Digital Elevation Models (DEMs) for Drone Missions - How do they work?


Overview


In recent years, terrain-following with drones have become a popular method to fly drones in hilly areas. Following the terrain means that the drone can fly safely and also collect high quality data. This can be useful for mapping missions with the end goal of producing a highly accurate orthomosaic map of the area. It can also be really helpful for magnetic surveying missions where the end goal is to survey an area's magnetic signature for mineral exploration applications.


A key component of flying terrain-follow missions is providing the drone with an accurate elevation map of the target area, so that it can fly to the correct heights during its mission.


This elevation map is sometimes referred to as a Digital Elevation Model (DEM)

But What exactly is a DEM? How do they work? What formats are they available in and how do you work with them in a drone context? We'll explore all of these questions below.

What is a Digital Elevation Model?

Simply put, a Digital Elevation Model is a representation of the Earth’s terrain elevation at specific locations. These Elevation Models (DEMs) are typically found in the form of

GeoTIFF (.tiff) raster files.


Here's an example DEM:



What’s the Data format?

Now that we understand DEMs on a high-level, let's try to understand how elevation data is formatted in these DEM GeoTIFF files?


Digital Elevation Models are commonly available in the GeoTIFF (.tiff) raster format . Other formats might be available, but in the context of drone missions, GeoTIFF rasters are more commonly used.

The GeoTIFF raster format is simply a grid of squares, with each square representing a square area on the surface of the earth and storing elevation data about that square.

Another way to think about this format is a digital image formed of rows and columns. Each pixel of this image represents a square area on the surface of the earth. The size (dimensions) of that pixel determine how big an area on the surface we are looking at. This pixel size is appropriately called resolution.

DEM Raster Format (.tiff files)

For example, if you come across a DEM file in raster format with 5m resolution, this means all pixels in that DEM file/image represent a 5 x 5 m square on the surface of the Earth.

How to Get Started?

Ok, so now we know that DEM GeoTIFF rasters files are simply images where each image pixel represents a square area on the surface of the Earth. The size of this area is determined by the image’s resolution (for e.g. — a 5m resolution represents a 5x5 square on the Earth).


To get started, we would highly recommend:

  1. Download a sample DEM file from OpenDEM or other sources.

  2. Download QGIS (a software tool that can visualise and process these files)

  3. Import the DEM file into QGIS and have a look at the file properties.

  4. Get used to playing around with file Origin, size, height and width.

  5. Visualise and transform files in multiple coordinate systems.


QGIS — to visualise/process DEM files


Where to find DEM Files?

You can find open datasets online for DEM files. Some links include OpenDEM or curated list of datasets online.

For commercial drone missions, we highly recommend working with a third party in obtaining highly-accurate DEM files (typically 2m in resolution) to ensure that your terrain models are up-to date and reflect the target site or area appropriately.

Third parties can include mapping agencies worldwide - USGS in the USA, Ordnance Survey in the UK, ESA in Europe and so on. These third-parties typically provide highly-accurate DEMs collected using LiDar technology.


It's important to check how recently was the data collected and how has the target area changed since. For instance, a common challenge in remote areas is the growth of trees or vegetation which has been not been appropriately captured in the dataset. Some of these datasets that include tree heights are labelled as Digital Surface Models (DSMs).

How do common Drone Mission Planning Software use DEM files?


Now that we understand DEM files better, let's have a look at how these files are used by common drone mission planning software.


At a high level, these files provide drone mission planning software with elevation information of an area encoded as a raster file (grid of cells). The software uses this information to plan the appropriate height for various drone waypoints in flight.




For instance, Hammer Missions uses this information together with the mission parameters to automatically generate terrain-following flight plans for the drone.

Here's how a mission planning software reads DEM files:


  • The software finds the origin (upper left coordinate), resolution, width and height of the DEM. Sometimes the origin is also referred to as Tile-point. All this information is either encoded in the DEM file itself or input by the user in the software.

  • For every waypoint at location N, the drone mission software will find the relevant cell in the DEM file using the following formula:

  • cell x = DistanceX(loc, origin) / res, where distance is in x direction.

  • cell y = DistanceY(loc, origin) / res, where distance is in y direction.

  • Once the required cell in the file has been found, the software reads the required elevation as value of cell at x , y

Locating an Elevation Value in DEM file



Did we get too technical? Ok lets back up a bit. At a high level we have learnt that DEM files encode elevation information for a given site or area on the surface of the earth. This elevation information is then read by drone mission software to create a flight path for the drone that follows the terrain, ensuring safety and high-quality data collection.


If you're curious to learn how Hammer Missions can be used for terrain-follow flights, we recommend visiting our guide on Terrain Following in Hammer Missions.

What are the Pitfalls to look out for?

  1. DEM files can be encoded using many different coordinate systems — UTM, WSG 84, BNG, etc. It's important to make sure your DEM file and mission planning software are using the same coordinate system. Files can be transformed from one system to another using QGIS or another GIS utility. We recommend using UTM coordinate system with WGS 84 datum where possible.

  2. The origin of the file is sometimes referred to as upper left coordinate or tile point. All of them refer to the same point in the top left corner of the DEM file.

  3. It might be tricky to retrieve the origin and resolution of the DEM file. However, QGIS should be able to help with this on most occasions.

Summary

In this guide we looked at DEM files from both a high level point of view and low-level formatting point of view. We learnt how DEM files can be integrated into terrain-follow drone mission workflows by using appropriate mission planning software and bearing in mind how the DEM file is used by the software.


If you'd like to learn more about Terrain-Follow in Hammer Missions, please visit our guide on Terrain Following in Hammer Missions. If you haven't got a Hammer account as yet, and would like try Hammer, you can get started on our free trial.


To learn more about our enterprise solutions, including mission collaboration, data processing and AI solutions, please contact us at team@hammermissions.com.


We look forward to hearing from you.


- The Hammer Team

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