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Bugfix for algorithm in get_distance()

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Fixed bug in the Haversine function in get_distance() based on algorithm
at https://www.movable-type.co.uk/scripts/latlong.html and added bounds
to the 'a' term to avoid floating point errors for antipodal points.

Signed-off-by: Doug Junkins <junkins@foghead.com>
This commit is contained in:
Doug Junkins 2019-04-30 06:53:31 -07:00 • committed by bstoeger
parent 154792ffd1
commit 14a763a6a0
2 changed files with 6 additions and 2 deletions
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1
CHANGELOG.md
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@ -1,3 +1,4 @@
- Core: fix bug in get_distance() to correctly compute spherical distance
- Desktop: For videos, add save data export as subtitle file
- Desktop: make dive sites 1st class citizens with their own dive site table
- Desktop: only show dives at the dive sites selected in dive site table

7
core/divesite.c
View file

@ -77,15 +77,18 @@ struct dive_site *get_dive_site_by_gps_and_name(char *name, const location_t *lo
// Calculate the distance in meters between two coordinates.
unsigned int get_distance(const location_t *loc1, const location_t *loc2)
{
double lat1_r = udeg_to_radians(loc1->lat.udeg);
double lat2_r = udeg_to_radians(loc2->lat.udeg);
double lat_d_r = udeg_to_radians(loc2->lat.udeg - loc1->lat.udeg);
double lon_d_r = udeg_to_radians(loc2->lon.udeg - loc1->lon.udeg);
double a = sin(lat_d_r/2) * sin(lat_d_r/2) +
cos(lat2_r) * cos(lat2_r) * sin(lon_d_r/2) * sin(lon_d_r/2);
cos(lat1_r) * cos(lat2_r) * sin(lon_d_r/2) * sin(lon_d_r/2);
if (a < 0.0) a = 0.0;
if (a > 1.0) a = 1.0;
double c = 2 * atan2(sqrt(a), sqrt(1.0 - a));
// Earth radious in metres
// Earth radius in metres
return lrint(6371000 * c);
}