# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Formula for dis. between two points on the Globe**

**From:**Jan van Puffelen

**Date:**1996 Oct 21, 18:08 EDT

Vincent (a dutch name?): At 11:33 20-10-96 -0400, you wrote: >Hello, > >Is there somebody that has the Formula to calculate the distance >between two points on the globe. > >eg. 52 29.66N and 52 32.86N > 005 02.45E 004 40.67E A very Dutch position! > >My GPS tells me that it is 13.6nm CTS106 > > Yes there are: 1. The straight line course (sailing a constant course from source to destination) als called "loxodrome" 2. The shortest distance between two points (but this requires a constantly changing course and can lead to unacceptably high latitudes). 3. The composite track (a good compromise between the two). I give the loxodrome course in the form of a small BASIC program, as developed for the CASIO FX7602P, a handheld computer with a small LCD screen. This machine could store up to 10 BASIC programs and has served me well over the years: ____________________________________________________________________________ ____________________ LOXODROME 218 ____________________________________________________________________________ ____________________ 1 INP "SR",R,"SS",S,"V",V,"HH",U SET F1 2 PRC 1,D-B Normalize difference in longitude RPC X,Y Z=180/pi*LN ((TAN C+1/COS C)/(TAN A+1/COS A)) Calculate integral of difference in longitude on the spheroid; an ellipsoid is a bit overdone 3 RPC Z,Y PRT "VH=";X/Z*(C-A)*60 distance K=Y-ASN (S/V*SIN (R-Y)) corrected for current 4 PRC V,K course vector Z=8*U/V/V leeway angle K=K-Z correct for leeway I=X J=Y PRC S,R current vector K=Y-360*INT (K/360) normalise course 5 RPC X+I,Y+J vector addition PRT "BV=";X resulting speed including current SET F0 PRT "DR=";Z,"WK=";K leeway and course ____________________________________________________________________________ _____________________ Distance and course are calculated between any two points on earth with coordinated (Lat, Long) in (A,B) and (B,C) following a straight line on the map (Mercator Projection) i.e. with a constant course (Loxodrome). The course is corrected for a given current vector and a calculated leeway angle. Leeway is calculated from the angle of heel and the speed. The CASIO has standard 26 predefined variables A-Z which are used as follows: The first column contains the variable, the second column the external name and the 3rd column an explanation ____________________________________________________________________________ _____________________ Var Disp Contains ____________________________________________________________________________ _____________________ A Latitude point of departure B Longitude point of departure C Latitude destination D Longitude of destination I J K Course R SR Direction of current S SS Speed of current V V Speed of the ship U HH Angle of heel +/- X Y Z Leeway angle ____________________________________________________________________________ _____________________ When the program is started the following information is started/displayed: ____________________________________________________________________________ _____________________ Display/Accept Description ____________________________________________________________________________ _____________________ SR Current direction SS Current speed V Speed of ship relative to water HH Angle of heel (negative to larboard, positive to starboard) VH= m.m Distance in NM BV= k.k Speed relative to the ground DR= g Leeway WK= True course to steer (to be corrected for variation and deviation on a magnetic compass) ____________________________________________________________________________ _____________________ N.B. the leeway angle is approximated with a formula from Sparkman & Stephens. Most BASIC instructions are self explanatory but the following: ____________________________________________________________________________ _____________________ Function Description ____________________________________________________________________________ _____________________ N.B. all trig functions work by default in degrees SET F1 Set display to 1 decimal position PRC (a),(b) Translates the Polar coordinates with length (a) and direction (b) into Rectangular Coordinates, stored in X and Y. The given expression is equivalent to: X=(a)COS(b) Y=(a)SIN(b) RPC (a),(b) Translates the Rectangular coordinates (a) and (b) into Polar Coordinates, stored in X (length) and Y(direction). The given expression is equivalent to: X=SQRT((a)^2+(b)^2) [plain old Pythagoras] Y=ATAN2((a),(b)) DMS a display a in format dd.mmss, i.e. DMS 3.5 displays 3? 