Why You Should Use
Terrain Awareness
Benefits of Terrain Awareness in Photogrammetry
Terrain Awareness, or terrain-following as some like to call it, eliminates the variance in above-ground flight height and thus eliminates the discrepancies in overlap throughout sites with high topographic relief. Consistent overlap leads to more reliable data overall, and Terrain Awareness guarantees more consistent overlap.
What is Terrain Awareness?
Contrary to popular belief, the world is not flat. Terrain Awareness accounts for that. It is an autopilot feature that keeps the drone at a set height above the ground, as opposed to a set height above the takeoff location, following the grade changes throughout a site. Ideally, you would use this feature on sites with significant topographic relief.
But what’s the point? Read on to find out!
Benefits of Terrain Awareness in Photogrammetry
Consistent Overlap
Overlap is the amount (in percentage) that each photo shares with its neighboring photos. It is calculated based on photo footprint and is displayed as frontlap and sidelap percentages.
Photo footprint is the real-world field of view of the camera. This “footprint” increases as height above ground increases. Therefore, the higher the camera, the larger the photo footprint and the further distance there can be between one photo and the next while still achieving the appropriate overlap.
As you may know, proper overlap is a keystone in good drone data collection workflows. Most autopilot software takes into consideration the planned flight height when calculating overlap to come up with the proper spacing for your preferred overlap (we recommend 75/75). Working as designed right?
Yes and no. This calculation only accounts for the height above ground from your takeoff location and does not account for any variance in elevation throughout the site. For example, if you take off for a 200’ flight, the drone is using that takeoff location to gauge altitude, ignoring any change in elevation on the site. The takeoff location is effectively zero; the drone knows the distance it travels vertically, and that is the altitude that you plugged in, or in practical terms altitude above takeoff. E.g. 200’ AGL is really 200’ above takeoff. The drone is also using that exact metric to calculate your overlap and it applies that same formula to the entire site as if it were flat. For sites with little to no topographic relief, this is good enough. However, you can start to see where there might be an issue if there is significant relief.
When elevation changes on a site, the overlap calculation is thrown off because the photo footprint changes. This leads to either too much overlap (when flying over lower elevations relative to takeoff, photo footprint is larger) or not enough overlap (when flying over higher elevations relative to takeoff, photo footprint is smaller).
Terrain Awareness or terrain-following as some like to call it eliminates the variance in above-ground flight height and thus eliminates the discrepancies in overlap throughout sites with high topographic relief. Consistent overlap leads to more reliable data overall, and Terrain Awareness guarantees more consistent overlap.
Consistent Resolution
Aside from consistent overlap, consistent resolution is another benefit of Terrain Awareness. As you fly higher your photos become more grainy and thus will be slightly more difficult to mark and verify features, leading to variable accuracies throughout a site.
Consistent height over the land results in the same resolution overall and aids in the same expected accuracies.
Single Mission/Time Savings = Cost Savings
Terrain Awareness is a cost/risk mitigation tool.
With Terrain Awareness, you can fly your entire site in one shot, with the peace of mind that your overlap is consistent throughout, compared to the worry or expense of having to re-fly the site due to overlap issues.
Terrain Awareness minimizes time spent in the field where terracing and multiple mission plans would be otherwise be required and minimizes processing times, as multiple stitched missions do not process as smoothly as those with Terrain Awareness. All of this saves you money and allows you to do more with your time.
Planning a Terrain Awareness Mission in Map Pilot
Map Pilot is our recommended autopilot/mission planning software. Map Pilot has a Terrain Awareness feature that will allow your drone to stay a uniform altitude above the ground. This is not a sensor-based solution. The Terrain Awareness feature uses SRTM (or Shuttle Radar Topography Mission) data that was collected in 2000. As of late 2015, the highest level of detail (30 meters/pixel) was made publicly available.
In Map Pilot, you will plan your mission as you normally would.
When done, drop down the top menu to expose the “Terrain Awareness” feature.
Tap it to activate it and follow the on-screen prompts to purchase it if you do not have it yet.
Once the software is done downloading the basemap for the site, use the “profile” button to see the waypoints the software created to follow the terrain.
It is always good practice to check the elevation change with the flight lines to make sure everything looks correct.
Once you are satisfied with your flight parameters, save the mission offline for upload to aircraft later.
