Checks should always be by an independent means/method and to the same or higher accuracy, whatever survey is being undertaken - otherwise they achieve nothing other than to waste time (and perhaps "earn" an hourly fee!)
We always carry out additional random RTK or total station checks on any UAV work we do, if only because there isn't any other method of "calibrating" the results for different types of vegetation cover. A good sample of points within each vegetation type (ground vegetation obviously, not trees) provides a set of average corrections which can be applied to the area of each.
Software tends to like to average up the ground points along the boundaries between different types (or at hard edges) over a couple of metres width, so the check points need to be well within the surface type.
Yes, individual points will never be as accurate as a directly surveyed ground point, but with the corrections as above applied then the volume results will be as good as or better than ground survey. Anything critical, such as road edge heights can be surveyed as part of the check survey.
No, the method isn't cheaper than a ground survey, IF DONE PROPERLY but it does give more information in a somewhat quicker time - and might be safer.
No, the method isn't cheaper than a ground survey, IF DONE PROPERLY but it does give more information in a somewhat quicker time - and might be safer.
And that is what 99% of clients, and about 98% of management types, do not understand.
Adding up all extras that a UAS survey entails, plus the required ground survey to perform proper QA/QC, means additional expense that won't be overcome until the project is of sufficient size.
Does the Pix4D have an IMU? AND is it tied to its own onboard GPS?
There has been discussions recently re the usefulness of classifying surface accuracy according to vegetation. bare, moderate ground cover, heavy ground cover, etc. Makes a lot of sense to me for remote sensing.?ÿ
I'm drone ignorant.
Its been dangled in front of me, and I've played with the toys versions.
I supported fixed wing OrthoPhoto and Lidar so got schooled heavily in that area.
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@norm ASPRS came out with some accuracy standards that address that a few years ago.?ÿ We follow these procedures for our large LIDAR projects. http://www.asprs.org/a/society/divisions/pad/Accuracy/Draft_ASPRS_Accuracy_Standards_for_Digital_Geospatial_Data_PE&RS.pdf
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Some of the main advantages of remote sensing and specifically photo based surveys are speed, safety, and that it obtains data that I don't know I need yet.
For sure - at an increased cost. Sometimes it is worthwhile, sometimes not. All depends on the project.
We have a remote sensing group in our geomatics department, and it's pretty rare for them to be brought in without a clear objective or when we know for sure there are features that cannot be observed through conventional means, specifically due to the costs of doing it right.
Personally, I would love to fly all of our projects as SOP, just to have a current ortho to place behind the linework. But my organization is large enough that money and profit is literally the only consideration when deciding whether something should or should not be done...
just curious what everybody's thoughts are on the last paragraph of that article? statistically it makes sense of course but for general topo work where ground data needs to be within say +/- 0.05' is that acceptable enough? Anybody have any experience using that method with photogrammetry? me personally I always like to pickup things either with GPS or conventionally with a gun where the vertical needs to be tighter than that for grade tie in purposes such as EOP, curb line, etc... as it is common place to have street grades with curb here that are at 0.5% and strip pavement where grades are 0.3%
I'm not a fan of doing hard surface topo with GPS. I use the total station for that. If my choice was between?ÿ well controlled photogrammetry or RTK for hard surfaces, I'd probably go with the photogrammetry.
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@ncsudirtman I believe the point is; if your error budget is +/-.05', it does not matter what tool you use (who says my least squared hand tape triangulation network with a degree of freedom >1000000 couldn't hit that), but rather can you prove that +/-.05' independently??ÿ For example, do your RTK curb topo, build a DTM, then check that DTM with a digital level or total station and do some statistics on your independent checks.?ÿ If you're under .05' @ 95%, then voila!?ÿ In my experience, getting +/-.05 vertical @ a 95% confidence level is very difficult.?ÿ That is usually my threshold for static control?ÿbefore the topo even begins.?ÿ Let's be clear, I am talking about network or absolute accuracy, not relative.
Getting under 0.05' vertical overall from a benchmark or datum can be difficult, but what I like about total station work is that relative to the previous shot 50 or 100 feet away, I am likely within 0.01' or less, vertically. That is meaningful in design work and especially in staking. The biggest issue I have with RTK is that the real world expected precision is perhaps 0.1'...PLUS OR MINUS...meaning that two shots 5 feet away, I am likely to have one down 0.05' and the other up 0.07'...and when you add that up, it can make for ugly curbs.
And to follow up...if you had a large site, and you wanted to be ACCURATE to a certain benchmark, RTK might be exactly the tool for the job. The precision shot to shot might be less, but it that precision and accuracy is maintained over a much larger area than a total station (that isn't moved).
