Custom Elevation Models (DEM) for Terrain Follow

Terrain Follow (AGL) flies the drone at a constant height above the ground. By default, Dronelink gets the ground heights from the ESRI World Elevation Service. In many areas, this public data has a low resolution, or it is years old. It does not show recent changes on a site, such as a new cut, a fill or a stockpile.

You can import your own elevation model from a survey or a lidar scan. This can be a digital elevation model (DEM), a digital terrain model (DTM) or a digital surface model (DSM). The plan then gets its ground heights from your model.

To see which plans include this feature, find "Custom elevation models" in the plan comparison table.

WARNING: Make sure that the coordinate system, the vertical datum and the unit of your model are correct. If one of them is wrong, the ground heights are wrong. The drone can then fly too low and hit the ground or an object. Do the checks in "Check the plan before you fly" before each flight.

How it works

• The mission planner reads the ground heights from your model when you edit the plan. The plan keeps these heights.
• The drone flies the heights in the plan. The mobile app does not need the model file during the flight.
• The model belongs to your account. Each member of the account can select it in a plan while the account has an active subscription.
• A model does not change after the import. For a new survey, import a new model and select it in your plans.

Supported files

• You can add a .prj file with the model file. The coordinate system in the .prj file replaces the system in the model file.
• For an OBJ file from Pix4D, you can also add the _offset.xyz file of the project.
• A model holds up to 4 million grid cells, for example 2,000 × 2,000 cells. For a file with more detail, the mission planner makes the cells larger.

Import a model

Import a model in a desktop browser at app.dronelink.com. The mobile app can use your models, but it cannot import them.

1. Open a plan, and move the map to the site of the model.
2. Click the plan icon at the top left of the plan panel. The plan Settings open.

   

3. In Elevation Source, select Model Library…. The Elevation Models list opens.

   

4. Click Import New Model.
5. Select the model file. To add a .prj file, select it together with the model file.
6. Make sure that the Coordinate System is correct:
   • If the file includes its coordinate system, the mission planner shows it with "from the file".
   • If not, select the system of the file. Suggested shows the systems that put the file near the map center. Other Systems shows the other regional systems at the map center.
7. In Vertical Unit, select the unit of the heights: Meter, Foot or US Survey Foot.
8. If the mission planner shows settings for the file format, set them:
   • LandXML: select the Surface.
   • LAS / LAZ: in Points to Use, select All Points or Ground Only.
   • OBJ: set the Up Axis and Swap X and Y. If the file uses local coordinates, enter the Coordinate offset.
9. Click Build Preview.
10. Make sure that the colored surface is at the correct location on the map. If it is not, change the coordinate system or the axis settings.
11. Wait while the mission planner compares your heights with public terrain:
    • If the median difference is 5 m or less, continue.
    • If the median difference is more than 5 m, the mission planner shows the possible causes. First, make sure that the coordinate system and the Vertical Unit are correct. Do not use the offset to correct a wrong unit.
    • If your heights use a different vertical datum, such as ellipsoidal heights, click Apply Offset of and the value. This moves all heights of the model by the median difference.
    • If you know that your heights are correct, select The heights are correct.
    • If the mission planner shows "Unable to compare with public terrain", it did not check the heights. Compare some heights of the model with known heights at the site before you save.
12. Make sure that the Name is correct. The mission planner starts with the name of the file.
13. If you know the survey date, enter it in Survey Date.
14. In Surface Type, select Bare Earth or Surface with Objects. If you used Ground Only, select Bare Earth.
15. Click Save Model.

The mission planner uploads the model and closes the Elevation Models list. If no altitude changes, the plan uses the model immediately. If altitudes change, a dialog shows the largest change. To apply the model, click Use and the name of the model. To keep the current source, click Cancel.

Select a model for a plan

1. Open the plan Settings.
2. In Elevation Source, under Custom Models, select the model.
3. If altitudes change, a dialog shows the largest change and its location. To apply the model, click Use and the name of the model.

Under Elevation Source, the mission planner shows the surface type, the survey date and the cell size of the model.

Under each AGL altitude, a line shows where the ground heights come from. Click this line to go to the Elevation Source setting.

All components of the plan use the model of the plan. Altitudes with the Takeoff Location (ATL) reference do not change.

Check the plan before you fly

• Coverage: The map shows your model as a colored image with a green outline. Blue is low, yellow is middle and red is high. The legend at the bottom left shows the lowest and the highest height.

• Outside the model: Ground heights outside the model come from public terrain. The mission planner adjusts them to the height level of your model. The plan issues show how many heights are outside the model. Keep the plan inside the model, or check these altitudes.
• Gaps in the model: Inside the outline, areas with no data also use public terrain. The plan issues do not count these heights. Check the altitudes over gaps in the colored image.
• Takeoff: Set the Takeoff Restriction to Specific Area, and keep all of the area inside the model. AGL altitudes use the model height at the takeoff location. If a part of the takeoff area is outside the model, the plan issues show a warning.
• Objects: A Bare Earth model does not include trees, buildings or other objects. Set altitudes that clear all objects on the site.
• Survey date: Make sure that the site did not change after the survey date.
• 3D preview: Preview the plan in 3D. The ESRI 3D view uses your model inside its outline. The Google 3D view shows public terrain, but the flight altitudes use your model.

Go back to public terrain

1. Open the plan Settings.
2. In Elevation Source, select ESRI World Elevation Service.
3. In the dialog, click Use ESRI World Elevation Service.

The model stays in your Elevation Models list.

Delete a model

The owner or an admin of the account can delete a model. The member who imported a model can also delete it. Other members get an error.

1. In Elevation Source, select Model Library….
2. Click the delete icon next to the model.

   

3. Click Delete.

If the open plan uses the model, the mission planner offers to move the plan back to public terrain. To keep the current altitudes, click Cancel. The plan then shows that its model is not available.

Other plans that use the model keep their altitudes, and the drone can fly them. The mission planner cannot calculate new ground heights for these plans. Before you change such a plan, select a different Elevation Source. The Elevation Models list cannot show which plans use a model.

When a model is not available

A plan shows "The model was deleted, or you do not have access to it" in these cases:

• The model was deleted.
• The plan is open in a different account, for example after you share it.

The plan keeps its altitudes, and the drone can fly them. Do not move waypoints or change altitudes in such a plan, because the mission planner cannot calculate new ground heights. To edit the plan, select a different Elevation Source first.