[Qgis-user] True North

Wed Apr 4 09:41:24 PDT 2018

And the image… https://imgur.com/a/l8Zpq

Fra: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] På vegne af Kristian Evers
Sendt: 4. april 2018 16:37
Til: qgis-user at lists.osgeo.org
Emne: Re: [Qgis-user] True North

I made it within the Layout Composer/Editor (what it’s called in QGIS3 escapes me now – it used to be Map Composer in 2.x). That was the simplest way to achieve it quickly I found. You have the possibility to add more than one graticule and you can also change the CRS of the graticule. That is what I did. One was kept in UTM, the other used my custom Mercator CRS.

This is just one way of doing it. There many ways to achieve the same thing, but this should outline the general idea at least.

My last post was not let through to the mailing list, so here’s a copy of the image I attached in the previous mail. Should anyone be interested…

/Kristian

Fra: Springfield Harrison [mailto:stellargps at gmail.com]
Sendt: 4. april 2018 16:19
Til: Kristian Evers <kreve at sdfe.dk<mailto:kreve at sdfe.dk>>
Cc: qgis-user <qgis-user at lists.osgeo.org<mailto:qgis-user at lists.osgeo.org>>
Emne: Re: SV: [Qgis-user] True North

Right, this looks promising.

I have been changing projections and then enabling the grid decoration which is always oriented straight up regardless of the projection.

I assume that you were inserting your grids as a vector feature using a drawing tool?

What was the CRS of the grid layer? I have been changing the project projection with OTF on. Maybe I should be adjusting the projection of the layer in question?

Thanks again . . .

Cheers . . . . .   Spring
Samsung Tab 4
On Apr 4, 2018 5:23 AM, "Kristian Evers" <kreve at sdfe.dk<mailto:kreve at sdfe.dk>> wrote:
I had a quick go at it after I wrote the email. What I did was to first create a custom projection with the proj-string “+proj=merc”. Then I opened the layout editor, inserted my map and added to sets of grid lines to the map. One of them was left with the original CRS (the UTM one) and for the other I used my custom CRS. This immediately gave me two grids, one where the meridians is oriented at the grid north (black in the attached image) and one oriented at true north (red in the attached image).

/Kristian

Fra: Springfield Harrison [mailto:stellargps at gmail.com<mailto:stellargps at gmail.com>]
Sendt: 3. april 2018 22:52
Til: qgis-user at lists.osgeo.org<mailto:qgis-user at lists.osgeo.org>; Kristian Evers <kreve at sdfe.dk<mailto:kreve at sdfe.dk>>
Emne: Re: [Qgis-user] True North

OK, that sounds like the solution I'm looking for.

It sounds like the answer is to draw the lines in one projection and then transform it to the other, depending upon which orientation is desired for those lines.

I hope I have this correct…
Cheers . . . . . Springfield Harrison

On 03-Apr-2018 02:46, Kristian Evers wrote:
You can use the Mercator projection to digitize true north lines. It is conformal and thus preserves angles. The meridian convergence is 0 (that is, true north == grid north) all over the map. If you then transform those lines to your chosen CRS you should have a set of lines pointing towards true north instead of grid north.

/Kristian

Fra: Qgis-user [mailto:qgis-user-bounces at lists.osgeo.org] På vegne af Alexandre Neto
Sendt: 3. april 2018 11:15
Til: Springfield Harrison <stellargps at gmail.com><mailto:stellargps at gmail.com>
Cc: qgis-user <qgis-user at lists.osgeo.org><mailto:qgis-user at lists.osgeo.org>
Emne: Re: [Qgis-user] True North

Maybe something like this :

https://gis.stackexchange.com/q/115531/6191

Alexandre
A ter, 3/04/2018, 10:11, Alexandre Neto <senhor.neto at gmail.com<mailto:senhor.neto at gmail.com>> escreveu:
Ah sorry,

I got lost in translation. You are right, I was talking about the magnetic north, not the geodetic one.

Meanwhile,  remembering that all UTM are Transverse Mercator, I don't think there's a CRS that will solve you question.

I think you need to use some formula to convert the grid north measures to true north according to the CRS, and the latitude and longitude of the place.

Alexandre

A ter, 3/04/2018, 09:25, Springfield Harrison <stellargps at gmail.com<mailto:stellargps at gmail.com>> escreveu:

Alexandre,

I believe that true north is actually a constant oriented towards the North Pole, i e 90 degrees north latitude. Also where all meridians converge in the northern hemisphere.

The other North orientations are magnetic north and grid North and these do vary as you suggested.

I do need to use a UTM projection but need bearings to be in degrees true north.

Thanks Alexandre...

Cheers . . . . .   Spring

Samsung Tab 4
On Apr 2, 2018 4:27 PM, "Alexandre Neto" <senhor.neto at gmail.com<mailto:senhor.neto at gmail.com>> wrote:
AFAIK, true bearing differs in time and location because of magnetic declination. So I don't think there is a such thing as a CRS with true bearings.

Assuming you are working at regional level, find the magnetic declination for that region and remove it from your observations. Then draw the azimuth using the chosen CRS. I think 26910 will work just fine.

If you are at a larger scale, then you may need to use different magnetic declination values for each observation.

This may help:

https://www.ngdc.noaa.gov/geomag/declination.shtml

Best Regards,

Alexandre Neto
A seg, 2/04/2018, 23:54, Springfield Harrison <stellargps at gmail.com<mailto:stellargps at gmail.com>> escreveu:
Hello,

I am using CRS 26910, NAD83 / UTM zone 10N, and need to draw lines at
exact true bearings.  The Advanced Digitizing Tool seems to do this but
which CRS would I use to ensure that the bearing is True?

Thanks very much . . .

Cheers . . . . . Springfield Harrison

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