Tuesday, September 12, 2017

Clipped or Un-clipped - That is the Question

The question asked: "Steve, should we leave our coordinate system icons clipped or un-clipped?"

My Answer: As we know, the Project Base Point (PBP) and Survey Point (SP) can be un-clipped. If they are untouched we'll find them clipped.

When these are clipped the symbols for each of these are attached to the coordinate systems they belong to. That means moving either while clipped will alter the coordinate system. If this is done unintentionally, or by someone who does not realize they have been adjusted intentionally already, the coordinate system(s) will be changed.

Therefore I'd say it is not unreasonable to leave these in their NOT clipped or un-clipped state at all times, especially after they have been adjusted intentionally to align models or survey data. If they are not clipped then accidental movement of these icons do not alter their related coordinate systems. It merely changes the symbol's position relative to their coordinate systems.

I regard these symbols as markers or annotation when they are not clipped. In this state they are harmless to our coordinate machinations. Clipped they pose a danger to our careful adjustments to align models and site information.

My opinion: Keep them Not clipped, un-clipped.


Dave Baldacchino said...

I so agree with you for suggesting to have them unclipped :)

AnthonyB said...

Thanks for the thoughts. I think I want it unclipped in my template too.

Our survey origin points are usually 6000km south of our sites. When we Specify Coordinates at a Point the Survey Point moves this great distance away. Zoom Extents will produce a blank view with only the SP and PBP showing. If I have my SP unclipped, it won't move on Specify Coords but does report the coordinates I want. This to me seems preferable. Plus it has the advantage you note that if it does inadvertently get shifted, it will not adjust the coordinate system.

Kimby said...

"Our survey origin points are usually 6000km south of our sites." Isn't the Survey Point supposed to be a point near the site used by the surveyor to set up his coordinates?

We keep our Project Base Point Clipped at all time, this way, it stay in the same place as the Internal Origin Point. We fill in the coordinates, so we can still derive real-world coordinates from our building elements. Our Survey Point will be a point near the site, which was used by the surveyor and allows us to overlay other surveyor plans.

This way, everything points stay within a 20-mile radius from the Internal Origin.

Steve said...


Initially, in stock templates, the Survey Point, the triangle symbol we see, is meant to identify the origin of the Shared Coordinate system. The Shared Coordinate system was designed to account for the fact that the WCS origin of survey files is usually very far from the actual site boundaries and surface contours. Revit is quite comfortable with the transposition required to recognize that the building model's own Project Coordinate system is related to a much larger range of values for coordinates.

As such, when a survey is linked into Revit we can use Acquire Coordinates (AP) or Specify Coordinates at Point (SCaP) to inform Revit of the WCS origin location of that survey. If clipped the Survey Point will disappear, move off, into the distance to mark said origin. If we unclip it before using AC or SCaP then we can move it to mark something more relevant to our site conditions. I encourage everyone to do that. It helps avoid Zoom All vanishing of the project in views too.

The Project Base Point being clipped doesn't ensure that it stays put. It only ensures that if it is moved the coordinate system of the project will move along with it...and the point of the post is that is usually BAD. It is safer to leave both the Survey Point and Project Base Point unclipped because accidentally changes to them does not alter the underlying coordinate systems they are related to. That's very different from the Push Pin tool being applied to them to pin them in place, making it slightly harder to move them.

The 20 mile threshold relates to the extents of linked files as well as being careful to create our model near the Project Base Point (origin) of the project. Revit recommends we link survey files using Auto - Center to Center to ensure that the scope of the survey lands close to the Project Base Point (project origin). If it is still far away it should be moved closer. Then AC or SCaP can be used to align Revit's Shared Coordinate system with the Survey's WCS origin.

Shorter response, model at and around the PBP. Link files so relevant geometry is located at and around the PBP.