Geodesy Package
Geodesy is the branch of science that deals with such topics as determining positions and areas over large parts of the Earth, the shape and size of the Earth, and the variations in the Earth’s gravitational and magnetic fields. The primary functions in this package are used for determining the distance between two points.
| SphericalDistance[pos1,pos2] | calculate the distance between the posi using a spherical model |
| SpheroidalDistance[pos1,pos2] | calculate the distance between the posi using a spheroidal model |
Finding the distance between two points on a sphere.
Each position can be given in degrees as a latitude‐longitude pair. A coordinate can also be expressed in the form {degrees,minutes,seconds}. A negative value for a coordinate indicates that the coordinate is south latitude or west longitude. Distances are returned in kilometers as the default.
The spherical model is based on an exact formula, while the spheroidal model is an approximation algorithm that is fairly good for distances of less than 10,000 kilometers. Because of the nature of this approximation, all computation with SpheroidalDistance is done with machine‐precision numbers.
| Radius->radius | specify the radius of the sphere in the spherical model |
| SemimajorAxis->length | specify the length of the semimajor axis in the spheroidal model |
| Eccentricity->value | specify the value of the eccentricity in the spheroidal model |
Options to control the size of the shape in the models.
The distances between points can be modified by using different values for the radius of the sphere, or the length of the semimajor axis and the eccentricity of the spheroid using the options Radius, SemimajorAxis, and Eccentricity, respectively. The default values for these options are those for Earth from the WGS‐84 standard, in kilometers.
| ToAuthalicRadius[semimajor,eccentricity] | compute radius of authalic sphere given spheroid’s semimajor axis and eccentricity |
| GeodeticToAuthalic[{lat,long},eccentricity] | compute coordinates of corresponding point on authalic sphere given latitude and longitude of point on spheroid with specified eccentricity |
| ToDegrees[{deg,min,sec}] | convert coordinate in degree‐minute‐second format to degrees |
| ToDMS[deg] | convert coordinate in degrees to degree‐minute‐second format |
Conversions between models and coordinate systems.
The simplest way to convert to the sphere from the spheroid is to use the authalic sphere. This sphere has the same surface area as the reference spheroid.
Note that the eccentricity used in the diagram was .6. The actual eccentricity of Earth’s spheroid is approximately .081.