This article explains how to select camera focal length and flight altitude while taking into consideration of the desired Ground Sampling Distance (GSD) and Terrain. 

It is important to consider the following facts:

• At a defined altitude, increasing the focal length:
• A smaller area is captured, and therefore:
• The Ground Sampling Distance (GSD) value will decrease: one pixel will capture a smaller area and therefore the image will have a higher spatial resolution.
   

  Important: The image rate needs to be increased to maintain a good overlap.
• At a defined focal length, decreasing the altitude:
• A smaller area is captured, and therefore:
• The GSD value will decrease: one pixel will capture a smaller area and therefore the image will have a higher spatial resolution.
  
   

  Important: The image rate needs to be increased to maintain a good overlap.

Terrains:

• Forest and dense vegetation: Flying higher helps for the reconstruction, but the spatial resolution will be lower.
• Buildings: Flying higher reduces the artifacts at the building edges, but the spatial resolution will be lower.

Overlap:

• Low altitude with big focal length will request a very high image rate in order to maintain a good overlap.
• For a given focal length and a given image rate, increasing the altitude will increase the overlap.

For more information about selecting the flight plan type, computing the flight height for a given GSD and computing the image rate to get a given overlap: Step 1. Before Starting a Project > 1. Designing the Image Acquisition Plan.

For more information about configuring the camera settings: Step 1. Before Starting a Project > 2. Configuring the Camera Settings.