After spending years with this type of software, and comparing RTK results to photogrammetric results of volumes using close-range photogrammetry software, I have the following personal opinion:
1. Despite the advertisements of some, this type of software can give erroneous volumetric results significantly exceeding 10-20 percent even if the control points check in very closely. This is especially true when doing volumetric work on stockpiles or quarries that are not very tall or deep (on the order of 25 feet more or less).
2. The statistics offered in the results in some of the softwares are generally provided, but the ramifications of the statistical results are often misinterpreted and are often vague.
3. It is pleasing to get a beautiful 3D model that looks right and checks into control points virtually perfectly. It is likely that beauty and checking to control will cause end users to simply accept the results. But the results can be disastrously incorrect. This condition is as bad or worse (and far more frequent) than GPS multipath errors that go unchecked.
4. I have flown stock piles on three successive flights on the same day, with the pile unchanged and have seen errors that exceed the gross error threshold. And I was using the same camera, equipment and software. I have found that RTK and Carlson yield that are incredibly consistent for on-the-ground RTK volumetric work, and the photogrammetric software results varied by over 10% from RTK/Carlson frequently.
5. The solution is the understanding of exactly how the software determines the volume from the spray of computed surface points, and their variation in accuracy of the surfaces of the finite elements from which the volumes are computed. The problem is that the top of the internally computed finite elements along the digital surface are not as sharp or accurate as one might think. The software publishes the statistic, and the end user should not forget the value of that statistic. The shorter the finite element, the higher the percentage of volumetric error in the finite element. The error could average itself out, but not necessarily.
I am sure that some of software vendors might try to refute what I am saying and say I don't know what I am talking about. However, I can and have demonstrated it on projects for the last 6 months on a controlled study.
I hate it when computational software provides something good looking that looks right, but is wrong.
My main concern is that GIS and others will use this technology and become the extreme button pushers (without verification of results) while at the same time violating the reason that land surveyors are professionally registered--one reason the protection of the public from damages. I'd say a quarry volume off by 4 times the allowable tolerance would amount to public damage/financial loss, and further degradation of the profession of land surveying and engineering.
Some of the software out there is better than others, and I am not knocking any brand. One brand I am aware of has a very good statistical output that assesses probable error, and it has an excellent of accuracy consideration and statistics, as one would expect from a professional package. But my concern is that some of the end users don't know the ramifications of the statistic and don't give it the attention it needs, all exacerbated by trying to make a profit and meet the deadlines.
One this is for sure: Drone volumetric software is not necessarily the wonderful solution for all projects. I do think a surveyor who checks his work and takes time to understand the limits and statistics of the output can use the software very effectively for volumetric work in many cases.
The critical action is to ensure that for each survey random ground checks are also taken across the site, BEFORE THE FLIGHT. If temporary marks are left at some of these points then they can be added as control, if the first model created shows significant errors. It's no good making them all control points initially, as then you have nothing to compare with. Once the error range is known then they can be added in to tighten up the final model as necessary (or accept that the photography is poor and walk the whole job with GPS!)
Key is to remember traditional survey theory, apply good geometry to the control with fixed ground points and don't believe statistics - real measured checks have far more significance. This is a "blanket" technology which relies on large numbers of "near-enough" points rather than many fewer "spot-on"s. Typical accuracies of 5cm. should be attainable as long as care is taken.
I was wondering if anyone tried Agisoft PhotoScan, how is it ?. I does lens calibration too, this should add towards accuracy.
Agisoft works quite well, although you need a lot of computing power if you have several stages open at the same time and the models are big (say 500 plus photos). For high quality level image manipulation you need really serious computing power. (That probably applies to any package, but others may have better data handling. Very large jobs need cutting into smaller chunks. Biggest pain is on high overlap jobs where each control point might appear on 10-15 photos. Although it will identify the relevant photos and mark the points, each needs to be adjusted manually on each photo for best quality results.
The stats are a little suspect (compared to real life) but I assume they represent internal residuals rather than the fit to control. Providing the image GPS coords does speed the process up and reduces the number of images it is unable to fit. As with any software it generally struggles with low texture areas and has, on occasion, managed to match up a sliver of the gimbal (which appeared in some frames by accident - incorrectly mounted camera) and create some interesting artefacts in the orthophoto.
It does have quite good facilities for masking out dubious sections of images. It copes well with vertical photography and the semi-obliques captured around each turn.
Thank you, I tried demo and it died on my current old computer, so I could not even try anything. I noticed from this program you can print out circular bar code markers (may be on a plastic sheets), and the program will automatically recognize the markers on all images they are present, I imagine this would reduce amount on manual work significantly.
chris mills, post: 346192, member: 6244 wrote: The critical action is to ensure that for each survey random ground checks are also taken across the site, BEFORE THE FLIGHT. If temporary marks are left at some of these points then they can be added as control, if the first model created shows significant errors. It's no good making them all control points initially, as then you have nothing to compare with. Once the error range is known then they can be added in to tighten up the final model as necessary (or accept that the photography is poor and walk the whole job with GPS!)
