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Azimuth and Altitude of a body from earth
Posted: 20.11.2003, 12:13
by alexibu
Hi,
Ive created a method to calculate the position of a celestial body in the sky given the observers location on a planet and the time.
Im not sure how accurate it is because I don't know what to trust more (celestia and my calcs - possibly with precision probs) or other sky map programs.
This would be useful to people trying to control telescopes with celestia.
Code: Select all
void Body::getDirectionOfBodyFromSurface(const Vec3f& lonLatAlt,Body * body,double when,
double& Dist,double& Azimuth,double&Altitude)
{
//get the bodies position in geographic coodinates
Point3d bodyPointd = body->getHeliocentricPosition(when)
*getGeographicToHeliocentric(when).inverse();
//get the position of the observer in geographic coords
Vec3f observerVector = planetocentricToCartesian(lonLatAlt);
//create a plane tangential to the earths surface where the
//observer is
Vec3f FloorN = observerVector;
FloorN.normalize();
Planef FloorPlane(FloorN,observerVector.length());
//find a point north of the observer by finding the floor planes
//intersection with the geographical y axis
Point3f northPoint = Planef::intersection(FloorPlane,
Planef(Vec3f(1,0,0),0),Planef(Vec3f(0,0,1),0));
//Do some precision conversions
Point3f bodyPoint(bodyPointd.x,bodyPointd.y,bodyPointd.z);
Vec3f bodyVector(bodyPoint.x,bodyPoint.y,bodyPoint.z);
Vec3f northVector(northPoint.x,northPoint.y,northPoint.z);
//calculate the vector from the observer to north
northVector -= observerVector;
//create a plane that splits the planet north south
//from the point of view of the observer
Vec3f NSPlaneN = northVector^observerVector;
NSPlaneN.normalize();
Planef NorthSouthPlane(NSPlaneN,0);
//Do the same for east west
Vec3f EWPlaneN = northVector;
EWPlaneN.normalize();
Planef EastWestPlane(EWPlaneN,0);
//Calculate the Azimuth
float bodyDistanceFromNSPlane = NorthSouthPlane.distanceTo(bodyPoint);
float bodyDistanceFromEWPlane = EastWestPlane.distanceTo(bodyPoint);
Azimuth = atan(bodyDistanceFromNSPlane/bodyDistanceFromEWPlane);
if (bodyDistanceFromNSPlane > 0) {
if (bodyDistanceFromEWPlane > 0) {
//do nothing
}else{
Azimuth += PI;
}
}else{
if (bodyDistanceFromEWPlane > 0) {
Azimuth = 2*PI + Azimuth;
}else{
Azimuth += PI;
}
}
//Calculate the Altitude
Altitude = asin(FloorPlane.distanceTo(bodyPoint)/Dist);
//work out the distance from the observer to the body
bodyVector -= observerVector;
Dist = bodyVector.length();
Dist -= body->getRadius();
}
Alex
Altitude and Azimuth calcs updated
Posted: 02.12.2003, 12:16
by alexibu
Hi,
Ive changed the function that I posted for calculating the Alt/Azimuth of a body from the surface of a planet.
It now works for any object not just other bodies in the solar system, and is double precision for the important parts. Changed body argument to a universal coord.
