Geoida Help

Azimuth and distance in Measure option

Question: In Geographic display mode, when displaying the azimuth and distance between two points using the Measure option, azimuths that should be 90 degrees or 270 degrees are shown as values greater or less than the correct values and get worse with greater distances.

Answer: This is normal - but the apparent error in the azimuths can become alarming to the unaware. The azimuth between two points on the same parallel of latitude is NOT 90/270 degrees except perhaps over very short distances where the difference is below the level of detection or display precision. The azimuth computed between any two points on the Earth's surface is along the great circle through the points and only when both points are actually on the Equator are the azimuths going to be 90/270. Also, when the points are separated by 180 degrees or greater longitude, the computed azimuth and distance will actually be for the complimentary sector of the great circle, ie, around the other way! The Vincenty formulae which are used for geodetic computations within Geoida always return the LESS than 180 degree longitude values, ie those for the shorter great circle arc. Unusual values may also be more likely in polar regions - for example, between 2 points on the same parallel of latitude but separated by nearly 180 degrees longitude, the azimuth may actually be very close to 0 or 180 degrees! The screen display in Geoida in geographic mode is a simple non-conformal square grid which does not correctly represent the spatial relationship between points on the Earth's surface, so the displayed values from the Measure option may appear to be incorrect when large areas are displayed. But the actual values displayed should be the correct azimuth and distance between the points along the great circle between them.