Author Topic: Bearing to and From  (Read 1304 times)

Offline carlseibert

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Bearing to and From
« on: August 30, 2021, 10:26:23 AM »
Hi,

I have found two issues in Bearing To and Bearing From. One MIGHT be a bug or it might be my mind operating backwards to everybody elses'.

The Bearing From and Bearing To variables may or may not be transposed. In my mind, "bearing from" the GPS home location would be which way to go to get from the GPS home location to the photo. And vice-versa for "bearing to". In which case, yeah, they're backwards.

... On the other hand, one might be standing AT the home location and "bearing to" would be which way to look to see the photo location. In which case the variables are right-way-round.

This kind of reminds me of Boolean AND and OR.

I can't easily find anything on the web that sheds any sort of standard light. All the references I find assume that one is standing somewhere looking - or shooting - at something somewhere else.

Maybe a line of explanatory text in the Set GPD Home dialog is called for. No biggie though, because people can simply try it until they get it the way they want it. Which is exactly what I did and plan to tell my viewers.

Meanwhile, there does appear to be a convention that says that bearings are supposed to be three digits before the decimal. "Three-six-zero" for north, but still "zero-nine-zero" for east and so forth. My memory from when I had to be able to speak pilot was that way.

And I would love to know what the original use case for Bearing To and Bearing From was. :-)

Cheers.

-Carl


Offline Odd Skjaeveland

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Re: Bearing to and From
« Reply #1 on: September 03, 2021, 03:11:36 AM »
...
... On the other hand, one might be standing AT the home location and "bearing to" would be which way to look to see the photo location.

Interesting. I have always thought that was the meaning of bearing to, but it is probably on par with the North wind. Bearing to will also have two solutions, in the sense that I would naturally travel North to get the Arctic, but I could travel southwards via the Antarctic. Does PM's bearing refer to the shortest distance along the Great Circle or Loxodrome?

I do not know the answer. I did however, check out with one of my images with GPS set to a place in Manila (PH), 14.x deg North, 121.x deg East. My home position is SW part of Norway 58.x deg North, 5.x deg East. PM calculates the distance =1005km, about 1/4 of the Earths circumference, thus it must be the shorter distance between the two locations.

Looking at a world map I would guess a SE bearing from home to Manila, something between 120 and 170 deg on the compass. But those things are hard to judge from a flat map. PM thinks differently, and calculates a bearing from home to Manila is 61 deg, meaning NE. I may perhaps arrive in Manila travelling that way, and it may even be shorter route. But it feels unnatural. PM finds the bearing home from Manila to be 332 deg, that is NW, which feels right I believe the distance is different than the 61 deg route to Manila. Subtracting 180 deg from 330 makes 150 deg, which is indeed SE from home and it feels more natural to me than PMs 61 deg does.

Conclusion: Use PM's bearing from/to with care?
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Odd S.

Offline carlseibert

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Re: Bearing to and From
« Reply #2 on: September 03, 2021, 07:32:33 AM »
I don't think any of these features are exactly meant for navigation. :-)  I was just stuck on the point of view thing (am I conceptually at the reference point or the photo?) until I realized that it didn't matter. If I just tweak my setup until the variable works the way I want it to, it's all good. Nobody sees what the variable is anyway.

Offline dennis

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Re: Bearing to and From
« Reply #3 on: September 03, 2021, 02:22:14 PM »
I don't think any of these features are exactly meant for navigation. :-)  I was just stuck on the point of view thing (am I conceptually at the reference point or the photo?)

These are based on the GPS home as you noticed and relative to the photo.  Bearing "to" means bearing (or heading) from GPS home TO the photo.  Bearing "from" means bearing (or heading) FROM photo to GPS home.  You can call this backwards if you like but that is how I coded it.

They should be accurate.  I compared using Google Earth Pro and I get the same values.

NOTE: this is initial bearing and would change of course during any "flight" between GPS home to/from photo.  That is why the numbers aren't precisely 180º apart (especially for long distances).

This is using formulae from https://edwilliams.org/avform147.htm

HTH...

--dennis

Offline Odd Skjaeveland

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Re: Bearing to and From
« Reply #4 on: September 04, 2021, 12:59:44 AM »
...That is why the numbers aren't precisely 180º apart (especially for long distances).

This is just pure curiosity, I do not use PM for navigation :-)

The to-from bearing difference I demonstrated is more like 270º. I need to get a globe to see if PM produced reasonable to-from bearings. I wonder if PM "travels" NE060 to Manila  and then back along NW330 to keep the orbital direction (keep the same spin vector if you like). If so, the distance is wrong for one of the bearings, a full circle is roughly 40000km. Right now the 270º does not make complete sense to me.

Edited to add:
I verified the math and I did a few calculations to mark some locations along a great circle on a flat map. The great circle is of course a curve on the map, The tangent (compass direction) is approximately 60 deg in my end and gradually increases along the route to become 150 deg in the Manila end. It makes perfect sense after all :-)
« Last Edit: September 04, 2021, 08:06:20 AM by Odd Skjaeveland »
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Odd S.

Offline carlseibert

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Re: Bearing to and From
« Reply #5 on: September 04, 2021, 03:52:34 PM »
Hi Dennis,

Point taken. I just recast my tooltip to be more explicit:

Bearing to this image from GPS home location: {bearingto}°
Bearing to GPS home location from this image: {bearingfrom}°

As opposed to "Bearing to GPS home {bearingfrom}°" (assuming "...from this tooltip")

Explicit is good.