At some point I figured I'd delve into this and finalize my own contribution as to what these tools do -- they are very useful indeed for dealing with the need to move Revit's Shared Coordinates (SC) and Project Coordinates (PC) origins around.
First of all, you have to have Reveal Hidden Elements toggled on, or enable the visibility of these icons under the Site Category. Once you do, you can see the two icons:
You can select these icons (note that if the icons are in the same spot, you'll need to hover-Tab-click to get at the Survey Point) and once selected, toggle them between clipped and unclipped modes.
- Project Base Boint icon -- a circle with an X thru it
- Survey Point icon -- a Triangle with a + in the middle of it
Let's look at the different modes of these two icons.
First of all, let's look at moving the Survey Point icon around. In its clipped mode, moving the Survey Point moves Shared Coordinates around. If you never move the icon in unclipped mode, this will set the origin of Shared Coordinates. This way, you can coordinate your 0,0 point that your CAD or BIM consultant team is using using Shared Coordinates. This is a very useful new feature in 2010. Note that if you acquire the coordinates of a linked CAD or RVT file, this will move the clipped Survey Point icon as you might expect. If you have linked CAD or Revit files inserted via Shared Coordinates, moving the clipped Survey Point around will also relocate those linked files.
The unclipped mode allows you to move the Survey Point away from the 0,0,0 location of Shared Coordinates. When you export your Revit project to the new .ADSK interoperable file format (big R > Export > Building Site), the origin in that file will respect the location of the Survey Point whether its location is clipped to the Shared Coordinates origin, or unclipped and moved away from the Shared Coordinates origin. Note that in the unclipped mode, the Survey Point icon shows the X,Y and Z offsets of the Survey Point with respect to the Shared Coordinates origin. There doesn't appear to be any way to move the Survey Point back to SC 0,0,0 except by typing 0 into each of the N/S, E/W and Elev fields.
If you're not working with the ADSK file format, then you'll always move the Survey Point icon in clipped mode. You'll need to move it in plan to set the N/S and E/W (X and Y) origin for the site, plus in an elevation view you'll need to move the icon vertically to geodetic Sea Level -- and then I would create a Revit level datum (and plan view) at that same elevation for bringing in the topo from the surveyor so your terrain model will be built at the correct elevation with respect to your project.
Project Base Point
As of Revit 2010, you can unclip and relocate Project Coordinates without moving your level datums. This is a huge increase in functionality as you can move them in X, Y and Z. But let's look at the clipped mode first.
In clipped mode, moving the Project Base Point moves the project with respect to Shared Coordinates. Project Coordinates are not being moved. So moving the clipped Project Base Point down 50' is like moving the clipped Survey Base Point 50' upwards. When moving the clipped Project Base Point, the Shared Coordinates origin (the Survey Base Point) stays where it is, and you move the project with respect to the Shared Coordinates origin. Revit's help files say that moving the clipped Project Base Point is the same thing as using the Relocate Project tool.
In unclipped mode, moving the Project Base Point moves the Project Coordinates of the project. So when you export the project w.r.t. Project Coordinates, moving the unclipped Project Base Point will change the Project Coordinates origin. However, this does not affect the cardinal rule about modeling your project within a one-mile radius from the startup location of Project Coordinates. Revit's help file instructs users to confirm that the model is within this radius. If the PC origin has been moved in this manner, you can return the Project Base Point back to its original location by right-clicking on the unclipped icon and from the right-click menu choosing "Move to Startup Location". The startup location of Project Coordinates cannot ever be changed, and your model must be built within a one mile radius of this point. Note that in Revit 2011 the limitation has been lessened: you may now design inside a 10-mile radius around the startup location -- a substantial improvement for larger projects!
The ability to move the unclipped Project Base Point solves some problems for our office. In our template, we set the main floor to 100' Project Coordinates (so subgrade storeys won't have negative level datums), and Shared Coordinates is the geodetic elevation of the project. However, when exporting to Google Earth, Ecotect, or other apps, the project comes may come in 100' too high because exports like these seem to deal with Project Coordinates rather than Shared. So now you can try unclipping the Project Base Point and move it to your main floor elevation, export, then move it back again. Note that the Globe Link export utility looks at your project and exports based on the Startup Location of Project Coordinates, so moving the unclipped Project Base Point will do nothing for this workflow.
So all in all, this new functionality is welcomed, even if it is a little confusing. I hope this treatise helps someone out.