I got this question over the weekend and thought it was worthy of a post here on the Forum. The user stated that they had a "Finished Grade" model from the Engineer and an Existing Ground from the Engineer. They used the Merge Surfaces Command to Merge the "Finished Grade" into the "Existing Ground" to create a nee surface called "Merged" . They used the setting Finish replaces Existing in the Merge command.
After the Merge was completed the Exiting Ground outside of the Finished Grade Area no longer matched the Initial Original Ground Surface - the slopes were "off" when you reviewed the results in the Cross Section Editor in relation to the alignment for the project. Below is one of the screen grabs that they sent through to me for review.
The Magenta Line is the Finished Grade, The Green Line is the Existing nd the Red Line is the Merged Surface - so beyond the limits of the Finished Grade surface you would expect the red and Green Lines to Match however they do not - why is that and what is going "wrong here"
See the response below for the answer - this opened up a range of points that I think are worth clarifying here.
When I first reviewed this project for the User, the first thing I noticed was that both the Finished Grade and Original Ground models were tagged as "Design" classification. While on the face of it these "Surface Classifications" may not be that important, they do help the software make some decisions about
Take the example below (exaggerated example to show the point)
we have two breaklines (Red) with 4 corners with elevations. At the base of the diagram the cross slope is 0% and at the top of the diagram the cross slope is 20%. If you slice across the middle - it is "clear" that the intent would be that the cross slope would be 10%. If you slice the two breaklines only you will get a 10% cross slope. However if you create a Triangle that joins as shown (Green Dashed Line) then if you draw a slice cross section it will have 3 nodes - the left side 100, the Mid point 100 and the Right side 110 - this gives two quite different results for the same exact data.
Now reverse the triangle side and you will have this
The slice line would now have elevations 100, 110, 110 - a completely different result, again for the exact same data.
For this reason, when we have a surface Tagged as "Design" classification, if we find the sequence Breakline, Triangle side, Breakline in a surface cross section, we have a property of the surface cross section to "Remove Diagonal" which if set to "Yes" then computes the slope between the breaklines only and if set to "No" computes the slope from the triangles and the breaklines. (The default behavior for "Design Surfaces" is to set this property to "Yes" and for Existing Surfaces is to set this property to "No").
Note - once the surface cross sections have been formed, they have inherited this setting default - you can change it in the properties of the Surface Cross Sections - i.e. changing the classification of the surface alone will not automatically updated Surface Cross Sections that have already been created. If you change the Classification then you would need to "Select and Delete the Surface Cross Sections" and then open the Surfaces list and turn them on again (this forces them to be rebuilt and in that process they will pick up the new classification and the correct default behavior. However simply changing the properties of the Surface Cross Section will also do the same job.
So in this example - the Existing Ground (Original Version) surface cross sections had this Setting set one way and the Merged Surface had the same setting in the alternative mode and as a result the two surfaces did not match as found by the user. Changing the setting on the Merged surface made it match the Existing (Original), however that was actually the "Worst Answer", changing the original Existing Ground to match the Merged surface actually gave a better result because it was now computing the slopes between the provided breaklines and not always from the triangles.
Here are the two surface cross Sections as defined in a Corridor Template (with different settings) - Note the Cross Section Areas in Top Left
Here is the corrected view where both surfaces have the same setting - again note the Cross Section Areas in Top Left (Including the Diagonals)
Note: You cannot correct a Surface Instruction in a corridor model in the same way, it is 100% controlled by the source TIN Model and the TIN model itself cannot be corrected in the same way. You can correct the surface referenced as Existing in a corridor model provided that it is also tagged as a Reference Surface.
Arc to Chord Offset Adjustment Property
The other setting in the Properties of "Surface Cross Sections" that is also not well known or understood is the Property named Adjust for Arc to Chord Offset. when we form a Corridor Surface Model we are chording the curved sections of the road based on the Template Detail and the Interval / Densification settings for the corridor / templates. These chords are used to form the TIN Model for the road. When you slice the Corridor Surface Model, this will however mean that all the nodes would be slightly off their true location unless you slice specifically at one of the "Drop Locations" in the corridor surface model - i.e. slicing between the Interval drop locations. If you set this parameter to Yes however, then when we display the surface cross sections in Corridor Template Editor or in the Cross Section Editor, then we adjust the locations of the computed nodes for the Arc to Chord Offset based on the Offset and curvature of the alignment at that station so that they are drawn in the correct locations not at where the TIN Edges say they are but where the curved breaklines would truly be at those locations.
Cross Section along the Slicing Line