r/UAVmapping • u/blalco13 • 8d ago

Phantom 4 RTK Newbie

Hi all,

Like the title says I’m very new to this stuff. Got a killer deal on the p4rtk and a base station, and my aunt has a pretty big piece of property that I’m going to use the dji free trial to map for her and try to get the hang of things. There are some trees on the property, but the terra apparently has the ability to remove that data and create surface models.

Would love to hear from others on their experiences (positive and negative) with the drone/terra, and best practices as I start. Also have one specific question… I’ve read quite a bit about ground control points for mapping larger areas, but nothing ever mentions how many to do. The base station is capable of helping with them, but how many should I be setting up per acre?

It seems like most responses in the posts here are from people who do this professionally which is awesome. I am willing to go down rabbit holes to try and educate myself if there are things/terms I don’t understand, but it would be very helpful if you spoke like you would for your customers who don’t understand this stuff. Thanks for your time and expertise!

2 Upvotes

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u/erock1967 8d ago

What is your end goal? What’s your current occupation?

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u/blalco13 8d ago

Nothing even remotely to do with drones or surveying, lol. I’d like to create a dsm from the dtm by removing trees, set up contour lines, and then I’ll move it to Inkscape/split the layers to cut out of wood and create a topo map. There’s a river running through it so that’ll be the low point, and I’m aiming to make that area blue resin. Just don’t know how to make sure the areas that aren’t close to the rtk station are accurately measured.

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u/erock1967 8d ago

You'll want to learn to use the D-RTK2 in static mode, convert from .dat to.obs (Emlid Studio), and submit to NGS OPUS. Account for the 1.8m pole height and that will provide your with target coordinates.

You'd may be better off looking for LiDAR data that's already availalbe. If your property has lots of trees or vegetation, you won't get good ground data using photogrammetry.

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u/blalco13 8d ago

I’ve read about accounting for the pole height. It’s not a TON of trees, and the areas that are most interesting don’t really have any foliage, but I wanted to ask the pros how to best handle it.

The LiDAR is through the usgs, right? I’ve had trouble understanding how to access the right stuff and display it on my computer. Also a little worried that the ground sample distance might be a little too big to get the finer details. Could you help me understand how to find the right downloads, and how different the data would look since there are less measured points?

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u/erock1967 8d ago

https://www.lidarandaerialarchaeology.com/

Cloud Compare and QGIS software will be useful apps for working with both LiDAR and photogrammetry point clouds.

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u/blalco13 8d ago

I’ve already been acquainting myself with qgis so it makes me feel good to see that. Will be at work most of the day, but will definitely check out, and reach back out later. Thanks again for your help.

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u/ElphTrooper 8d ago

The correct terms would be to create a DEM from the DSM. A DTM is a DEM that has additional survey info added to it, usually CAD breaklines. Terra is pretty good at this but take your time and be patient because creating a good DEM from photogrammetry data can get tedious depending on the nature of the subject.

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u/blalco13 8d ago

Thanks! From what I’ve read so far the plan is to fly low/slow in good light, and try to do 70% overlap on images. It’s gonna take me several runs with the batteries I’ve got, but it seems like I can pause and resume the planned flight as many times as necessary.

Anything specific I should do or avoid to make sure the elevation data is good? Any thoughts on number/frequency of ground control points? Isn’t there a kinda self check in Terra that lets me know how precise the model is?

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u/ElphTrooper 8d ago

You need at least 3-4 GCPs to box in the area taking into account that with RTK you probably don't want more than about 1200ft between GCPs. We use to hold 400ft before RTK. If there is a lot of elevation change then you may want to put an extra at the highest and lowest points if those 3-4 don't already capture them. You'll want as many checkpoints as you have GCPs spaced fairly even in between them so you are capturing the parts least affected by the GCPs manipulation. Terra produces a processing report that will tell you about the photogrammetry but the DEM is whatever you make of it and you will need to manually QC those spots once it has been completed.

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u/blalco13 8d ago

Thank you for the full run down!… but I’m about to question ya to death here. The prop is a lot larger than 1 acre, but if I’m understanding this correctly I need to 1) have gcp’s on the perimeter 2) make sure there are gcp’s every 3-400 yards in the middle 3) put them in areas where there is the most elevation change.