Alex
Code: Select all
void Body::getDirectionFromSurface(const Vec3f& lonLatAlt,const UniversalCoord & TargetPos,
const double & TargetRadius,const double when,
double& Dist,double& Azimuth,double&Altitude)
{
//get the position of the observers sun in universal coords
Point3f sunPos(0.0f, 0.0f, 0.0f);
const Star* sun = getSystem()->getStar();
if (sun != NULL) sunPos = sun->getPosition();
//get the targets position in geographic coodinates
Point3d bodyPoint = astro::heliocentricPosition(TargetPos,sunPos)
*getGeographicToHeliocentric(when).inverse();
//get the position of the observer in geographic coords
Vec3f observerVectorf = planetocentricToCartesian(lonLatAlt);
Vec3d observerVector(observerVectorf.x,observerVectorf.y,observerVectorf.z);
//create a plane tangential to the earths surface where the
//observer is
Vec3d FloorN = observerVector;
FloorN.normalize();
Planed FloorPlane(FloorN,-observerVector.length());
//find a point north of the observer by finding the floor planes
//intersection with the geographical y axis
Point3d northPoint = Planed::intersection(FloorPlane,
Planed(Vec3d(1,0,0),0),Planed(Vec3d(0,0,1),0));
//Do some conversions
Vec3d northVector(northPoint.x,northPoint.y,northPoint.z);
//calculate the vector from the observer to north
northVector -= observerVector;
//create a plane that splits the planet north south
//from the point of view of the observer
Vec3d NSPlaneN = northVector^observerVector;
NSPlaneN.normalize();
Planed NorthSouthPlane(NSPlaneN,0);
//Do the same for east west
Vec3d EWPlaneN = northVector;
EWPlaneN.normalize();
Planed EastWestPlane(EWPlaneN,0);
//Calculate the Azimuth
Azimuth = atan2(NorthSouthPlane.distanceTo(bodyPoint),
EastWestPlane.distanceTo(bodyPoint));
//work out the distance from the observer to the body
Point3d Temp = bodyPoint - observerVector;
Dist = Temp.distanceFromOrigin();
//Calculate the Altitude
Altitude = asin(FloorPlane.distanceTo(bodyPoint)/Dist);
//Subtract the radius of the target
Dist -= TargetRadius;
//The distance to the planet the observer is on is sometimes slightly negative
//due to precision errors - this looks bad so make sure that distance is positive
Dist = fabs(Dist);
}
Posted: 02.12.2003, 16:29
by Christophe
Hi Alex,
I haven't tried your code yet, but I think it's a nice addition. Once 1.3.1 is released, I'll try and add it for display in the popup menu.
Something that would be really useful too would be Alt/Az cursor coordinates. Do you think you could do that as well?
Alt / Az code
Posted: 02.12.2003, 18:58
by alexibu
Err,
The code I posted doesn't work,
Ill post a new one soon
sorry
Alex
Alt / Az code
Posted: 04.12.2003, 14:02
by alexibu
Hi,
Ive now fixed this code.
Code: Select all
void Body::getDirectionFromSurface(const Vec3f& lonLatAlt,const UniversalCoord & TargetPos,
const double & TargetRadius,const double when,
double& Dist,double& Azimuth,double&Altitude)
{
//get the position of the observers sun in universal coords
Point3f sunPos(0.0f, 0.0f, 0.0f);
const Star* sun = getSystem()->getStar();
if (sun != NULL) sunPos = sun->getPosition();
//get the targets position in geographic coodinates
Point3d bodyPoint = astro::heliocentricPosition(TargetPos,sunPos)
*getGeographicToHeliocentric(when).inverse();
//get the position of the observer in geographic coords
Vec3f observerVectorf = planetocentricToCartesian(lonLatAlt);
Vec3d observerVector(observerVectorf.x,observerVectorf.y,observerVectorf.z);
//create a plane tangential to the earths surface where the
//observer is
Vec3d FloorN = observerVector;
FloorN.normalize();
Planed FloorPlane(FloorN,-observerVector.length());
//find a point north of the observer by finding the floor planes
//intersection with the geographical y axis
Point3d northPoint = Planed::intersection(FloorPlane,
Planed(Vec3d(1,0,0),0),Planed(Vec3d(0,0,1),0));
//Do some conversions
Vec3d northVector(northPoint.x,northPoint.y,northPoint.z);
//calculate the vector from the observer to north
northVector -= observerVector;
//create a plane that splits the planet north south
//from the point of view of the observer
Vec3d NSPlaneN = northVector^observerVector;
NSPlaneN.normalize();
Planed NorthSouthPlane(NSPlaneN,0);
//Do the same for east west
Vec3d EWPlaneN = northVector;
EWPlaneN.normalize();
Planed EastWestPlane(EWPlaneN,0);
//Calculate the Azimuth
Azimuth = atan2(NorthSouthPlane.distanceTo(bodyPoint),
EastWestPlane.distanceTo(bodyPoint));
//work out the distance from the observer to the body
Point3d Temp = bodyPoint - observerVector;
Dist = Temp.distanceFromOrigin();
//Calculate the Altitude
Altitude = asin(FloorPlane.distanceTo(bodyPoint)/Dist);
//Subtract the radius of the target
Dist -= TargetRadius;
//The distance to the planet the observer is on is sometimes slightly negative
//due to precision errors - this looks bad so make sure that distance is positive
Dist = fabs(Dist);
}