In some cases it is easier to edit and create your toposurface using points based on sea level elevations, you however still want your project datums (levels) to report elevations relative to the project for example the first floor datum should start numbering at 100’-0”. By manipulating the project levels datums and the project base point you can enter toposurface points using sea level elevations and set the project datums where you wish.
This technique will work best in a brand new project, but could be used in an existing project, but extra care may need to be taken in order to keep all of the building elements related to each other correctly.
Begin in a new project. In an elevation view select the level markers in the project. Move the level markers to the position as measured by sea level to establish where the project is located. In this example Level 1 in the project is going to have a finished floor level of 256’-0” above sea level so the levels are moved up to this position. Once moved the levels should be reporting the position above sea level.
Use the reveal hidden elements tool on the view control bar and then zoom to the extents of the view. In the reveal hidden elements mode you will be able to see the project base point and the survey point of your project. Select the project base point (the circle shaped point); you may have to use the tab key to select it. Watch the tooltip indicator near the cursor to inform you what you will currently select when the mouse is clicked. Once the project base point is selected, click on the paperclip icon to unclip the point from the project. This allows you to change the position of the point without affecting anything else in the project. Now change the elevation of the point relative to the sea level position of the project. In this example the sea level position of the project is 256’ and the project datums need to read starting at 100’. In this case I set the elevation for the project base point to 156’. The difference between the sea level elevation and the project base point will establish where the levels begin numbering from.
Once the project point has been moved, click the paper clip icon again to clip re-attach it to the project. Toggle off reveal hidden elements mode and zoom in to confirm the project levels are now reading as you wish, 100’ in this example. You can edit the type properties of a level in order to have them report the sea level elevations if required.
Now in a site plan view the toposurface can be created using sea level elevations for point entry. The contours can be labeled and they will report sea level elevations.
Now the model of the building can be built and it will be based on the project datums established. Further edits to the toposurface can be made using sea level elevation points.