Dear Kavitha,
Thank you for your query, both via ADN and in your thread the Revit API discussion forum:
https://forums.autodesk.com/t5/revit-api-forum/issue-with-topography-surface-boundary/m-p/8044069
Please note that the preferred method to submit non-confidential ADN queries or requests on the Revit API like this is via the Revit API discussion forum, so I will answer you here.
Any thread that you submit here using your email address registered as an ADN member will be recognised as such and automatically escalated to us in the ADN team, so there would be no need to submit the additional ADN case.
In the forum, you obviously address a larger audience, more of your peers will see it, be able to chip in and help, and more people will see and profit from the answers we provide.
We therefore prioritise cases from the discussion forum.
Please submit all your non-confidential queries via the Revit API discussion forum.
Thank you for your understanding and cooperation.
Concerning the question you raise:
Can this be achieved in the user interface?
If not, it is probably not supported by the Revit product at all, and therefore obviously not by the API either.
Here is the knowledgebase entry on toposurfaces:
https://knowledge.autodesk.com/support/revit-products/learn-explore/caas/CloudHelp/cloudhelp/2015/EN...
Why do you need it to be concave?
In reality, only the points count, and the lines are imaginary.
That is highlighted by this note from a different question: Topo lines are always an interpretation. You measure points. For the measured points the topo will be correct, regardless of what the topo lines show. Topography lines are not a connect-the-dots process. If you need more accurate data, place more points. They don't have to be even elevation numbers. If that topo line is "wrong" and it should be e.g. 100.1 instead of 100, then put 100.1 there.
I hope this helps.
Best regards,
Jeremy