Alternatives to Terrain Awareness
Although Terrain Awareness is by far the best option, there are alternative methods for dealing with elevation change on a site.
Terracing Missions
Terracing or “stepping down” your missions is one way to deal with elevation change. The idea here is to fly an area with 50-100’ of elevation change at a time as separate missions. This ensures that the takeoff location changes and the height/overlap are recalculated with every flight. This works with the elevation change in a more gradual manner than flying the whole site at once and ending up with an altitude discrepancy of 400’+.
If you don’t have Terrain Awareness, this method works fine for mapping a hillside with consistent, gradual grade change. However, as soon as you introduce multiple hilly areas and other variable elevation features in one site, this terracing becomes increasingly difficult to plan and can often end up with more guesswork than you would ever want if you are mapping for business purposes.
Takeoff from Highest Elevation on Site
This method is useful if you have a grade change of less than 100’ or so and do not have access to Terrain Awareness. Taking off from the highest elevation ensures that you at the very least, do not end up with too LITTLE overlap. At worst you will end up with higher than normal overlap where some photos may need to be disabled to process properly but this is better than having to re-fly the site due to too little overlap.
Even by that description, you can see that this is not the most efficient method when it comes to processing as there will often be hiccups due to overlap issues.
Any more than 100’ of elevation change and this method becomes impractical due to overlap issues becoming more severe.
FAQ
How do I know if it’s working before I fly?
Within the Map Pilot app, you can tap on the “profile” tile to visualize the change in altitude throughout the flight. If you slide your finger along the profile, you can see the point in the flight that is represented. Trust but verify. Have faith in the autopilot app but always have a plan B.
What if the elevation change is more than 400’? Am I breaking the law?
No, the Part 107 regulation states that you cannot be more than 400’ above the ground, therefore as long as your Terrain Awareness mission is not set above 400’ you will not be breaking the law
Will this work on sheer Cliffs?
Terrain Awareness is based on the 2000 SRTM model of the earth which was a dataset collected from space. Therefore the resolution of the data is about 30 meters/pixel. Exact cliff edges will not be defined by the data so caution should be used when working with sheer cliffs as opposed to gradual elevation change.
Does Terrain Awareness work in Cities for building clearance?
Terrain Awareness will NOT work on anything constructed after 2000, therefore dynamic environments like downtown cities should not rely on Terrain Awareness to avoid obstacles while mapping. It is always best practice to set a minimum height of 100’ above the tallest structure on your site whether using Terrain Awareness or not.
Why shouldn’t I use terrain awareness below 250’?
Map Pilot adds a 250’ buffer as a safety net for the data. This buffer adequately compensates for the 30 meters/pixel data which is obviously cruder than real-world data and also compensates for tree cover. 250’ is actually a perfect Goldilocks Zone for drone data collection where excellent accuracy can be attained while minimal time is spent collecting it.
When using Terrain Awareness you should always be verifying that the drone is doing what it’s supposed to be doing and be prepared to recall the drone if something doesn’t look right, while the data is really good for this use case, it is not perfect.
Will I still get accurate elevation data?
Yes, in fact, you will get the most accurate elevation data as the data collection perspective will be identical throughout the mission.
Are there other autopilot apps that i can use?
Yes, other autopilot applications have the Terrain Awareness feature. Regardless of which app you choose, it should deliver the same benefits described in this article, but Aerotas does recommend Map Pilot.
What about Terrain Awareness with the Phantom 4 RTK?
The Phantom 4 RTK has a Terrain Awareness feature as well! Although it is not quite as user-friendly as Map Pilot. As you likely know, the controller of the Phantom 4 RTK does not allow you to use any software other than DJI GS RTK for mapping. The Terrain Awareness feature on this app has the same function as this article describes, however, you are required to import your own DSM into the controller in order to get elevation data.
We have a workflow to create and import your own DSM using the National Elevation Data from USGS. Follow the link here — https://www.aerotas.com/phantom-4-rtk-terrain-awareness
FYI, DJI has released an SDK Controller for the Phantom 4 RTK, but availability is extremely limited right now. Users can plug in a compatible Android or iOS device and install any compatible DJI Mobile SDK-enabled app. Expect updates in the near future as 3rd party software developers (such as Map Pilot) begin to offer support for the Phantom 4 RTK SDK Controller.
Questions? Contact us anytime at (949) 335-4323, or info@aerotas.com