30' 00.00" pi the famous constant, 3.14159265358979 etc. ____________________________________________________________________________ _____________________ The other program is slightly more complex. It calculates not only the distance and course according to the loxodrome, but also distance and initial heading (since the course changes all the time) of a great circle course, The coordinates of the vertex (the highest latitude reached, this is the reason why transatlantic flights fly over the north pole) and many more calculations: ____________________________________________________________________________ _____________________ Courses 628 ____________________________________________________________________________ _____________________ 1 PRC 1,D-B Normalize difference in longitude RPC X,Y K=Y Remember sign of difference in longitude Z=180/pi*LN ((TAN C+1/COS C)/(TAN A+1/COS A)) meridional parts on sphere 2 RPC Z,Y X=X/Z*(C-A) distance PRT "LX"; print "LX" to indentify loxodrome GSB 100 print distance and course 3 X=ACS (COS (D-B)*COS C*COS A+SIN A*SIN C) Great Circle distance Y=ACS ((SIN C-SIN A*COS X)/COS A/SIN X Initial heading IF SIN (D-B)<0; normalize initial heading Y=360-Y 5 PRT "GC"; print GC to identify Great Circle GSB 100 print distance and course Z=ATN (1/(TAN Y*SIN A)) difference in longitude with vertex I=ATN (TAN A/COS Z) latitude of vertex 6 PRC 1,B+Z normalize longitude vertex RPC X,Y J=Y PRT "VTX: LAT="; print to identify vertex DMS I print latitude vertex PRT "LON="; DMS J print longitude vertex 7 INP "GC CALC: LATX",R input latitude X IF ABS R>=ABS I; if this is greater than the latitude of the vertex PRT "NO LONX" there are no points on the great circle GOTO 10 skip remainder of calculation 8 S=ACS (TAN R/TAN I) difference in longitude with vertex PRC 1,J-S normalize western point on GC RPC X,Y PRT "LONX1="; print first longitude X DMS Y 9 PRC 1,J+S normalize eastern point on GC RPC X,Y PRT "LONX2="; print second longitude X DMS Y 10 INP "LONY",U input longitude Y T=ATN (COS (J-U)*TAN I) compute latitude Y PRT "LATY="; print latitude Y DMS T SET F0 set display to 0 decimals 11 M=ACS (SIN (J-U)*SIN I) course in point LatY, LonY N=ASN (SIN M*SIN (R-T)) minimum distance from point LatX, LonY to Great Circle IF K<0; direction along Great Circle M=M+180 course is opposite 12 O=ATN (1/(COS (R-T)*TAN M)) bearing from point LatX, LonY to nearest point on Great Circle IF N>0 N of the Great Circle O=180-O bearing is complement GOTO 14 13 O=-O S of the Great Circle IF O<0; normalize bearing O=O+360 14 PRT "COURSE(YY)=";M,"BEARING(XY)=";O print course in YY and bearing in XY SET F1 set display to 1 decimal PRT "MIN(XY)=";N*60,"LATM="; print minimum distance in NM and coordinates of this point of the great circle 15 PRC ABS N,O sail this distance in the direction of the bearing DMS R+X print the latitude of LatM PRC 1,U+Y/COS (R+X/2) normalize the longitude of LonM RPC X,Y PRT "LONM="; print LonM DMS Y GOTO 7 return for more calculations 100 IF Y<0; subroutine to normalize a given course Y=Y+360 101 SET F1 set the display to one decimal PRT ": DIST=";X*60;" COURSE=";###;Y print the distance in NM and Course RET ____________________________________________________________________________ _____________________ This program calculates a loxodrome course, a great circle course and several things around great circle courses Between the point of departure (latitude in A, longitude in B) and the destination (latitude in C, longitude in D), course and distance according to the loxodrome, distance and initial heading of the Great Circle and the Latitude and Longitude of the Vertex (point with the highest latitude on this great circle) are calculated. With