More often, you are needing to conduct your flights with terrain following feature. Terrain follow is suggested for sites with varying elevations and helps to ensure a consistent ground sampling distance across the survey area. It is also needed for TreeTools VirtualPlot to ensure a consistent plot size is collected for the real-time forest inventory. Using the same DTM during processing of the imagery in TreeTools VirtualPlot will also yield improved precision of the tree stocking results.
DJI’s Enterprise drones use DJI Pilot 2 flight software for the DJI Matrice 4/300/350 RTK and the DJI Mavic 3 Enterprise. This includes a feature to allow for terrain following. But how do we provide the terrain to the software and in what format?
If you want to find out more info on flying terrain awareness flights with other drone types and applications, our Tutorial on TreeTools VirtualPlot provides an overview.
Terminology
To help with terminology used, here are some quick definitions used in this article.
| Term | Definition |
|---|---|
| DSM | Digital Surface Model (DSM): a DSM represents the Earth’s surface and includes all objects on it, such as buildings, trees and vegetation. |
| DTM / DEM | Digital Terrain or Elevation Model (DTM / DEM): a DTM/DEM represents the bare ground surface without any objects like buildings or vegetation. |
| GeoTIFF | A raster format used in geospatial datasets. Tagged Image File Format (TIFF) which is georeferenced. In this context, a GeoTIFF DTM or DSM file will be a raster of elevation heights across an area. |
| Vertical Datum | A vertical datum is a reference surface used to measure elevations or heights of points on the Earth’s surface. It defines the meaning of the heights provided. |
| NZVD2016 | New Zealand Vertical Datum 2016 (NZVD2016) is the official vertical datum used in New Zealand to define heights. It provides a consistent reference surface for measuring elevations across the country and its offshore islands. The reference surface for NZVD2016 is the New Zealand Quasigeoid 2016 (NZGeoid2016), which provides the difference between GRS80/WGS84 Ellipsoid and the NZVD20161. |
| WGS84 Ellipsoid | Global positioning systems (GPS), Global navigation satellite systems (GNSS) units natively report heights relative to the World Geodetic System of 1984 (WGS84) ellipsoid. This is a smooth, mathematically defined surface that approximates the shape of the Earth. It is an oblate spheroid, meaning it is slightly flattened at the poles and bulging at the equator. For DJI drones, the heights stored in the images taken are displayed in this vertical datum. |
| EGM96 | This is a global reference vertical datum geoid and stands for Earth Gravitational Model 1996. DJI drones use this to display in the user interface on screen the Above Sea Level (ASL), which is being calculated using EGM96 geoid model. |
- Due to the minimal difference in their parameters, the calculated ellipsoid heights between GRS80 and WGS84 will be very close for most locations on Earth, with the largest discrepancies occurring near the poles. Though the difference is usually negligible for most applications; typically, only a few millimeters at most. ↩︎
Types of Terrain Following
There is two Terrain Follow modes in DJI Pilot 2.
- DSM Follow
- Use Download from Internet (this uses the global ASTER DEMs, but the potential accuracy and time of collection are important to consider when flying in areas with recent terrain changes or development. The downloadable “ASTER” DEM was produced between March 1, 2000 and November 30, 2013. Take this into serious consideration when planning around terrain or features that may have changed).
- Use Local File (use a custom GeoTIF DSM or DTM loaded onto the controller).
- Real-Time Follow (Mavic 3 Enterprise and Matrice 4 only).
Note that an RTK connection is NOT required for either of these modes on the M3E and Matrice 4, but is suggested for lower-altitude flights or areas with large altitude changes. An RTK connection IS required for DSM Terrain Follow missions with M300 and M350.
Do I Use a DSM or DTM?
If you want the drone to follow the terrain (bare ground model), you will be choosing to use a DTM. If you want the drone to follow the vegetation / building surface, then you will select to use a DSM. While DJI Pilot 2 just displays “Select DSM”, providing a DTM GeoTIFF will ensure your drone follows the terrain (bare ground), maintaining a consistent above ground level (AGL). This article will focus on DTM, but the process is the same for a DSM.
What Coordinate Systems Does DJI Pilot 2 Need?
