Magnetic Declination - To be or not to be set manually?

[Razvan Dumbrava]

I saw that it was another topic in Italian almost 6 months ago and I have read the information with the help of the automatic translation. If you want you can move or block the topic...
I just want to clarify...

If I import a survey from TopoDroid with the declination set, is it affected by the automatic reference by cSurvey when I activate the geographic functions?

I mean,  in the properties, now I have North - Magnetic north and at the declination - 5.55 at one session dated NOV 2019 (not editable). Is it OK to have the declination completed here? If I change to Geographic North the sketch is modifying I think with the automatic declination.

The question is, if I had the declination inserted manually, when I select Geographic north, will be OK or the declination will be doubled? Is better to not set any declination on the surveys and only to set the date and activate the geographic function? (setting only the coordinate of one station)?

Than you!

Razvan

[Razvan Dumbrava]

And... another question, can we know (find somehow) the magnetic declination applied automatically on each branch / session by cSurvey/Therion?

[cepe]

For declination: I have to confess that I never tryed setting declination from Topodroid.
I take some time do some test. Even if I use TopoDroid I don't set this data here but set it only "on computer".
By the way: in cSurvey declination is defined by session.
You have a 'default session' used for shot without session but, for a correct survey management, you have to assign to each shot a session.
In sessions you could set how nord is managed: magnetic or geographic.
If you enable geographic function you could only set this and no declination cause, with geographic function enabled, cSurvey calculate mag declination from date/location.
Withou geographic function you could manually manage declination but, if you want to use it set in file->property->calculate option to do north correction.

If you wanna know what declination is used you could read it in Survey->Bearings->North. In this tab there's a table with years and declination used.
But another precisation: real declinations is managed internally by Therion. What you see here is a medium value by year.

[Razvan Dumbrava]

I tested the functions on a cave that we have surveyed from 2014 to 2019... First segments are from 2014-2015 and the last 2 from 2019...

This means that in 2014 we had 4.86 - 4.96 degrees E declination and in 2019 we had 5.55 E degrees declination.

I tested the survey with the automatic calculation (geographic function enabled) and i got the blue line (i exported the kml from cSurvey and imported in ArcGis to compare).
When I have set all the declinations manually ( I have 7 segments surveyed in different periods) - I got the red one...

What do you think? The length (planar) is about 1550 m. Difference between final points is almost 100m.

I think that is better to set the declination manually for all the segments.

Razvan

[cepe]

How do you get the 2 trace?
If are both exported from cSurvey I think that you have found a bug but not in declination...

[Razvan Dumbrava]

What do you mean? What kind of bug?

Yes, they are exported as track - kml - from cSurvey. Why do you think they are bugs?

First segments are only the centerline. Only the last gallery is with the splays.
I can send print screens...  The KML contains lines, points and poligons. I deleted the points and polygons components.

Razvan

[cepe]

Problem may be (but I have to check) that for the first case cSurvey use meridian convergence to compensate UTM north 'deformation' (correct) creating a track but in the second case this don't have to be compensated (to be honest, in the second case, has I wrote, you could not export track cause data are not projected in UTM).
But, has I wrote, I have to check it.

[Razvan Dumbrava]

Hmm... In the second case, the box with use geographic positions for all stations is checked, and the origin have coordinate.
I think the projection is still UTM but it is corrected with the declination for every branch. So the projection is OK... (i presume... you have to know better how the centerline is projected in this case).

Razvan