Dense Vegetation - Conflicting instructions from Pix4D support

[leonard]

I am developing a flight plan with the hopes of making a topographic map of an area with a lot of vegetation, and I am assuming that means I will generate a DTM from Pix4D.

My intended area is approximately 8 acres of forest that should have minimal leaves. I plan to fly at 90% overlap for both frontal and side.

Looking through the support and community pages, I seem to be getting a lot of conflicting information.

NADIR vs. Angled images:

In this post, the P4D project manager recommends Nadir Grid Flights:

Criss Cross Obliques With Less Sidelap Versus One Direction?

Normally for a topographic survey you only need nadir pictures and single grid flight. The oblique and/or double grid it’s for having good building or side reconstructions (normally for 3d bases). No need and it’s a waste of time if your output it’s only a topographic survey.

In this post, the community manager recommends an image angle of 10 to 35 degrees:

Flight plan to generate best DSM and DTM of dense vegetation

Hi, In case I want to generate both DSM and DTM from pix4dmapper of a dense vegetation. Which flight plan should I use, is it the double grid plan, [image] or it is sufficient to use the normal grid with at least 85% frontal overlap and at least 70% side overlap? [image] Thanks

First question, which is the best for what I am trying to do?

Flight Height:

This article says that for a forest, “best results are obtained with a GSD higher than 10cm per pixel.”

Step 1. Before Starting a Project > 1. Designing the Image Acquisition...

However, using this tool set for my Mavic 2 Pro, it says that for a GSD of 10+, I need to fly at over 400m (1200 ft).
https://www.handalselaras.com/calculator/

Lets assume that I fly at the maximum allowable 400 ft allowed in class G Airspace, will I really get better results than at 200 ft? And if I could get clearance to fly at 1200ft, would the Mavic 2 Pro even see anything from up there?

I am completely new to DTM generation and topographical maps, so if I am missing something, I know it is my inexperience. It just seems that the recommendations I have found from Pix4D are confusing.

Daniel

[Jose.Arnaudo]

Hi,

A Digital Terrain Model will try to generate the elevation model without the vegetation.
A Digital Surface Model, will generate the elevation model including the vegetation.

NADIR vs. Angled images:
As you have found there is no real truth on this topic, as the area you need to fly is not that big, you can try both nadir and oblique flights, have it compared and see what gives you better results. Personally I would go with Nadir and at least 85% overlap, front and side.

Flight Height:
Regardig the flight height, it is recommended to have 10cm as it’s an homogeneus surface, will give better results, but you can fly the highest you are able to, and afterwards use 1/2 Image Size when setting the Keypoints image scale: https://support.pix4d.com/hc/en-us/articles/202560159-How-to-improve-the-outputs-of-dense-vegetation-areas-using-Pix4Dmapper

Hope this helps,
Best

[leonard]

Thank you for the quick response. If I have time, I will give both of them a try