Projecting Waypoints With Greater Precision

[binthair]

Projecting Waypoints With More Precision

 

by binthair

 

4 December 2002

 

I have developed a technique on my Garmin GPS 12 which enables me to project waypoints with greater precision than the 100 meters directly available with the unit*. (The technique should also work with suitable modification on other units than the Garmin GPS 12.)

 

The technique involves making use of the built-in Simulator mode. (Simulator mode is called Demo mode on some other brands.) By simulating movement away from a waypoint at a given bearing and speed and watching the trip time, one can mark a waypoint when the trip time reaches a required value. For instance, travelling at 36 km/hr covers 10 meters per second. So if the required projected distance is, say, 270 meters, then marking a waypoint when the trip timer reaches exactly 27 seconds will create a waypoint 270 meters from the present waypoint. Greater precision can be achieved with longer waits. (18 km/hr covers 5 meters / sec; 9 km/hr covers 2.5 meters/ second; and the ultimate precision with this method is 1 km/hr covering 0.2777 meters/sec.).

 

To use the technique, make sure that there is no active route, otherwise, the track may curve attempting to go to a route waypoint. When finished with the projection, make sure to take the unit out of Simulator mode and put it back in Normal mode.

 

----------------------------------------

* It is of course possible (but more awkward) to project an approximate waypoint in even 100 meter increments, proceed to that waypoint, and then do a GOTO back along the reciprocal bearing in 10 meter increments. However, the technique which is the subject of this note is more direct and can be more precise.

 

Example (for Garmin GPS 12):

 

In this example the initial position is N45 25.365 W075 42.993; the desired distance to the projected waypoint is 270 meters and the desired bearing to the projected waypoint is 216 degrees. (Desired accuracy is 10 meters in this example, so use a simulator speed of 36 km/hr.)

 

1. Clear the active route.

2. Go to SETUP MENU; Select SYSTEM; Set MODE to Simulator.

3. Go to the position display screen.

4. Initially set SPEED to zero. (This stops any simulation while other values are being set.)

5. Set position to desired start position for example N45 25.365 W075 42.993 .

6. Set desired bearing (i.e., TRACK), for this example equal to 216 degrees.

7. Reset trip time (not elapsed time) TTIME to 00:00 .

8. Set Speed to desired speed, for this example, 36 km/hr, but DO NOT hit ENTER button until ready

9. When ready, hit ENTER and watch the TTIME display.

10. Quickly hit the MARK button when the time reaches the desired time, 27 seconds in this example.

11. The MARK POSITION display will show the projected waypoint position of N45 25.245 W075 43.117 . Save it.

12. PAGE back to the position display and stop the simulated track by setting the SPEED to zero.

13. PAGE to MAIN MENU; Go to SETUP MENU; Select SYSTEM; set MODE to NORMAL.

 

When familiar with the procedure, the above example takes about 2 ½ minutes to perform.

 

MapSource calculates the range to be 275.1 meters at a bearing of 216 degrees from the first waypoint to the projected waypoint. Since 1 second increments correspond to10 meters travel at 36 km/hr this answer is acceptable. For more precision, a rerun for 108 seconds of simulation at 9 km/hr corresponding to 2.5 meters / second of travel yields N45 25.247 W075 43.115 . In this case, a check with MapSource calculates the range as 270.6 meters at a bearing of 216 degrees.

[+RPaske]

Interesting. Anyone know how to do this using Garmin gpsmap76?

[+PDOP's]

I believe that the older Garmin units are unique in being able to set the speed in simulation mode. The Etrex series uses a set speed of 20 mph in it's demo mode. Not sure about the others.

 

Checkout Dale's online manual for more on this mode.

 

[This message was edited by PDOP's on December 04, 2002 at 03:53 PM.]

[+CharlieP]

quote:

---

Originally posted by RPaske:

Interesting. Anyone know how to do this using Garmin gpsmap76?

---

 

With my GPS 76 there is an easier way. Go to the map page, press menu, select Measure Distance, then just play with this function to learn how it works. It can be used to measure distances, project waypoints, or to determine bearings. Note that you press enter to reset the measuring reference to the pointer location, otherwise it uses the current location.

 

You can also project waypoints by pressing menu from the waypoint page and selecting Project Location, but this limits you to .01 mile accuracy.

 

FWIW,

CharlieP

[+apersson850]

Changing the units for bearing to mils instead of degrees gives you a resolution of 6400 mils around the compass rose, instead of 360 degrees.

 

Doesn't affect resolution of distance, of course.

 

Anders