To start modeling you want to import your site survey.
Having a bunch of different layers for the site is good for the organization.
Default layer - Default layer that should always be active
Google earth - Layers will contain images and terrain from google earth
building model - Building model to be placed in the site
object layers - Objects such as terrain, trees, and rocks
Special Layers - Property lines, setbacks, and easements
Importing CAD site survey, Before opening the DWG file into SketchUp it is best to first open it in CAD to do some cleanup to the file.SketchUp will automatically delete things like dimensions, text, hatching, etc. Once it is imported into SketchUp you will want to do some cleanup and start to assign layers.
Before you begin organizing the imported CAD, its best to make each layer have its lines show the same color that the layer is set to. pg-64-65
Once you assign everything to each layer you then want to purge the model. This essentially removes all objects that are not assigned to layers as well as layers that have nothing assigned to them. This likely won't remove all the entities you want to get rid of so you have to manually go through and delete objects you wish to remove.