Heading line drawn @ wrong angle when using degrees magnetic

Carl Mascott shared this problem 7 years ago

Locus Free 2.2.3

Use hardware compass: OFF

Use true bearing: OFF

Screen mode: North up

When using degrees magnetic the heading line is drawn on the map @ the wrong angle. It appears that the magnetic declination (15.1 degrees W in this case) is being added (West = positive). A picture is worth a thousand words. In this picture of an aircraft taxiing down a runway, the course line is red and the heading line is orange. When using degrees true both the course line and the heading line point straight down the runway.

UPDATE 04/23/12

I forgot to mention that in degrees magnetic the GPS cursor has the same problem as the heading line. In the screenshot you can see the GPS cursor pointing along the incorrect heading line.

Comments (14)


I was setting incorrectly magnetic declination as

bearing (direction of travel in degrees East of true North) - declination.

Which is wrong. Declination is define as positive angle from true north to east. So thanks for report, fixed now


one more note. Carl please, because declination in your area is so huge, may you test next version if all is correct? Not just these headings lines but also compass and map rotate, rotated by hardware compass and also by GPS bearing. Thank you. Declination in my area is around 2° and it`s really useless for testing ...



I don`t understand your first reply. Course line and course readout were both OK in degrees magnetic. Only heading line was incorrect. For purpose of drawing course & heading lines on (true) North-up map screen, magnetic declination should not be used at all (EDIT: Incorrect. When heading is coming from magnetic compass, heading must be converted to degrees true to get screen angle.). Regardless of degree mode (true or magnetic) the angle on the (true) North-up screen, as measured from 12 O`clock position, should be simply degrees true.

RE heading line


It occurred to me that with Use hardware compass: OFF, as I have it set, it is impossible to know the heading: GPS gives only the course. I can think of two ways to deal with this:

1. Disable heading line when Use hardware compass: OFF. Perhaps heading line menu item should be grayed out, with comment that HW compass is required to get heading line.

2. Fake the heading line: just align it with the course line.

I`ll be happy to test what I can but my only Android device (Nook Color) doesn`t have a compass.

P.S. Don`t forget GPS cursor.



You said above: "with Use hardware compass: OFF, as I have it set, it is impossible to know the heading: GPS gives only the course".

That is true.

Then you said: "my only Android device (Nook Color) doesn`t have a compass". If your Nook Color doesn`t have a magnetic sensor, how could it ever sense heading?

You would think that Locus would check for the presence of a magnetic sensor, and if false, or if it is set to OFF, the orange heading line would be suppressed.


My Nook Color is running CM7.2-RC0 MiRaGe 12/12/2011.

Droid System Suite shows, under Available Features:



and several other hardware features that the device doesn`t actually have.

If Locus is checking, it may be getting misinformation.

One of my suggestions above was to disable the heading line with Use hardware compass: OFF. To extend that, if Locus can detect "no compass present" (perhaps not possible with this device), that would be another case in which to disable the heading line.

Note 1: Just for the heck of it I tried a compass app from Google Play Store. It reported a magnetic sensor error and did not work.

Note 2: Device has no GPS either. I`m using a Bluetooth GPS simulator.


This is really getting comical !!

Although Locus is a "Navigation" app, none of the computer geeks at Locus know anything about navigation.

The developer lives on the Agonic line, so he cannot test magnetic versus True.

They`re asking you to do real-world testing with a book reader with no compass and substituting a GPS SIMULATOR for the GPS.







I`ve been thinking: Perhaps map rotation by GPS bearing shouldn`t be allowed. That gives a course-up map, not a heading-up map. In land navigation course and heading are the same except in abnormal circumstances (e.g., vehicle in a skid). In marine, and especially air, navigation, course and heading generally are not the same. I don`t know if a course-up map display is desirable.

Also, since you are calculating the track yourself, and since there is no track until the vehicle has moved, there is a jump in map rotation when the vehicle first starts to move. I don`t know if using the track from the GPS would solve this: I`ve seen this referred to as "course made good" and "track made good" in unofficial documentation (I`ve never seen the official NMEA 0183 spec.), which implies that there`s no valid data there either, until the vehicle has moved (because there`s no "made good" until the vehicle has moved).

One thing seems clear: if "Use hardware compass: ON", but there is no compass, map rotation shouldn`t be allowed. Indeed, on my device, Locus doesn`t allow it.

One more thing: GPS cursor currently shows heading. Should it show course instead? Again, GPS reports only course, not heading.

I don`t know the right solutions -- I`m just raising the issues.


I just tried v2.3.1.

Everything still OK in degrees true.

In degrees magnetic there are two differences from v2.2.3:

1. Heading line (yes, I know: meaningless w/o compass) has flipped to the other side of the course line by the same angle.

2. Course readout is now wrong (was correct in v2.2.3): with 15.1 degrees W declination and 135 degree true course, course readout is now 120 degrees. Correct magnetic course is 150 degrees.

Also, the GPS cursor still points down the heading line (same as v2.2.3).


I just tried v2.3.4.

No change from v2.3.1 (see above).

Please notify me (i.e., by posting here) when there is something new to test RE degrees magnetic heading line, course readout and GPS cursor.

In the meantime I will continue to use v2.2.3.


thanks for notification by email, I already forget on this incomplete topic ...

firstly about declination. In first post you wrote that it work incorrectly. I fixed computed orientation to

magnetic value from compass - declination (from previous + declination)

and it`s still wrong?? I know you don`t say exactly where is problem but I think that this definition is correct! I did also some improvements now, so you should not be able to turn on compass if your device don`t have any. So please check next market version and let me know. Then we`ll continue ...


It sounds like you may be confusing two things:

1. The angle of the heading line on the map (original post)

2. The numeric course readout

When converting from true to magnetic, the mnemonic is:

"East is least, West is best."

That means that with W declination , magnetic > true.

Here is what is happening with numeric course readout in Locus:

True course: 135

Magnetic declination: 15 W

Locus v2.2.3 magnetic course: 150 (correct)

Locus v2.3.0-v2.4.1 magnetic course: 120 (incorrect)

Your modification (in v2.3.0) to attempt to fix the angle of the heading line made the numeric course readout incorrect (W declination, magnetic < true).

I hope this clears things up.


Hello Carl,

even that you wrote me email, I almost forget on this topic, sorry. Some very similar issue is already discuss here https://getsatisfaction.com/locus/top...

so please check next Google Play version 2.5.5 and let me know what`s current status of this issue if you`re still interested in fixing