[Qgis-user] True North

[Kristian Evers]

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>
Cc: qgis-user <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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