DJI Pilot 2 needs to have a GeoTIFF in WGS84 Coordinate System where heights are also represented as WGS84 Ellipsoid Heights. In NZ though most datasets have their heights in NZVD2016. The difference between these two systems for example at the Interpine Office in Rotorua is +29m. In Northland this could be +40m which could be a significant impact on your flight planning.
So, if you don’t be careful, you might think you’re planning a flight for 120m AGL but will be flying at 91m AGL (120-29) as the heights in the GeoTIFF provided could be NZVD2016 instead of WGS84 Ellipsoid.
Download a LINZ Data Service DEM to Use with DJI Pilot 2
STEP 1 – Find a Suitable DTM/DSM on LINZ Data Service
Find a suitable DEM to use. You can use this search for example: https://data.linz.govt.nz/data/?q=dem. Once located select to add to map using the [Map +] button. There are a range of 8m or 1m DTM across NZ to select from.
STEP 2 – Export Cropped Area
Select the area you are planning to fly using the cropping tool, and then select Export. HINT: You can hide the layer to help refine the cropped area.
and Select Export, choosing WGS84 (EPSG 4326) and ensuring your GeoTIFF file size will be below 20MB which is the preference of DJI Pilot 2 app. This will transform the horizontal datum to WGS84 Lat Long format, but the altitude will remain in NZVD2016 (or at least that of the source file).
STEP 3 – Get Offset for Flight Area to WGS84 Ellipsoidal height.
Then also add to the map in NZ LINZ Data Service, NZ Quasigeoid 2016 Raster | LINZ Data Service which provides the offset to convert the data to WGS84 Ellipsoid. With the layer visible click on the centre of your area of interest, and the spatial query window will display the offset. In this case +14.09m. You will notice that selecting around your area of interest often only changes it this value by a couple centimeters.
STEP 4 – A shortcut: You could just now plan your flight for AGL + Offset Value.
If you are not a GIS user, and do not have Raster tools to adjust the raster, you could just now plan your flight, adding the offset to planning AGL flight height. Example: Planning to fly at 100m AGL, then 100m + 14.09 = 114.09m will ensure you are maintaining 100m AGL across the flight area.
STEP 5 – Adjust the DTM with the Offset in ArcGIS Pro or other GIS Software.
Open the Raster in ArcGIS Pro (or QGIS) and use the raster calculator analysis tool to edit and adjust the elevation values. Simply select the Raster + Offset. If you have other raster in the map, just make sure you set output raster cell size to equal the DEM or change it to 5 or 10m to reduce the file size to keep the file under 20MB limit to keep DJI Pilot 2 happy.
The resulting file can now be used in DJI Pilot 2.
The raster file can also be used in TreeTools VirtualPlot software as well.
What About Your Own DTM?
The process is the same for your own DTM derived from a LIDAR dataset. It might be better to just do a full conversion of your DTM to WGS84 and Ellipsoid height using the method below for large areas, and then you can just crop areas out as needed for flight planning or TreeTools VirtualPlot application.
Software like ArcGIS Pro can convert the horizontal coordinate system for you as part of the output settings when using the raster calculator.
What About Adjusting a Large Area?
Just download the NZ Quasigeoid 2016 Raster | LINZ Data Service for all of NZ (it is only 33MB). And then use this to dynamically calculate using the raster calculator. Just ensure you set the output raster cell size to that of your DEM or a custom value as the NZVD2016 grid is much larger.
More Information – Extra Tutorials
Here is a range of external links to useful articles on this topic.
Geoid vs Ellipsoid: What’s the Difference and Why Does it Matter?
Geoid vs Ellipsoid: What’s the Difference?
How to Plan a Terrain Follow Mission with DJI Pilot 2 – Propeller Aero
Reference ellipsoids | Geodetic Guidance
How to do Terrain Following with DJI Pilot 2 for the R3 Pro V2 and R3 Pro
Vertical datums—ArcGIS Pro | Documentation
Vertical coordinate systems—ArcGIS Pro | Documentation
Further Support
We wrote these articles to equip you with everything you need to get the job done on your own, but we understand that sometimes this isn’t sufficient.
If you’re stuck, the Interpine support team may be able to help. You can contact them by submitting a contact request.
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