Thursday, November 13, 2014

Relocate Project is Sleight of Hand

Try these steps:
  • Create a new empty project
  • Open the Site plan View
  • Make sure the PBP (Project Base Point-circle) and the SP (Survey Point-triangle) are visible
  • Use Relocate Project, "move" the PBP 1 meter to the left
  • We'll find the SP is now 1 meter to the right, left behind marking where the origin was
  • The SP identifies the origin of an alternate coordinate system, roughly equivalent to AutoCAD's WCS (World Coordinate System) origin, consider that using Acquire Coordinates aligns Revit with the WCS of the source DWG file.
  • The previous steps are essentially the same as moving the SP (clipped) to the right 1 meter instead (use Undo and try it)
  • Using Relocate Project you see the PBP move but its really the SP that's changed, it just doesn't look like it because the PBP is reporting a different 0,0 coordinate offset now (more on that below).
In a sense Revit just shifted the world over, underneath our building, and the origin never really changed. If we try the steps above and make sure we can see elevation symbols it becomes more apparent when they don't change their relationship to the PBP after using Relocate Project.

The PBP and SP start out at the same location in stock templates, but they are NOT marking the same information.

The Project Base Point always (when clipped) identifies where the project's origin is. The Survey Point identifies one alternate coordinate system's origin location, when it reports 0,0.

I believe it causes confusion when we examine the PBP coordinate values (when selected) because it displays values that are relative to where the SP defines the WCS origin, NOT the project origin. Since the project origin is never really changing it would be more accurate or consistent to continue displaying 0,0 and only begin showing different coordinates when it is moved un-clipped.

The following image shows a SP that has been moved by Acquire Coordinates to mark the WCS origin of a source survey DWG. The PBP now shows coordinates that match the offset from this alternate coordinate system's origin. To be fair it does say Shared Site: just above the values but it isn't as meaningful to most users as we'd hope.

The following image shows both PBP and SP moved while un-clipped. It is tempting to think of them as points when they are moved like this but they are really annotation referencing coordinates that are only meaningful when compared with where the origins they are referencing are, which I believe contributes to the confusion about what they display when selected.

General Comments and Advice
  • The Project Base Point never displays coordinates that reference anything but the Shared Coordinate origin location.
  • The Survey Point initially identifies an alternate coordinate origin but it can be un-clipped and moved to show coordinates that reference its origin location.
  • The Project Base Point and Survey Point start by marking their own origins, at same location in stock templates but they are NOT the same coordinate systems.
  • Don't use Relocate Project for X/Y axis project changes, it's really just establishing an offset relative to the Survey Point, not changing the project origin.
  • Don't move the Project Base Point clipped
  • Relocate Project can be useful in the Z axis when you need to show an arbitrary elevation value without placing the building at the actual elevations, see True Elevation and Position, it is still sleight of hand though, using Shared Coordinates to achieve the difference.
  • Moving the PBP un-clipped can be used with the Spot Coordinate annotation. It can reference the Project Base Point location when it is desirable to mark locations, using the Spot Coordinate tool, that all reference where we've placed the un-clipped PBP.


Anonymous said...

Hi Steve,

I know this is an old post but, I've always thought the opposite of what you said in your 'advice' section regarding the 'do not move project base point clipped'. I was hoping you could explain that a bit more, if possible. Always have trouble wrapping my head around this topic.


Steve said...

We can't do anything productive by moving it clipped. The only useful purpose they provided for the PBP is based on it not being clipped. We can position a linked file by PBP to PBP now and Spot Coordinates can set to reference the location of PBP versus the SP.

If you think there is something useful that can be done while moving a clipped PBP...what might that be?