computes the angular heading at the starting point of a geodesic path.Returns the starting point in the CRS specified at construction time.void
Sets the start point and starting azimuth to the current end point and ending azimuth values.void
setEndGeographicPoint
(double latitude, double longitude) Sets the destination as geographic (latitude, longitude) coordinates.void
setEndPoint
(Position position) Sets the destination as coordinates in arbitrary reference system.void
setGeodesicDistance
(double distance) Sets the geodesic distance from the start point to the end point.void
setStartGeographicPoint
(double latitude, double longitude) Sets the starting point as geographic (latitude, longitude) coordinates.void
setStartingAzimuth
(double azimuth) Sets the angular heading at the starting point of a geodesic path.void
setStartPoint
(Position point) Sets the starting point as coordinates in arbitrary reference system.Returns a string representation of start point, end point, azimuths and distance.

Method Details

create
Constructs a new geodetic calculator expecting coordinates in the supplied CRS. AllGeodeticCalculator
methods having aPosition
argument or return value will use that specified CRS. That CRS is the value returned bygetPositionCRS()
.Limitations
Current implementation uses only spherical formulas. Implementation using ellipsoidal formulas will be provided in a future Apache SIS release. Parameters:
crs
 the reference system for thePosition
objects. Returns:
 a new geodetic calculator using the specified CRS.

getPositionCRS
Returns the Coordinate Reference System (CRS) in whichPosition
s are represented, unless otherwise specified. This is the CRS of allPosition
instances returned by methods in this class. This is also the default CRS assumed by methods receiving aPosition
argument when the given position does not specify its CRS. This default CRS is specified at construction time. It is not necessarily geographic; it may be projected or geocentric. Returns:
 the default CRS for
Position
instances.

getGeographicCRS
Returns the coordinate reference system for all methods expecting (φ,λ) asdouble
values. This CRS always has (latitude, longitude) axes, in that order and in degrees. The CRS may contain an additional axis for ellipsoidal height. Returns:
 the coordinate reference system of (φ,λ) coordinates.

getStartPoint
Returns the starting point in the CRS specified at construction time. This method returns the last point given to asetStartPoint(…)
method, transformed to the position CRS. Returns:
 the starting point represented in the CRS specified at construction time.
 Throws:
IllegalStateException
 if the start point has not yet been specified.GeodeticException
 if the coordinates can not be transformed to position CRS. See Also:

setStartPoint
Sets the starting point as coordinates in arbitrary reference system. This method transforms the given coordinates to geographic coordinates, then delegates tosetStartGeographicPoint(double, double)
. If the given point is not associated to a Coordinate Reference System (CRS), then this method assumes the CRS specified at construction time. Parameters:
point
 the starting point in any coordinate reference system. Throws:
IllegalArgumentException
 if the given coordinates can not be transformed. See Also:

setStartGeographicPoint
public void setStartGeographicPoint(double latitude, double longitude) Sets the starting point as geographic (latitude, longitude) coordinates. The starting and ending azimuths, the end point, the geodesic distance and the rhumb line length are discarded by this method call; some of them will need to be specified again. 
getEndPoint
Returns or computes the destination in the CRS specified at construction time. This method returns the point specified in the last call to asetEndPoint(…)
method, unless the starting azimuth and geodesic distance have been set more recently. In the later case, the end point will be computed from the start point and the current azimuth and distance. Returns:
 the destination (end point) represented in the CRS specified at construction time.
 Throws:
IllegalStateException
 if the destination point, azimuth or distance have not been set.GeodeticException
 if the coordinates can not be computed. See Also:

setEndPoint
Sets the destination as coordinates in arbitrary reference system. This method transforms the given coordinates to geographic coordinates, then delegates tosetEndGeographicPoint(double, double)
. If the given point is not associated to a Coordinate Reference System (CRS), then this method assumes the CRS specified at construction time. Parameters:
position
 the destination (end point) in any coordinate reference system. Throws:
IllegalArgumentException
 if the given coordinates can not be transformed. See Also:

setEndGeographicPoint
public void setEndGeographicPoint(double latitude, double longitude) Sets the destination as geographic (latitude, longitude) coordinates. The starting azimuth, ending azimuth geodesic distance and rhumb line length will be updated as an effect of this call. 
getStartingAzimuth
public double getStartingAzimuth()Returns or computes the angular heading at the starting point of a geodesic path. Azimuth is relative to geographic North with values increasing clockwise. This method returns the azimuth normalized to [180 … +180]° range given in last call tosetStartingAzimuth(double)
method, unless thesetEndPoint(…)
method has been invoked more recently. In the later case, the azimuth will be computed from the start point and the current end point. Returns:
 the azimuth in degrees from 180° to +180°. 0° is toward North and values are increasing clockwise.
 Throws:
IllegalStateException
 if the end point, azimuth or distance have not been set.GeodeticException
 if the azimuth can not be computed.

