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Error of height on my maps

Hi, I want to use MME in order to calculate some volume of materials (stockpiles). I tried different parameters (altitude from 50 to 150 meters, overlaps from 70 to 85%) and there is always an error in the result on the heights of the final model (sometimes more than 2 times smaller than the reality !). Also the average AGL altitude is always different than the real altitude of the flight, I don't know if it's normal or if it's linked to my problem... In parallel I tried to upload the same projects (same photos) on other software and I did not have this problem of height... All the results were at the good scale. Maybe I do something wrong or I forgot something but I think there is a reason why MME builds me a wrong model along the Z axis ?

Ugo Suardini

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In general, for volume calculations we recommend 60 meters and 80/80 overlap to ensure reliable results. 

Using a ground reference image (image collected on the ground at the takeoff location) is the best way to get absolute accuracy. We use that image to correct against the notoriously inaccurate elevation numbers from the GPS. 

Read more about that here:

https://support.dronesmadeeasy.com/hc/en-us/articles/206208583-Ground-Reference-Images

You do have a bit of a weird situation in that you are covering piles that are right next to water. Water is bad which is why its avoidance is the first thing listed in our Data Collection guidelines that are agreed to at the time of processing. Its reflectivity and motion tends to throw things off. 

Your most recent dataset in particular was collected during a time of highly variable lighting which doesn't help the system in its effort to match up like feature. 

Your best bet is to try to cut the water out of the area you are covering as much as possible to keep it from throwing off the results. 

Zane
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Thank you for your answer. Is there a way to correct the height of my model after the process ? I usually import the .las in CloudCompare to work on it so I can edit the scale on the Z axis but is it possible to know or calculate the coefficient to put in order to resize it ? I tried to put the coefficient "real altitude of flight" / "average AGL altitude of MME" but I'm not sure if it is a reliable method. Does this value of average AGL altitude should be the same as the real altitude of flight or is it a completely different thing ? How is calculated this average AGL altitude ?

Ugo Suardini 0 votes
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You can adjust the height offset of everything or do a little adjustment to the tilt using GCPS but that doesn't change the shape of the model itself, it just moves it around. 

The average AGL is the average AGL of each image but it is susceptible to error if the model is not correct. I wouldn't use it in anything. 

Zane 0 votes
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So the average AGL is the average altitude of each EXIF data in the photos ? I try to understand if my problem of heights comes from my drone or from MME, but i checked all my EXIF data and the relative altitude of each photo is good so my drone flies at the correct altitude... I will try to do another map with all your recommendations (no water, 60m high, 80/80 overlaps...) to check if I can manage to obtain a reliable result.

Ugo Suardini 0 votes
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No, the "Average AGL" number is a value that takes the relative altitude and subtracts off the distance to the surface of the model. As noted, it is prone to error. We should probably take that out to avoid confusion and attempts to use it. 

Zane 0 votes
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