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Calculating the azimuth
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JavaScript
(see
here for description of these functions
)
(default values are for Baltray [point A] and Rockabill [Point B], both in Ireland)
(Remark: north and east are positive, south and west are negative)
WGS84 Longitude/latitude method
WGS84 Long A:
[
o
]
WGS84 Lat A:
[
o
]
WGS84 Long B:
[
o
]
WGS84 Lat B:
[
o
]
GPS error
:
[m]
Light line
1
FAI Rhumb line
2
FAI Greater circle line
2
Map grid method
Grid:
Irish grid
UK grid
OSLong A:
[m]
OSLat A:
[m]
OSLong B:
[m]
OSLat B:
[m]
Grid error:
[m]
Sometimes the Irish map grid holds a letter: Instead of using a letter add the below numbers to it OSlong and OSlat:
Example Rockabill (Ireland): O 32177, 62660 -> 332177, 262660
Distance from A to B:
[m]
Geocentric angle:
[
o
]
Geodetic Azimuth
l
from A to B:
[
o
] +/-
[
o
]
Grid distance:
[m]
Grid azimuth:
[
o
]
Convergence angle:
3
[
o
]
To get a reliable difference, you NEED to input
LongA and Lat A
!!!
Geodetic Azimuth
m
:
[
o
] +/-
[
o
]
The light line distance is based on converting
WGS84 into xyz ECEF 3D Cartesian coordinates
.
The light line azimuth for the light line comes from conversion of xyz ECEF 3D Cartesian coordinates to
local ENU coordinates
(pers. comm. E. Williams [2010]).
An approximation formula (based on FAI spheroid) is used to determine the rhumb/greater circle distance/azimuth (from:
Calculate the great circle and rhumb distance
). It can be
shorter
than the light line distance (which uses ellipsoid based Earth)!
A spreadsheet for rhumb/great circle distance/azimuth, using the more accurate ellipsoid formula (worksheet
spheroid
), can be
found here
. A greater circle web-based calculator can be
found here
.
The formula to determine the Convergence angle comes from
A. Thom's [1967], page 168
The error in azimuth is somewhat smaller than derived by Prendergast (2001, page 180), as the d
i
on both sides have uncorrelctaed errors:
dA
=atan(2*d
i
*0.707/L)
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Last major content related changes: July 18, 2000
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