Elevation change effects on orthomosaic


I have been asked to survey a site to generate an orthomosaic and contours. One end of the site is 20m higher than the other end.

I am going to split the site into two flights as it is quite large but am wondering if i need to modify the height of the capture by the 20m or is 20m a small enough change that the stitching will still work seamlessly? ie one flight plan at 270 feet and the other at 335 feet or just do them both at 270 feet?

Thank you

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I would try to be no more than 85m AGL just for resolution purposes. If you split it into two missions does that alleviate some of the elevation change?

Sounds like 270 should work for both unless you have tall trees or objects on the high side. You might just want to compromise and fly 300 ft.

Sorry, i might not be clear.

I am splitting the site into 2 flights. The homepoints will be roughly 20m vertically apart.

Do i need to try and keep both flights in the same horizontal plain or does it matter if i set the height to 270 feet for both so one will be 20m higher than the other one (due to differences in homepoint) when it comes to stitching.

Thank you

I would use the same AGL for both flights to keep your exposures and focus consistent. If you vary too much you may see slight differences in exposure, color and sharpness.

Hi Michael,

Thank you for your help. I will set the altitude the same on both flights so they are both at the same height above ground level.

I just wasn’t sure if one flight being 20m higher than the other would have any affect on the stitching process. Many thanks.

You’re most welcome. This is actually a feature that is being worked on that some other softwares have called Follow Terrain. Happy flying!

I’d be interested to learn your findings James. The one time I’ve tried to do something funky by uploading images of varying heights, the processed elevational values were all over the place, despite uploading oblique images to assist.

The multi-mission I carried out last week had a 30m fall across the site but I bottled it and maintained a constant height throughout the mission. I was 40m AGL at one end and 70m at the other. The only issue with this is that I was only 10m or so above the tree line at the upper end. DD did remarkably well producing the orthomosaic above the tree line but lost the plot when it came to the elevational values where the close tree line meant it’s field of view was not so wide. Understandably.

I did think of the Follow Terrain functionality at the time I turned up to site but didn’t have time to consider it properly.

Thank you for your input. If i have enough batteries i may try flying the same AGL at both ends and also a constant height and compare the results.

I have flown several with 20-30m difference across the site from one position and the images of the lower tier was not near as sharp and my GCPs we’re almost unusable (in my opinion) at 325’.


Thank you for your continued input into this.

I am planning to split the site into two flights using the same AGL. Should i include an overlap between the two flights so they capture the same area? I read somewhere to use a 50m overlap but that seems quite a lot? What is your experience?

Thank you


I would create a map with the entire plot and then make a copy and break them into the halves by removing vertices from one end to the other a little overlap of the two couldn’t hurt, but it’s not necessary to have anything more that what you would see on a normal flight. When you split it you will end up with a lane at the edge of the perimeters anyhow.

I am curious to see your results. The orthomosaic may come out fine but as far as contours go, that’s another question unless you are using GCPs. We’ve flown multiple flat locations with resulting contours 10+ feet off over a 600-700 feet length. The Phantoms are known for losing altitude while thinking they are maintaining a constant elevation.

I have looked at several of my log charts and none of them show this behavior. They are as good as they’re going to get with a 1-3m device. More like a radio wave. multi battery flights work fine because you are generally taking off from the same location and width in inches of your previous elevation so the only thing that concerns me is what will happened from taking off from a different location 10 to 20 m above where the first mission was flown. Technically it should be fine, but you know…

We too see contours off by almost 10 feet for large, flat fields. But I believe it is due to the photogrammetry not the Phantom 3 and 4 we are using. There is a crown in the elevation plot that is 7’ higher which would be hard to explain due to losing altitude. But it is reported as a common weakness of several photogrammetry programs. Using GCP’s irons out the crown but does not tell us its exact cause. Both could be contributing but my data is better explained by a photogrammetry fault rather than a drone weakness especially for the Phantom 4 Pro I use.


You know, 10 feet is about 3m…

Our fields are 1300 x 2600 feet and show 7 feet of crowning. How big are your areas Michael? We are seeing about the same amount of crowning on three 80 acres sites.

Most of them are right at about a hundred to 120 Acres. I have one that’s 200 right now, but doesn’t seem to make any difference.

So are you seeing flat contours without GCP’s?

Once I get over about 50 acres I always use GCP’s so I couldn’t tell you. I have no issues with projects around that size without GCP’s.