Project Base & Survey points - 5000km discrapency

[SamuelAB]

We're working on a project where the Shared coordinates have a very large discrepancy between the Project Base Point and the Survey Point.

The files of the consultants all have the same discrepancy of about 5000km between the two points.

Will this affect accuracy or performance on our Revit files? Should we make a point of getting all the consultants to "fix" their files by dragging the unused Survey point on top or close to the Project Base Point?

We have not encountered any adverse effect yet, but I am wondering if it worth speaking out or not.

[Steve_Stafford]

The goal is to keep the "project" (model) near the Revit origin. Shared Coordinates will let you reconcile the large coordinate offset you have. As long as the geometry of the building is near Revit's origin you should be fine.

Imagine the relationship that Revit has with the site as the building "fixed", in-place... and you move the "world" underneath to get it aligned. Not the reverse, this keeps the building near the origin. HTH

[SamuelAB]

So you are saying to not bother moving the survey point?

The building is close enough to the Project Base Point, but the Survey point is far off.

[Steve_Stafford]

You can turn off the survey point so you don't have to see it. If you are concerned about syncing your project with survey data then you'll want to have a look at the shared coordinate features.

[SamuelAB]

I'm not worried about seing the survey point and no survey data is used in the project. I am just wondering if it will cause issues (double point precision and such forth).

If it would be an object such as a wall, I would say for sure, it will cause issues, but since it is a Survey Point, I am unsure.

This articles illustrates the issue I am talking about:
http://www.revitstore.com/index.php?option=com_content&view=article&id=139:-cadfile-link-options&catid=37:tips-and-tricks&Itemid=55

"In Revit 2010, there is now a limit to the extents of Linked/Imported CAD file geometry. There has always been one, however it is now more apparent; this limit is set to 2 miles/3.218Km. The limit reduces inaccuracies and instabilities in display, as well as the further any geometry is away from the origin, the more error starts to creep in (this is true of any product, not just Revit).

This limitation of 2 Miles has been increased to 20 miles in Revit 2011."

[Steve_Stafford]

The geometry of the model is what Revit is concerned about being near the origin. If it is, you should be fine. If your model is larger than 20 miles in overall extent then you might start to see degradation in the perceived accuracy of snapping and graphics.

[SamuelAB]

The geometry mmh? That makes more sense as a rule of thumb, thanks for the advice!

[SamuelAB]

Actually, i am starting to see some corruption. The Revit technical performance white paper says that the Project point and the survey point should be no farther than 1km from each other.

[Steve_Stafford]

Can you be clearer when you write "corruption". Is that file crashing, snapping sensitivity, angle or length values aren't correct?

The white paper is recommendations, best case scenario. I've worked with files that place the survey point and project point hundreds of miles away from each other. The goal of using the survey data is to provide property boundaries, provide a common reference point between project files (survey, building and others).

Technically if you told me what coordinate the corner of a building is supposed to be or a grid intersection I could properly identify it in Revit to allow for file coordination. What else are you attempting to do with the survey data?

[SamuelAB]

The linked files are moving slightly. I had my grid unpinned and they moved by different amounts.

My copy monitored elements are slightly off. This may also be cause by model elements that are further than 32 miles in linked models.

They placed the survey point at 0,0 from their CAD file (the survey engineers). This was to define they property lines. I played around with the main topographic file and moved the survey point closer to the project and the annotation on the property lines did not change.

I may proposed that we move the location of the survey point closer, not 100% sure if we should do it pinned or unpinned. I will investigate probably next week.

[David Conant]

The metaphors used in this feature are:
The survey point marks the origin that appears in the surveyors documents. It might be the origin of a state plane, national grid, or some other point on the globe. Revit does not really care how far away it is from you building. It also marks the origin of your shared coordinate system so that any other project located with respect to the same survey base can be located correctly with respect to this project.

The project base point represents some point on the site from which local measurements will be made. This might be a survey stake, a grid intersection, or site monument. If you do not move the project base point from its default location, it is located at Revit's internal origin. Revit does not like geometry to be constructed outside the 20mile (32km) circle around this internal origin. Beyond that limit, you will find that geometric errors increase, snapping fails, patterns lose their pattern, etc.

These errors are due to the cumulation of rounding errors and the large difference between the overall size of numbers and the precision at which the geometry engine must operate. Essentially, systems like Revit can only store numbers with a maximum number of digits. The more digits there are to the left of the decimal point, the fewer there can be to the right. Revit's calculations are all based on its internal coordinate system. Shared or survey coordinates are not used in calculation so can be as large as needed.

[Steve_Stafford]

Love it when you chime in David! Thanks!