a given LatX the corresponding LonX1 and LonX2 are calculated (for a given latitude there are either 2, 1 or no points on the great circle). With the given LonY the corresponding LatY is calculated as well as the course of the ship in point LatY, LonY. >From a point with given LatX, LonY, the minimum distance to the great circle is calculated as well as the (radio/radar/visual) bearing to the ship. The coordinates LatM, LonM of the point on the great circle with minimum distance to the given point LatX, LonY are calculated as well. ____________________________________________________________________________ _____________________ Variable Display Contains ____________________________________________________________________________ _____________________ A Latitude point of departure (N/S=+/-) B Longitude point of departure (E/W=+/-) C Latitude destination (N/S=+/-) D Longitude destination (E/W=+/-) I Latitude vertex (N/S=+/-) J Longitude vertex (E/W=+/-) K Difference in Longitude (E/W=+/-) M Course in LatY, LonY N Minimum distance from LatX, LonY to great circle O Bearing from LatX, LonY to point with minimum distance on great circle R LatX (N/S=+/-) S Diff Longitude LonX (E/W=+/-) T LatY (N/S=+/-) U LonY (E/W=+/-) X Y Z ____________________________________________________________________________ _____________________ The conversation goes as follows: ____________________________________________________________________________ _____________________ Display/Accept Description ____________________________________________________________________________ _____________________ LX: DIST= m.m COURSE= g Distance and Course loxodrome GC: DIST= m.m COURSE= g Distance and init. Course great circle VTX: LAT= gg mm ss.s Latitude Vertex (N/S=+/-) LON= gg mm ss.s Longitude Vertex (E/W=+/-) _____________ Is repeated: _____________ GC CALC: LATX Input LatX (N/S=+/-) NO LONX Latitude is too high LONX1= gg mm ss.s Longitude of first point on great circle LONX2= gg mm ss.s Longitude of 2nd point on great circle LONY Input LonY (E/W=+/-) LATY= Display corresponding LatY COURSE(YY)= g Course in point LatY, LonY BEARING(XY)= g Bearing from LatX, LonY to point on GC with min dist. MIN(XY)= m.m Minimal distance great circle to point LatX, LonY LATM= gg mm ss.s Latitude point on GC with min dist to LatX, LonY LONM= gg mm ss.s Longitude point on GC with min dist to LatX, LonY ____________________________________________________________________________ _____________________ Example: San Francisco (37? 25'N, 122? 30'W) to Yokohama (35? 30'N, 139? 40'E) Loxodrome 4722.1 NM, course 269? Great Circle 4479.0 NM, initial course 303? Vertex 48? 22'5N, 169? 40.2'W For LatX, LonY we enter the coordinates of Dutch Harbour (53? N, 166? W). This gives the following results: ? There are no points with the given latitude LatX=53? N on the great circle (53? has a higher latitude than the vertex), normally there are two points on the great circle LonX1 and LonX2. ? For the given longitude LonY=166? W the latitude LatY=48? 19'N ? The course in point LatY, LonY is 273? ? The bearing from Dutch Harbour to the nearest point on the great circle is 183? ? The minimum distance the ship on the great circle passes Dutch Harbour is 280.7 NM ? The point of minimum distance on the great circle to Dutch Harbour is LatM=48? 19.6'N, LonM=166? 21.3'W. N.B. Both programs work accurate to any distance as the loxodrome is calculated based on the meridional parts formula (Dutch: Vergrotende Breedte). According to this last program, the loxodrome distance between your given two points is 13,6 NM and a course of 284 true. According to the great circle, the distance is the same (the distance is very short) and the initial heading is 284 too. N.B. If you are interested I can send the Composite track program as well. Such tracks are used for long courses between relatively high northern or southern latitudes , for instance from cape town to melbourne, from which the great circle would lead to unacceptable high latitudes (through the polar region). > >THANKS! You are welcome, Jan van Puffelen <puffelej{at}XXX.XXX> 52d 24.5N 4d 55.0E > > > >*** Jippie Kajee KaJoo *** > > >