setStartingAzimuth
public void setStartingAzimuth(double azimuth) Sets the angular heading at the starting point of a geodesic path. Azimuth is relative to geographic North with values increasing clockwise. The ending azimuth, end point and rhumb line length will be updated as an effect of this method call. Parameters:
azimuth
 the starting azimuth in degrees, with 0° toward north and values increasing clockwise. See Also:

getEndingAzimuth
public double getEndingAzimuth()Computes the angular heading at the ending point of a geodesic path. Azimuth is relative to geographic North with values increasing clockwise. This method computes the azimuth from the current start point and end point, or from start point and the current starting azimuth and geodesic distance. Returns:
 the azimuth in degrees from 180° to +180°. 0° is toward North and values are increasing clockwise.
 Throws:
IllegalStateException
 if the destination point, azimuth or distance have not been set.GeodeticException
 if the azimuth can not be computed.

getConstantAzimuth
public double getConstantAzimuth()Computes the angular heading of a rhumb line path. Azimuth is relative to geographic North with values increasing clockwise. Returns:
 the azimuth in degrees from 180° to +180°. 0° is toward North and values are increasing clockwise.
 Throws:
IllegalStateException
 if the start point or end point has not been set.GeodeticException
 if the azimuth can not be computed.

getGeodesicDistance
public double getGeodesicDistance()Returns or computes the shortest distance from start point to end point. This is sometime called "great circle" or "orthodromic" distance. This method returns the value given in last call tosetGeodesicDistance(double)
, unless thesetEndPoint(…)
method has been invoked more recently. In the later case, the distance will be computed from the start point and current end point. Returns:
 the shortest distance in the unit of measurement given by
getDistanceUnit()
.  Throws:
IllegalStateException
 if the start point or end point has not been set.GeodeticException
 if the distance can not be computed. See Also:

setGeodesicDistance
public void setGeodesicDistance(double distance) Sets the geodesic distance from the start point to the end point. The end point, ending azimuth and rhumb line length will be updated as an effect of this method call. Parameters:
distance
 the geodesic distance in unit of measurement given bygetDistanceUnit()
. See Also:

getRhumblineLength
public double getRhumblineLength()Returns or computes the length of rhumb line (part of constant heading) from start point to end point. This is sometime called "loxodrome". This is not the shortest path between two points. The rhumb line distance may be up to 50% longer than the geodesic distance. Returns:
 length of rhumb line in the unit of measurement given by
getDistanceUnit()
.  Throws:
IllegalStateException
 if a point has not been set.

getDistanceUnit
Returns the unit of measurement of all distance measurements. This is the ellipsoid axis unit. Returns:
 the unit of measurement of all distance measurements.
 See Also:

moveToEndPoint
public void moveToEndPoint()Sets the start point and starting azimuth to the current end point and ending azimuth values. The ending azimuths, the geodesic distance and the end point are discarded by this method call; some of them will need to be specified again. Throws:
GeodeticException
 if the end point or ending azimuth can not be computed. See Also:

