Difference between Grid and True North

magicalcloud_75magicalcloud_75
edited February 10 in General

I think we should use and talk about Grid North **and **Project North. And forget about 'True North'. In Revit True North is Grid North and Geo-positioning can be challenging enough with just one 2 north, We don't need three. Is this something BuildingSmart made up?

Comments

  • MoultMoult

    The purpose of true north is not for georeferencing. It's for:

    1. Architects doing solar analysis to know where the sun is, so that they can do shadow and light simulations properly (see the solar analysis panel)
    2. People who think they understand georeferencing, to start a conversation to make them realise perhaps they don't actually understand georeferencing, and that the "True North" feature is a lie they've been fed their entire life.
    3. Surveyors who actually do understand georeferencing, to make them realise that there are people in the second category who are gullibly measuring the "true north" in their DWG plans and thinking it's "grid north", and realising the importance of being responsible for georeferencing in BIM and not just hoping someone else transcribes your work properly.
    NigelMassimoJohnsteverugiviktorsemhustejKoAraGerardTwalpa
  • MoultMoult

    Oh, and no, it's not something buildingSMART made up, it's a legitimate concept. For example if you had a solar-north aligned map projection like WGS 84, then true north = grid north, but most other map projects will see a deviation.

    There's also magnetic north which also exists, but it doesn't exist in IFC because it serves no useful purpose (what with wandering around all the time).

    zoomer
  • magicalcloud_75magicalcloud_75
    edited February 10

    I think i would be using the same value for both Grid and 'True' North. Rotation for Just north as one important value.

  • brunopostlebrunopostle

    In some map systems grid north really doesn't align with longitude/true north:

    MassimovdlsemhustejtlangzoomerGerardTwalpaAce
  • NigelNigel

    If only the earth was flat, life would be so much easier. Wait you're saying it's not!

    MassimoJohnsteverugiRoelsemhustejcarlopav
  • MoultMoult

    In Australia , they are also not the same. That's a fantastic image. Another way I like to demonstrate it visually is to draw a segmented vertical line in QGIS then change projections and watch how it becomes a curve and is no longer vertical.

    JohnNigel
  • steverugisteverugi

    from what I heard:
    true north is the direction to the geographic north which in most cases roughly coincide with the grid north, much depends on the projection of the grid used, like the one nicely showed by @brunopostle
    in UTM for instance it gets a bit tricky when you get closer to the edge of the zone but the two north are almost the same when at the center
    for small projects True and Grid in most cases they practically tell the same story, the same when a high accuracy is not required
    interesting topic, thanks
    PS @Moult 's explanation is not just very hilarious, it's actually true :)

  • magicalcloud_75magicalcloud_75
    edited February 11

    For the Netherlands the correction is only about half a degree. And Blender like all BIM software only use imagery and 'flat' content only, all based on a grid north situation. In respect to the argument of 'sun study' software is perfectly able to calculate the correction based on grid north position anywhere on the earth using EPSG code and rough position. Why bother users with it? Not something you would love to dive into. And the problem i see is in the naming . 'True North' can really confuse a lot of users. Revit calls Grid North True North. This might lead to putting the values in the wrong place. Miscommunication in the field. This fuzz can be avoided by using them both the same value. Nevermind the sun-study. Maybe there a (ISO) BIM standard that describes something and help us out?

    JanF
  • MoultMoult

    IFC is an ISO BIM standard and it is using the correct terminology.

    If Revit is using the wrong term, that is something for Autodesk to fix.

    steverugiRoelsemhustejMassimoGerardT
  • magicalcloud_75magicalcloud_75

    Autodesk 'fixing' this is not going to happen anytime soon. Which section are you refering to?

  • MoultMoult

    https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcCoordinateReferenceSystem.htm

    See also the subtype IfcProjectedCRS and IfcMapConversion.

  • magicalcloud_75magicalcloud_75

    I'm reading this as: defines the rotation in a flat situation. There is only one in IFC. Or am i missing something?

  • GorgiousGorgious

    @magicalcloud_75 Do you mean you would like to be able to define a coordinate system that is not a flat projection ?

  • magicalcloud_75magicalcloud_75
    edited February 11

    I only use CSR EPSG 28992. Thats for the Netherlands. I have no intention to use more than one CRS, nor more than one rotation for North direction. .. I think i made a clear point :)

  • GorgiousGorgious

    Well judging by all the back and forth and after re-reading everything several times I beg to differ ><

    Is this thread actually adressing a legitimate issue or asking a real question or is it just a rant about the semantics of the IFC schema regarding georeferencing ?

  • magicalcloud_75magicalcloud_75
    edited February 11

    Well

    JanF
  • MoultMoult

    The earlier link was for grid north, true north is defined here: https://ifc43-docs.standards.buildingsmart.org/IFC/RELEASE/IFC4x3/HTML/lexical/IfcGeometricRepresentationContext.htm

  • magicalcloud_75magicalcloud_75

    Dion, if i edit the 'true north' value and save it as .blend file this value is not saved. Will be 0.0 after re-open it.

  • MoultMoult

    Nothing is saved in the .blend file so this is expected. See https://docs.bonsaibim.org/guides/troubleshooting.html#saving-and-loading-blend-files

  • magicalcloud_75magicalcloud_75
    edited February 11

    Thanks for pointing that out. Saved as IFC it will saved!
    As IFCDirection

    Also, this note was given by BuildingSmart in case of inconsistancy! (which will happen..)

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