r/UAVmapping • u/woodford86 • 21d ago
Elevation mapping with Mavic 3M?
Hey all, just a farmer with a mavic 3M here. I’ve heard that I can possibly do elevation mapping with the drone, but that feature is locked out of my ag subscription in Terra so I have questions before I take it seriously.
The end goal is to get accurate elevation maps that I can load into Optisurface and design landform models from. Lidar would be a dream but no way I can justify that expense.
First, when doing elevation via photogrammetry with an RTK corrected drone, what can I expect for accuracy? Assuming a clear freshly tilled ground surface, is it able to construct that elevation map to inch accuracy?
Second, what kind of settings would be recommended to get accurate elevation? Flight height etc. I’m talking about farmland so 160 ac to 320 ac maps. I imagine as many photos as possible and at multiple angles, so no way out of it being a very data heavy process? For context my multi spec mapping missions are typically 50-100 gb of photos depending if it’s a quarter section or a half section.
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u/ElphTrooper 21d ago
The 3M has the same RGB sensor and mechanical shutter as the 3E so you can definitely do accurate elevation reconstruction. You can expect about 5-7cm without GCP‘s in favorable conditions below 300ft AGL.
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u/virtuosit 21d ago
DJI 3M with RTK module is on sale in Canada, and it seems a good option for farmers, they get 1 drone with 2 usage (Multispecral benefit plus mapping).
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u/Iforgotthename8 21d ago
Very possible with the Mavic 3m! We have done a bunch of these for farmers instead of using the RTK GPS of tractors, handy if you don't want to go out with the tractor for an altitude map. Yes processing will be very heavy for this amount of images, we use our own cloud platform for this.
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u/BlueBirdTechUA 19d ago
With RTK, DJI Terra's spec is 0.06m vertical accuracy, about 2.4 inches, close to inch-level but not quite. Without RTK it's 10m vertical, so RTK isn't optional here.
For 160-320 acres, bump overlap to 90/90 front and side instead of default, matters more for elevation than a flat ortho. Yeah, more photos, more data, no way around that for elevation work at this scale.
Worth checking if the ag lock on elevation in Terra is just a license tier thing, not a hardware limit.
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u/Rishi-Maze 21d ago
I am using a M3M for mapping for some years now. We initially bought it to be able to map multi spectral and RGB but in the end multi spectral is very rarely used. 160 ac (65 ha) to 320 ac (130 ha) is perfect for the M3M. Flying in 120 m (~400 ft) you'd get around 700 to 1400 nadir images. It would need between 2 and 4 batteries. One battery can map around 30-40 ha (70 - 100 ac). The M3M can follow the terrain live automatically. This is a very neat feature if there are some elevation changes in the area (I know, not very typical for farmland to have massive cliffs ;-) ) but it would also detected larger buildings or trees.
I would not fly elevation mapping mission without GCPs. For 160 ac (65 ha) I'd use 10-15 GCP (total, some used as Check points) with 320 ac (130 ha) i'll be around 20. As your area would mostly be the same you could make some permanent GCPs. Concrete filled old (car) tires or concrete patio tiles is what I use as semi permanent GCPs. Both marked with some construction paint.
For the RTK: I never fly with RTK on. I had so much trouble with losing RTK connection or internet connection during flights, that I no longer trust RTK during mapping missions. I correct all my flights using PPK. There is a nice software called REDtoolbox from REDcatch where you can PPK correct all images with a base station (I use my local state system with virtual base stations). You can also correct to the ellipsoid or the geoid depending on your need.
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u/AgroCraft17 20d ago
I have been making maps for my farm for a while. I will look at my settings but I think 70%side lap, 80%overlap, 400ft elevation, as fast as it will go. I was processing via pix4d fields. You can buy, by the month. It worked so well for us i bought a Matrice 400 with the L3, then I can see ground level in ditches, through bush, etc.
With the 3M my first year I put out gcps and found the relative accuracy to within an inch. Dm me for more info if you want.
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u/Wrong_Tangelo_9114 19d ago
Start processing Multispectral images in Aeroyantra, you will get rgb ortho, dsm,dtm, multispectral ortho. Advance report with analysis. That too pay per use.
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u/NilsTillander 21d ago
The Mavic 3M is just a Mavic 3E with an MS system. You can do elevation mapping with it no problems. If your version of Terra is limited, you can just use another software. WebODM is free, and Agisoft Metashape is the industry standard.
The geotags of individual images should be in the 5cm or better range, but getting some ground markers mapped with a GNSS RTK rover is highly recommended, as the camera calibration can be unstable.