createGeodesicPath2D
Creates an approximation of the geodesic track from start point to end point as a Java2D object. The coordinates are expressed in the coordinate reference system specified at creation time. The approximation uses linear, quadratic or cubic Bézier curves. The returned path has the following characteristics: The first point is
getStartPoint()
.  The beginning of the curve (more specifically, the tangent at starting point) is oriented toward the direction given by getStartingAzimuth(), adjusted for the map projection (if any) deformation at that location.
 The point B(½) in the middle of the Bézier curve is a point of the geodesic path.
 The end of the curve (more specifically, the tangent at ending point) is oriented toward the direction given by getEndingAzimuth(), adjusted for the map projection (if any) deformation at that location.
 The last point is
getEndPoint()
, potentially with 360° added or subtracted to the longitude.
tolerance
parameter should not be too small for avoiding creation of unreasonably long chain of Bézier curves. For example a value of 1/10 of geodesic length may be sufficient.Note: this method depends on the presence ofjava.desktop
module. This constraint may be addressed in a future Apache SIS version (see SIS453). The "2D" suffix in the method name represents this relationship with Java2D. ThecreateGeodesicPath(…)
method name (without suffix) is reserved for a future version using ISO curves instead. Parameters:
tolerance
 maximal error between the approximated curve and actual geodesic track in the units of measurement given bygetDistanceUnit()
. This is approximate; the actual errors may vary around that value. Returns:
 an approximation of geodesic track as Bézier curves in a Java2D object.
 Throws:
IllegalStateException
 if some required properties have not been specified.GeodeticException
 if some coordinates can not be computed.
 The first point is

createGeodesicCircle2D
Creates an approximation of the curve at a constant geodesic distance around the start point. The returned shape is circlelike with the start point in its center. The coordinates are expressed in the coordinate reference system specified at creation time. The approximation uses cubic Bézier curves.Note: some authors define geodesic circle as the curve which enclose the maximum area for a given perimeter. This method adopts a different definition, the locus of points at a fixed geodesic distance from center point.This method tries to stay within the given tolerance threshold of the geodesic track. Thetolerance
parameter should not be too small for avoiding creation of unreasonably long chain of Bézier curves. For example a value of 1/10 of geodesic length may be sufficient.Note: this method depends on the presence ofjava.desktop
module. This constraint may be addressed in a future Apache SIS version (see SIS453). The "2D" suffix in the method name represents this relationship with Java2D. ThecreateGeodesicCircle(…)
method name (without suffix) is reserved for a future version using ISO curves instead. Parameters:
tolerance
 maximal error in the units of measurement given bygetDistanceUnit()
. This is approximate; the actual errors may vary around that value. Returns:
 an approximation of circular region as a Java2D object.
 Throws:
IllegalStateException
 if some required properties have not been specified.GeodeticException
 if some coordinates can not be computed.

createProjectionAroundStart
Creates an Azimuthal Equidistant projection centered on current starting point. On input, theMathTransform
expects coordinates expressed in the position CRS. On output, theMathTransform
produces coordinates in aProjectedCRS
having the following characteristics: Coordinate system is a twodimensional
CartesianCS
with (Easting, Northing) axis order and directions.  Unit of measurement is the same as position CRS
if those units are linear, or
Units.METRE
otherwise.  Projection of the start point results in (0,0).
 Distances relative to (0,0) are approximately exact for distances less than 800 km.
 Azimuths from (0,0) to other points are approximately exact for points located at less than 800 km.
D = √(x² + y²) — distance from projection center.
The following calculations are not exacts, because distances and azimuths are approximately exacts only when measured from (0,0) coordinates:
θ = atan2(y, x) — arithmetic angle from projection center to (x, y).
x = D⋅cos θ
y = D⋅sin θ — end point for a distance and angle from start point.
D = √[(x₂ − x₁)² + (y₂ − y₁)²] — distances between points other then projection center are not valid.
This method can be invoked repetitively for doing calculations around different points. All returned
θ = atan2(y₂ − y₁, x₂ − x₁) — azimuths between points other then projection center are not valid.
etc.MathTransform
instances are immutable; changingGeodeticCalculator
state does not affect those transforms. Returns:
 transform from position CRS to Azimuthal Equidistant projected CRS centered on current start point.
 Throws:
IllegalStateException
 if the start point has not been set.GeodeticException
 if the projection can not be computed. Since:
 1.1
 See Also:
 Coordinate system is a twodimensional

toString
Returns a string representation of start point, end point, azimuths and distance. The text representation is implementationspecific and may change in any future version. Current implementation is like below:Coordinate reference system: Unspecified datum based upon the GRS 1980 Authalic Sphere ┌─────────────┬─────────────────┬──────────────────┬─────────────┐ │ │ Latitude │ Longitude │ Azimuth │ │ Start point │ 9°39′06.1120″N │ 132°37′37.1248″W │ 17°10′37″ │ │ End point │ 70°32′45.0206″N │ 109°50′05.0533″E │ 119°03′12″ │ └─────────────┴─────────────────┴──────────────────┴─────────────┘ Geodesic distance: 9,967,530.74 m
