Yep. Click Modify Sectors after you highlighted a sector, hit 0 and the last sector of the circuit in the range and set Altitude to 0 for both the First and the Last sectors.
Set Banking to 0 as well to make the track absolutely flat.
EDIT: 'Expert' trick - set altitude to -0.01 or -0.02 when you want to work with an inserted Google Maps image (or any satellite image) as a base for a layout. That way, you'll directly see what's underneath each sector you create without the need to F10 your way in and out while also being able to see the frames of your .dat layout in Orthographic view.
My workthread - [www.grandprixgames.org]
Full of classic F1/non-F1 track layouts
My blog about F1 performance analysis - [thef1formbook.wordpress.com]Edited 3 time(s). Last edit at 09/27/2015 11:30AM by Atticus..