WSJT-X/lib/deg2grid.f90

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subroutine deg2grid(dlong0,dlat,grid)
real dlong !West longitude (deg)
real dlat !Latitude (deg)
character grid*6
dlong=dlong0
if(dlong.lt.-180.0) dlong=dlong+360.0
if(dlong.gt.180.0) dlong=dlong-360.0
! Convert to units of 5 min of longitude, working east from 180 deg.
nlong=int(60.0*(180.0-dlong)/5.0)
n1=nlong/240 !20-degree field
n2=(nlong-240*n1)/24 !2 degree square
n3=nlong-240*n1-24*n2 !5 minute subsquare
grid(1:1)=char(ichar('A')+n1)
grid(3:3)=char(ichar('0')+n2)
grid(5:5)=char(ichar('a')+n3)
! Convert to units of 2.5 min of latitude, working north from -90 deg.
nlat=int(60.0*(dlat+90)/2.5)
n1=nlat/240 !10-degree field
n2=(nlat-240*n1)/24 !1 degree square
n3=nlat-240*n1-24*n2 !2.5 minuts subsquare
grid(2:2)=char(ichar('A')+n1)
grid(4:4)=char(ichar('0')+n2)
grid(6:6)=char(ichar('a')+n3)
return
end subroutine deg2grid