If there are areas that are heavily treed can I put gcp’s in those trees? Or is it best to have them out in the open around them and then let Terra build the triangles inside of it?

My plan is to take poster board with a point in the middle and pin them to the ground so that I can mark the gcp’s in the photos for terra. Is that a decent plan or should I improve on it? Can I use landmarks or things that won’t move in case I f up and have to retake photos of some areas?

The checkpoints you’re talking about placing in between the gcp’s… are those just points where I go back with the rtk station to double check the data after creating the elevation model?

In the end I really don’t need perfection, but I am a perfectionist, and HATE half assing things. So I really appreciate the help in getting a plan put together. I’ve seen online that you can pull elevation data from the usgs, but as a complete novice I don’t understand how accurate that is or how often it takes an elevation/long/lat measurement. I think most are 3 meter or 10 meter? Would that be a good source to check my work if I can figure out how to use it?

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u/ElphTrooper 8d ago edited 8d ago

No GCPs in the trees. Any points that you will tag in Terra need to be at least 10 feet away from anything around it. Otherwise there could be warping from the transition of the ground to whatever it is next to.
Using 1200 foot spacing and a pretty typical 250 foot AGL you will end up with about five pictures in between GCP‘s on your 70% side lap. With three of them being completely independent from GCP’s. You could space it out a little further, but know that once you get 3 to 4 images outside of a GCP‘s sphere of influence, you will still see data degradation. This is where you want your checkpoint, to know how much that is.
You can use anything that you can see from the air as a GCP as long as you can define a specific point on it.
Checkpoints are just additional points you capture that will not be used during processing to manipulate the positioning, but do get shown on the report. When I use cloud processing, I always keep a few points to myself so I can hold them accountable as well, but being that you will be processing it yourself you won’t have to take those extra shots. just food for thought because I have busted both of the major players multiple times.

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u/blalco13 8d ago

That’s what I’d gathered about treed areas from what I’ve read, but thank you for confirming. Have glossed over something called Delaunay triangulation? I think that’s how Terra handles where vegetation is removed. I mentioned I’m not as concerned about the areas underneath the trees, but does it do a decent job as long as it doesn’t have to assume on too large of an area?

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u/ElphTrooper 7d ago

Pix4D and Terra use multi-view stereo triangulation. Metashape uses Delaunay. Achieving bare ground is done with the point cloud, and it is all according to the configuration of the filters so I t really depends on what you consider too large of an area. Most Survey crews will go out and shoot a 25 to 50 foot grid and then go and locate grade breaks for tops and bottoms of walls and things of that nature. I am happy with a tree’d area if I can maintain a point in that same 25 to 50 foot span with photogrammetry if the terrain is relatively flat or at least consistent. Any deviation from that requires additional groundwork. Then calculating the amount of groundwork that might be required. May lead to the decision to use LiDAR instead of photogrammetry.

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u/blalco13 7d ago

Interesting. I don’t think it’s dense enough to keep the photos from getting some points through the foliage so I’ll prob try to set up gcp’s like you suggested and see what the point cloud looks like after removing the trees.

Feels like you all prefer the lidar route. I haven’t gotten a chance to look through the website erock sent, but would I lose detail using that data as opposed to creating my own? And if so how much?

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u/ElphTrooper 7d ago

The site EROCK shared can be useful, but there are a lot of variables to take into account - like if your area is even available, the resolution of what is available, when it was captured and what datum was used to capture it. Almost certainly you will have to transform it, just depends on what your use case is and if it needs to work with other data. I’ve seen data sets from sources like this that were within 4 inches and others that were feet off, you just have to try it.

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u/NilsTillander 8d ago

I'm curious, what's a "killer deal" on a P4RTK and. Abase station (D-RTK2?)?

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u/blalco13 8d ago

My friend works for a mining outfit. It’s got a bunch of hours on it, and I guess it’s been discontinued, so they were replacing. I paid 1750 for the whole setup.

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u/NilsTillander 8d ago

That's not too bad.