# Forum

### Author Topic: GSD and GCP in very steep terrains  (Read 4191 times)

#### birdofprey

• Guest
##### GSD and GCP in very steep terrains
« on: March 07, 2017, 12:49:58 AM »
Let us consider a ground with large elevation changes in narrow spaces.

The flight can follow two approaches:

A) We can maintain a constant flight altitude relative to the underlying terrain and then we got a constant GSD with a drone that constantly changes its flight absolute altitude (relative to sea level).

B) We can maintain a constant HIGH flight absolute altitude relative to sea level and then we got a widely variable GSD according to the ups and downs of the morphology you fly over.

Since I can fly in both ways which is the best in terms of photogrammetric ? Maybe A) ?

In addition, since the GCP target should have about five to ten times the dimensions of the GSD, in case A) there are no problems because the GSD remains constant... but in case B) in which the GSD changes so much ?

So scenery A) is the best way ?
« Last Edit: March 07, 2017, 12:52:15 AM by birdofprey »

#### stihl

• Sr. Member
• Posts: 410
##### Re: GSD and GCP in very steep terrains
« Reply #1 on: March 07, 2017, 03:33:20 PM »
Hi,

In scenario A you mentioned that the drone can follow the terrain altitudes. However where does it get this elevation information from and how updated is this data? This is quite crucial for performing the flight according to scenario A. If this terrain information is inaccurate this will cause the drone to experience minimal or insufficient photo overlap for reconstructing high elevation areas because the programmed amount of photo overlap will be reduced when you get closer to your terrain.

If this terrain information is accurate then you will likely get the most uniform results using this scenario.
Also I believe you mean that the drone altitude is relative to the WGS84 ellipsoid which is different from relative mean sea level that you mentioned.

On a side note; empirically I've found out that the amount of GCPs needed depends more on the shape and dimensions of the project area than the value of the GSD. For instance long narrow project areas require more GCPs than a project area that's basically a square.

#### birdofprey

• Guest
##### Re: GSD and GCP in very steep terrains
« Reply #2 on: March 07, 2017, 04:10:09 PM »
If this terrain information is inaccurate this will cause the drone to experience minimal or insufficient photo overlap for reconstructing high elevation areas because the programmed amount of photo overlap will be reduced when you get closer to your terrain.

I'm able to fly, in a safe way, at a constant distance from the real terrain (± just a couples of meters) and I'm also able to guarantee the min overlaps I desire between photos.

I got minor battery due the continuous changes in flight altitude but the GSD varies very little even flying over very irregular terrains.

So its the best scenario ?

On a side note; empirically I've found out that the amount of GCPs needed depends more on the shape and dimensions of the project area than the value of the GSD. For instance long narrow project areas require more GCPs than a project area that's basically a square.

I agree.
« Last Edit: March 07, 2017, 04:17:47 PM by birdofprey »

#### stihl

• Sr. Member
• Posts: 410
##### Re: GSD and GCP in very steep terrains
« Reply #3 on: March 07, 2017, 04:19:10 PM »
If you can guarantee the points that you mentioned then I'm certain you will yield better results with scenario A.

Out of curiosity; which drone will you be using?

#### birdofprey

• Guest
##### Re: GSD and GCP in very steep terrains
« Reply #4 on: March 07, 2017, 09:59:41 PM »
Just a Phantom 4

The difference is in flight planning... I do not use the usual apps but a solution designed (by me) just for my needs...

• Jr. Member
• Posts: 74
##### Re: GSD and GCP in very steep terrains
« Reply #5 on: March 08, 2017, 01:51:04 AM »
Good thing to have designed it yourself! Actually you can use also "Ultimate flight", it has the ability to load waypoints with varying altitudes and also preload DTMs. Regarding your project, I have had success with scenario A in a relevant project concerning a steep faced quarry https://skfb.ly/YRPn

All went well and it was fantastic for the Phantom3 to be climbing the steep paths we designed!

#### birdofprey

• Guest
##### Re: GSD and GCP in very steep terrains
« Reply #6 on: March 09, 2017, 10:05:46 AM »

The example that you linked is very similar to what I usually survey with my drone.

In this scenario I'm taking off from the bottom and I follow the survey grid keeping the drone at a fixed (more or less) altitude from the ground. Im also able to calculate how often I must take a photo to keep the overlaps imposed even if the underlying terrain is not flat. In this way the GSD remains (almost) constant along the entire flight.

The alternative to this way of working would be to take off from the highest point, rise a few meters and then follow the survey grid at a fixed (absolute) flight altitude. Its a simpler flight but in this way the GSD varies dramatically. You get photos at 30 meters away and photos at 150 meters away...

With UltimateFlight you can just draw paths on tablet screen but is not so good to plan a photogrammetry mission...