Why cannot we set the topography to be based on survey point?
It's my own conjecture, first...the toposurface feature predates the existence of shared coordinates in Revit.
Since it is older, I believe it boils down to what they knew to be true when a toposurface is created. In the beginning there was only the project coordinate system, everything was derived relative to that.
With the addition of Shared Coordinates, they didn't redesign the toposurface tool base relationship. I imagine that's because to some degree they can't guarantee that a user will have established a site elevation relationship.
As you can see in the other thread, some of us get around it by using a separate site model and others create the surface and move it down so the correct site elevation is at the ground floor level of the project. Revit can't assume which level is the correct one to be at ground level, even if one of them happens to be at zero elevation. I've seen projects where ground floor is at X above zero and a level at zero was used for footings or somesuch. As such, the toposurface tool is referencing the easiest reliable thing it can, as it did in the beginning.
That written, it would be nice if we could choose between Project and Survey coordinate systems to define the basis for its points. Another item to toss on wish list mountain...among the thousands... They'd have to remember to change the Contour Label tool to work with it too.
Steve Stafford
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@pmartin, If it’s really bugging you and you’re not using shared coordinates; you could always build your project from sea level – starting with a very deep earthen foundation – and then place the Survey Point and building levels on top of it. 
For me though: I build from the ground up and anything placed below the pad elevation is relative to my building level – not to sea level. That includes Toposurface points.
I model the site in Revit (usually from the Civil Engineer’s CAD base for the Rough Grading Plans) in order to calculate the finish grade elevations that need be shown on Plot Plans. “True” point elevations don’t concern me because I’m usually sloping earth away from and around the house to meet drainage, driveway and other slope requirements. Having the pad points at 0.0 makes the math a lot easier. Still, my Spot Elevations accurately report the “true” elevations on Plot Plans because they are set relative to the Survey Point or the “Sea Level” Level.
So, I have no problem with moving the Toposurface down once it’s created. If I have to add “true” points to it later on, and I don’t want to bother doing the math; I simply unpin and move the Topo up to its origin point, edit the surface and I then move it back down and re-pin. It’s no biggie to me.
TIP 1: If your Topo is dependent on a duplicate for cut and fill computations, I suggest you move both at the same time to avoid breaking that association.
TIP 2: Have a Level at the base of your Topo. When the Topo and its Level are at the correct elevation for your project; pull a dimension between the Topo’s Level and the Project Base Point Level (e.g. the Topo’s origination level), and then lock that dimension. This Level will serve as a “Relative Base” for Spot Elevations, should your Survey Point be needed elsewhere in the project.
Just for the record...a building pad can slope (via Slope Arrow). That doesn't necessarily mean sloping them is easy, fun or effective for what you want to do.
I prefer to use them to create excavation and make them thin, imagine a vapor barrier. Then I place floor slabs over that.
Steve Stafford
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That blog post reiterates things I've been writing on my blog for 14 years. None of this is any mystery to me. Perhaps you were replying to someone else.
Steve Stafford
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Hah, flatterer... Sadly it just means I've been around so long, the old dog.
Steve Stafford
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The Survey Coordinate System is integral to positioning a model relative to a survey/civil DWG. The Survey Point itself is just a tangible icon, piece of the system.
That the toposurface element does not reference it is a separate issue. There is no guarantee the SCS will be sorted out/defined prior to a user creating it (topo) so the Project Coordinate System is used as its "foundation". I think it's a practical and reasonable decision from the developer's standpoint. If not then people would inevitably complain that they can't create a toposurface because they aren't ready to set the Survey Coordinate System yet.
I think it would be useful to be able to tell a toposurface which coordinate system it should reference. Maybe they'll do that in the future.
Steve Stafford
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If you create a toposurface by placing points they only reference the Project Coordinate System.
Steve Stafford
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Yup...I'm oversimplifying. The language Autodesk has been using has evolved over time. Most of the time my use of the terms Project Coordinate System and PBP are no different than the internal origin/coordinate system. It only manifests as different or special when it is un-clipped and relocated.
As for improved... I'd like to be able to specify actual contour elevations when I create a toposurface inside a building model even though the building is at "zero" elevation versus working in a separate site model where the values I provide are the actual elevation. In that context I'd define what the ground floor elevation should be using Relocate Project "up" to that elevation. When I create a toposurface I'd change a property that offers Survey Point or Project Base Point. Using Survey Point would allow me to enter a value like 465.5' instead of having to figure out what the project elevation offset would reduce that to.
Steve Stafford
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