Hiroshi Jacobs brought this item to my attention the other day. He also shared with me that he's been accepted to the Masters of Architecture program at Harvard!! Attaboy Hiroshi!! He currently works at RTKL in DC. Here's the issue.
There is currently a “bug/issue” (2009/2010) when you combine the use of these two features. First of all when you use the underlay feature you get to choose which Level of the project you’d like to add to your view as an “underlay”. The spot dimension tool lets us identify a point elevation or point coordinates on an element in the model.
In the image above you can see that combined they can have an undesireable result. The conditions for this are using an Underlay of a Level ABOVE the level of your current view and having a floor above in the same area as the intended location of a Spot Elevation.
When you place the spot dimension in an area where this underlay doesn’t compete for attention you get normal results. However when you put the spot dimension in an area where the underlay and its floor are present Revit will identify the spot elevation of the underlay, not the floor that it "should be" paying attention too.
Does tabbing permit you to choose the correct element? No it doesn't. If you use the Underlay Orientation: Reflected Ceiling Plan the Spot Elevation tool works as "expected", it does not "see" the floor above, if that makes any sense at all since the floor should be more visibleish? I'm confused...
Since I brought it up what happens when you switch between Underlay Orientation: Plan and Reflected Ceiling Plan? Let's see, watch the Spot Elevation that displays both top and bottom values for the underlay floor above. First with Underlay Orientation set to Plan.
Notice the values...now compare with Underlay Orientation: Reflected Ceiling Plan.
Does it matter if it is a floor or roof? Does the same thing for either. Interesting that the elevation values change depending on the Underlay orientation, increasing with the Reflected Ceiling Plan selection. Keep in mind that the floor is at Level 2 which is just 10'-0" above Level 1. I could understand the top vs. bottom display values switching places in the tag with the orientation change but not the elevation values becoming something they aren't.
Boiled down the underlay is “more important” than the current level’s elements when using the Spot Dimension tool. Keep this in mind when using the Spot Dimension tools and using an Underlay.
One more for the road: Spot Dimensions don't like Model Graphics Style: Wireframe. The tools won't "see" the surface unless you change to one of the other choices like Hidden Line. You can switch back to wireframe afterward and the values will stick. This is from an earlier "stump the chump" question and the answer.