經緯度的semicircle單位

今天在處理佳明fit文件的時候，發現佳明用的gps的經緯度的單位是semicircle，而不是常見的度。後來在MSDN中找到相應的換算方法。

The World Geodetic System (WGS-84) coordinate system is a global standard that plots locations on earth by using an earth-fixed global reference frame. WGS-84 provides coordinates in relation to the center of the earth, compared with latitude and longitude coordinates that are determined by ground-based mathematical calculations. WGS-84 also uses various parameters, such as the shape of the geoid, the earth mass, and gravity, to calculate coordinates with improved accuracy.
A semicircle is a unit of location-based measurement on an arc. An arc of 180 degrees is made up of many semicircle units; 231 semicircles to be exact. A semicircle can be represented by a 32-bit number. This number can hold more information and is more accurate for GPS systems than a simple longitude and latitude value. Semicircles that correspond to North latitudes and East longitudes are indicated with positive values; semicircles that correspond to South latitudes and West longitudes are indicated with negative values.
The following formulas show how to convert between degrees and semicircles:
degrees = semicircles * ( 180 / 2^31 )
semicircles = degrees * ( 2^31 / 180 )

MSDN WGS84原文