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Project Base Point and Survey Base Point, the 2 different Coordinate system in Revit. (02 of 10)

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In any project, one of the most important information is the base elevation. For any typical project, we usually use "0.00" as the Base level/Level 1. But there might be a time when you might need to find the actual elevation of an element or you might want to reflect the actual elevation values that you got from your site survey. So to accomplish that, you need to understand 2 hidden elements and their properties in Revit - Project Base Point and Survey Base Point.

Survey Base Point vs Project Base Point

Project Base Point

A Project Base Point can be considered as the origin point(0,0,0) of your project. All the elements that you model take the project base point as the origin and get their coordinates. But it is not necessary that the value of the Project Base Point is (0,0,0). In large-scale projects where multiple RVT project files are linked using Shared coordinates, each of the linked files has their own projects base point whose locations are in reference to the PBP of the parent file which has all the linked files.  Head to the following post to understand how to properly Change/Manipulate Revit coordinates and to understand it's importance in a bigger project with multiple buildings linked to one main file.

Visual representation: Circle with a cross(X) symbol at the center. 
The Project base point is associated with the Project North.

 Survey Base Point
On the Other Hand, a Survey base point is the point of reference for the actual world coordinates that are calibrated on the project site using surveying tools (like theodolite). Unlike Project Base Point, a Survey Base Point for an entire project will have a single value. and the values will be the same for all the individual Revit project files that are linked into one file for a large-scale project.

Visual representation: Triangle with a Plus(+) symbol at the center. 
The Survey base point is associated with the True North.
Example
To better understand what a project base point is, let's assume we're creating a 3D model of a building on the top of a Cardboard/Canvas with a size of "100cm  X 100cm" that is placed on a table with a size of "200cm X 200cm" whose height is 75cm from the ground placed exactly at the North Pole. 

Let's consider for this model, the Center of the table to calibrate the physical/actual location on site, the Survey Base Point would be N=0, E=90, Elev 75cm. So if you change the value of survey base point, it means that you've moved the table to another location on the earth.

And if we consider the Project Base Point to be the bottom-left corner of the Cardboard/Canvas whose values would be X=0, Y=0, Z=0. You can change this point by moving the Cardboard. All the model elements location is relative to that point. And if your model is a combination of several sub-models(which you created separately) that you combined(linked) to make the whole model(Revit links), then you can say the each of the sub-model has their own Project base point (0,0,0), but relative to the main cardboard, they each have their own different points they need to be placed on, which is the Shared Coordinates(explained in detail in a separate post).

Note: The symbols, X, Y, N & E are custom symbols and can be changed to anything in Revit. The default symbol for both points will be N & E

Orientation/Rotation 

If you observe in the image above, you can see that the Second-floor plan is oriented parallel to the screen orientation while the site plan is oriented in an angular way. The reason for that is that the orientation of "Site plan" is set to the True North(which is associated with Survey Base point) while the orientation of the "Second Floor plan" is according to the Project North(which is associated with Project Base point). For a more detailed explanation of True North & Project, head to the following link.


Watch the following video for visual representation.

The 3rd Hidden Coordinate system:

The overall project in a Revit basically has these 2 coordinate systems, but there's one more coordinate system(Grids) for each of the plan views similar UCS in Autocad. But unlike the UCS, when we rotate the Grid the orientation of the elements doesn't change. The following image shows a preview of the effect on rotating the view grid.

For a detailed explanation of the settings, head to my post on how to Change the orientation of Snap Grid in Revit.(Rotate Grid/UCS)


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