DTM creation

[AM]

Hi guys,

I'm looking to build a DTM in agi so that in arc I can use the difference of the DEM and DTM to assess tree height from a scan.

I built the DTM following these instructions:
"Creating a DTM is done by determining the ground points from non ground points (e.g. buildings/vegetation/cars/street poles). This is done by running the classify ground points option after you've generated a Dense Point Cloud.
Once this is completed you need to use the Build DEM option to generate a surface model, only you need to specify which points to interpolate.
In the Build DEM window you'll need to set the Select Point Classes set only to Ground. After this is completed you can export it the same way only you've ended up with a DTM instead of a DEM."

The calculation I used was DTM - DEM and the range of values I got gave me negative values, and I'm wondering if anyone had insight as to what would cause this in the landscape, or if it is due to an incorrect process in building the DTM.

Thanks
AM

[stihl]

Seeing as the quote you're using comes from a post of mine I thought I should answer your question as well.

The calculation you did (in, I'm assuming ArcGIS) has to be inverted. DEM minus DTM will give you the elevations of the objects.

As you subtract the DTM (ground) from the DEM, what you'll end up with are the elevations of trees and objects.