Fly a drone using Terrain Follow with ArcGIS Flight

Summary

Rather than flying a drone at a constant height, ArcGIS Flight allows autonomous flights at dynamic altitudes following site topography with the Terrain Follow feature. For survey areas around hills and other variable terrain elevations, terrain-following enhances the consistency and quality of imagery products from a drone flight by capturing images with constant resolution and proper overlap. Terrain Follow can be enabled in all flight modes except for the Inspection mode.

Caution:
Terrain Follow is intended to be used by experienced pilots only. Terrain Follow flights introduce additional risk due to the variation in Flight Height during the mission. It is the sole responsibility of the pilot-in-command to ensure safe operation throughout the entirety of the mission, and assume manual control to avoid collisions if necessary. 

Important Notes about Terrain Follow

• Terrain Follow requires either an offline basemap or a strong internet connection in the field. It is highly recommended to download an offline basemap if internet connection at the mission site is unreliable.
• Terrain is calculated from ArcGIS Earth data.
• The terrain database contains multiple data sources and has varying resolution and currency across the world. Therefore, terrain may be outdated or not have the resolution required to represent features like peaks, cliffs, and other terrain obstructions. 
• Obstacles such as trees, tall buildings, water towers, and power lines are not represented in the terrain database and therefore are not taken into consideration during the flight height calculations.
• Profile View only shows the ground elevation beneath the flight path. Higher elevations may exist in the flight area, and between the flight area and the drone’s takeoff location. This should be accounted for while setting the Minimum Return Height.

Procedure

To conduct a mission using Terrain Follow:

1. Update ArcGIS Flight to the newest version.
2. Open the app on the iPad.
3. Select an existing project, or create a new one within your organization.
4. Choose the desired flight mode. This tutorial uses Area Survey.
5. Provide a name for your mission. 
6. Type the address or city closest to the flight location. Use the map to zoom to the specific flight area.
7. Select Next → in the bottom left corner to go to the Mission Settings menu.
8. Drag the vertices (white circles) for the flight polygon to cover your area of interest. Add vertices to the flight polygon by tapping the smaller white circles between existing vertices.
9. Optionally, use Define Mission Area from Polygon to specify your flight polygon by with layers from your ArcGIS Online organization or from Site Scan project overlays.
10. Enable Terrain Follow. The map window switches to a 3D view.
11. In the map’s upper left corner, tap the graph icon to bring up the Profile View. Use the image below to familiarize yourself with the elements of the Profile View.

• • The red Drone Home icon is the drone’s takeoff location.
  • The green End icon is the drone’s height at its final waypoint.
  • The brown polygon represents the terrain height directly beneath the drone’s throughout the mission.
  • The green line is the drone’s height above takeoff during data capture.
  • The gray dotted line is the drone’s height while traveling to the first waypoint.
  • The yellow line is the drone’s height above takeoff during return to home.

12. Using the map and Profile View as guides, adjust the flight settings within the Mission Settings menu as necessary.
    • If the Profile View shows the flight path getting close to the terrain, consider increasing the Flight Height or decreasing the Tolerance Height.
    • The graph within Profile View updates after changing flight settings or adjusting mission area vertices. Vertices can be adjusted by switching back to 2D view
    • To optimize efficiency, it is recommended to set the hatch angle perpendicular to the slope, such as the example below.

• • Optionally, enable Geofence. When enabled, a return leg following the terrain is planned and uploaded to the drone before takeoff. Enabling the Geofence option reduces the area coverage per flight because Return to Home is triggered sooner to ensure the drone has sufficient battery to safely return.

Warning: 
If the pilot manually triggers a Return Home (from the App or the controller), or the drone automatically triggers a Return Home (due to signal loss for example), the drone does not follow terrain during the return. It instead returns in a straight line, at its current height or the predefined Return Height, whichever is higher.

12. Once satisfied with the mission, it is recommended to download an offline basemap.
    1. To save the flight plan, tap Save Flight Plan in the upper right corner and provide a name for the flight plan.
    2. Return to the homepage and tap Flight Plans.
    3. Ensure that there is a strong internet connection.
    4. Find your flight plan and tap Download.
    5. After download is complete, open the flight plan and confirm that you are using the offline basemap. To do this, tap the basemap icon in the lower left of the map area, toggle Offline, and find the newly downloaded offline basemap.
    6. Ensure that the offline basemap contains the entirety of the mission area as well as the planned take off location. You must take off within the offline basemap coverage so the drone can follow terrain on its way to the mission area.
    7. Alternatively, you may create a custom offline basemap directly within the offline basemap menu. While planning your mission tap the basemap icon in the lower left of the map area, tap Offline, and tap Create. Provide a name for the offline basemap and select Create to initiate download. As with the other procedure, validate that the newly created offline basemap covers the full area including the takeoff location.
13. After finalizing the settings in the Mission Settings menu tap FLY →.
    • Depending on the flight configuration settings, the Preflight Warnings screen may appear. Different notices may be shown:
      • Cautions inform pilots when the drone exceeds regulation heights above the ground or above the takeoff point. The pilot may modify the height at which a Caution is presented in Global Settings > Flight Settings > Caution Height. The flight can still be performed. The default setting is 400 feet.
      • Warnings inform pilots when there may be a risk of collision. For example, if the Return Height is set to a value that is lower than the highest terrain in the entire flight area, a Warning is presented. The flight can still be executed.
    • If presented with the Preflight Warnings screen, it is highly encouraged to tap Cancel and adjust flight settings to avoid risk of collision or exceeding regulation heights.
    • Depending on the flight configuration settings, ArcGIS Flight may not have the terrain data necessary to calculate the drone's proper altitude at all points during the mission. The Preflight Errors screen may appear for several reasons: 
      • The flight area is too far from the drone's current location. Move the drone closer to the flight area.
      • Internet connection may not be strong enough for ArcGIS Flight to retrieve the terrain data for an online basemap. Download an offline basemap when there is a strong internet connection, such as in the office.
      • The offline basemap may not cover the takeoff location. Move the drone inside the offline basemap coverage area.
14. Read the Safety Briefing screen. To proceed, select the pilot acknowledgement box and tap Continue.
15. Make sure the iPad is connected to the drone. Check one last time for any potential obstacles above and on the flight path to the mission area. When ready, from the Mission Checks screen slide the green bar to the right to initiate take-off.
16. Use the in-flight view to monitor the progress of your mission, and maintain visual line-of-sight with the drone. Mission progress will be indicated by the green bar at the bottom of the screen.
17. The Mission Complete screen will display when all imagery has been captured and the drone has landed. From the Mission Complete screen select Done to return to the project home screen.
18. For information on methods to import the captured imagery from your drone into ArcGIS Flight, refer to this article.