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Terrain Aware or Fixed altitude for Mapping (Ortho, 3D and cut/fill calculation)

chasco

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Yes. We set up a base station on a surveyed benchmark that provides an RTK stream to refine the aircraft's GPS numbers. I've flown both ways; both with an RTK stream supplemented with Aeropoints as checkpoints as well as without an RTK stream using only the Aeropoints as control points and checkpoints. I've flown with and without terrain avoidance ( and that term is not accurate in this case) which only became available on the FireFly sometime last summer?? It makes a difference.

I know by looking at the data and comparing it, then spot checking it in the field. I don't take anything for granted. I'm old school. Technology breaks when you need to rely on it the most. I don't trust anything I haven't field verified when it comes to the important stuff. A lot of things we collect fall outside that level of scrutiny. It doesn't matter if a tree is exactly at that given location of if its 5 inches east of where we drop the point. OTOH, a 13.2 KVA duct bank is down to the centimeter, as is a fiber trunk line. Some things you can't afford to find accidently. At that voltage everyone about 8 feet around the excavator will be a mort.

The bottom line is you have to trust the equipment. But I don't trust the equipment so I have more equipment to check the numbers multiple times. If they come up close (1-2cm) THEN I know we did it right.
I'm right there with you. Twenty years of surveying and the are some things that you just have to do for documentation and sometimes certification. It may even go as far as shooting GCPs with a robotics total station or running a level loop if accuracy requires it. Collection raw logs and point metadata can come into play to so I'm pretty sure we know how to get accuracy.
 

chasco

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Got a map back from today from a site with 90-100ft of fall that I flew 200ft TA. Results were pretty good most notably the consistency of resolution from top to bottom. Normally from my takeoff point 200ft would turn into 260-270ft at the bottom and resolution reduction was very evident. Disclainer is that lighting was just a little better today. Vertical faces look much better, the streets are smooth and the curb is present. Here's the overlap map. Very consistent.

1585188750531.png

On the last linear flight the difference was about 2 images/pixel more on the bottom. More overlap is ok, but it was approaching overkill. More importantly you can see a drastic change in resolution from the 60-70ft.

1585189084325.png

Linear

1585189412487.png

Terrain Awareness

1585189495228.png
 

yarrr

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... We set up a base station on a surveyed benchmark...

The bottom line is you have to trust the equipment. But I don't trust the equipment so I have more equipment to check the numbers multiple times. If they come up close (1-2cm) THEN I know we did it right.
Thanks for clarifying that- seems like a solid workflow. I'm not a RLS, but I work with and for surveyors, and have been using survey equipment for 20 years- and I've learned its extremely important to trust but verify your tools.

It's also important to know the limits of the equipment, and the workflows. Compounding error is a beast. You're only as good as the weakest link.

This means that your UAV 3d reconstruction can only be as good as your GCPs (or geotags).

So it is important to remember that the very best Z accuracy you can expect from a RTK GNSS is 1.5cm. (If you go PPP for 7 hours you could get sub centimeter.) RTK accuracy decreases with baseline distance, and more importantly user error. Wiggle the pole, take a short reading, sink the tip into the ground, translate to orthometric heights/change datums- and you're well beyond 1.5cm.

The sames goes for SfM. The best we can expect is 1-3x GSD in Z. So it's tempting to jack up the GSD, but keep in mind the overall accuracy is still tied to your GCPs; and subject to all kinds of potential user error- or in the case of cloud based processing- all kinds of simplification/generalization.

All of this is to say that TA may be a incremental improvement in your orthophoto (and frankly can save a ton of time in mission planning for large/complex sites); but it isn't a magic bullet that is going to noticeably improve the accuracy of earthwork calculations.
 
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chasco

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So it is important to remember that the very best Z accuracy you can expect from a RTK GNSS is 1.5cm.
You get 1.5cm if you take the GNSS for what it is and don't do the proper control protocols. This is also why it is crucial to run a level-loop on the primary control network. You can't expect to produce a proper control network with GNSS alone. The problem that most people have when trying to tie to existing control especially on construction sites is that you have to localize in order to prorate error across the network. PPP is not even an option. If you don't localize then the monumentation doesn't match site-wide. Sometimes the benchmarks that are provided don't even match each other and then it doesn't matter what you do. Once you have your localization completed you run the loop and analyze the network to remove any points that are out of tolerance. Our tolerance for the primary control network is 0.05ft so the majority of the points are more like 1cm. Setting secondary control points has its own protocol, but if at all possible the GCP's should always be populated from primary control.

Wiggle the pole, take a short reading, sink the tip into the ground, translate to orthometric heights- and you're well beyond 1.5cm.
These things should not even be factors in a professional survey program. That's why we have bi-pods, set monuments for all control points and orthometric heights aren't even a thing.

but it isn't a magic bullet
As I mentioned before the recognition of the limitations brings you to a point that advances are smaller and smaller as you approach "perfection" so obviously there is no magic bullet, but TA is now a part of the process that IMHO should always be used when the terrain varies more than 25ft. There's no reason not to.
 

yarrr

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You can't expect to produce a proper control network with GNSS alone...

These things should not even be factors in a professional survey program.
Precisely, and completely my point!
 

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