Key is to remember traditional survey theory, apply good geometry to the control with fixed ground points and don't believe statistics - real measured checks have far more significance. This is a "blanket" technology which relies on large numbers of "near-enough" points rather than many fewer "spot-on"s. Typical accuracies of 5cm. should be attainable as long as care is taken.
Chris, even when you do what you suggest, the surface model is a fuzzy group of many points. Random points might match the surface, but they can just as easily miss it by feet. I agree that the check point should not be control points, because the model will adjust or warp to the control points. That is not the problem though. The problem is that the digital terrain surface is the average of thousands of points, which are only so accurate, period.
Here is a link to a very recent article in a south african survey & geomatics publication were they did a 6km road strip survey using traditional ground survey with GNSS versus a photogrammetry survey using a UAV, no lidar used.
Each survey was done by two different companies and the results interpreted independently, I have only briefly read the article, so I don't have an opinion on it yet.
The article only compares the accuracies and precisions, I would also be interested in the costs comparision per km or hectare between the two methods.
I recently sub contracted out a 126 hectare farm survey out to some aerial guys, the time saving was in the order of 40 days and the cost saving about 50 %, as to the results... well its looks good and met all the checks and balances . But if I was going to use it for detailed engineering services and planning, I think I might go shoot it conventional, not the whole farm, but just areas of interest.
http://www.ee.co.za/article/rpas-vs-ground-survey.html#.VlfsqF9XenM
Here is a photo showing the type of terrain and the ground control we placed.
Frank is correct in saying the surfaces are fuzzy groups of points, but Photoscan has an option which allows you to set a search and slope parameter. Using this it will find the lowest point in the search area (say 1 foot) and eliminate all other points which lie at an angle of more than the slope parameter. This not only thins down the model substantially, but removes most of the outliers (either random, or plants/bricks, etc.). There still needs to be manual inspection and selection for feature lines etc. but on subjects with not much detail the time saved is large. To date, as long as a rigorous approach is taken, nothing within the parameter area falls outside a 30cm. (1 foot) band once the automatic thinning out has been done. Go outside the control and the errors spiral away.
PDOP 1.0 is right about taking engineering detail conventionally. I tend to do this using a total station, even when the rest of the site has been surveyed by GPS. The aerial method is best suited to work where access is difficult or the surface is very irregular, when a surveyor is having to make significant judgements about what are "typical" points, or large areas with little of interest - ie. bulk spots.
Incidentally, Photoscan have just released a new version, which has a number of improvements - on orthophotos it shows the boundaries of the areas used from each photo and it will display contours on the imagery (very useful for a quick check on any rogue points left in the model)
I haven't seen the bar code markers that Yurly mentions being used, but I imagine they would have to be quite large to be read satisfactorily from 400 feet. For terrestrial work from a range of a few tens of feet smaller markers like that would be satisfactory. The photo by PDOP 1.0 shows a very large marker. For SUA work I've never needed to try anything bigger than 60cm (2 ft) square, although for conventional LIDAR I put out 120cm (4ft) boards.
Ha ha - looks like a beer leg target!
Don't you shoot the point before laying out the plastic sheet?
the markers print out is in Tools->Markers->Print Markets, but I do not know if one can print on a larger sheet than A4 (Letter size)
Warren Smith, post: 346290, member: 9900 wrote: Ha ha - looks like a beer leg target!
Don't you shoot the point before laying out the plastic sheet?
We used 3 polypropylene woven sand bags filled with few rocks or stones to keep them in place , positioned to form a triangle in the middle , then wack in a spike and record , cost for 3 bags about US $1
pdop 1.0 I was just wondering what kind of a radio antenna are you using with your R? (5800) in the picture above, do you have p/n ? Sorry for bugging. Intuitively looks like it has a good gain antenna, and the fiberglass pogo does not interfere much with the reception unlike regular metallic pogos.
Yuriy Lutsyshyn, post: 346322, member: 2507 wrote: pdop 1.0 I was just wondering what kind of a radio antenna are you using with your R? (5800) in the picture above, do you have p/n ? Sorry for bugging. Intuitively looks like it has a good gain antenna, and the fiberglass pogo does not interfere much with the reception unlike regular metallic pogos.
We are on a VRS network , no radio antenna plugged into the R4 receiver, what looks like an antenna is the bipod leg that we attach to the top of the pole , so we can lay it down on the ground while fixing the ground control
Yuriy Lutsyshyn, post: 346322, member: 2507 wrote: I was just wondering what kind of a radio antenna are you using with your R? (5800) in the picture above
I think you may be mistaking a 12-inch bipod for a radio antenna. It looks to me like he has the Seco bipod pictured below mounted just underneath the receiver.
Spot on Jim, that's it
That bipod is one of my favorite tools. I use it with an 8-foot Seco carbon-fiber adjustable pole for both GPS and total station work.
Yuriy Lutsyshyn, post: 346194, member: 2507 wrote: I was wondering if anyone tried Agisoft PhotoScan, how is it ?. I does lens calibration too, this should add towards accuracy.
Lens calibration on a plastic lens?? Really? All internally? Really??
I'll bet you a dollar right now that if the government maintains the same lens calibration criteria that they use on "Real" Aerial cameras, that there is not ONE light weight digital camera made, that a drone can lift, that would pass such testing. I have a Leica digital camera that takes awesome photos, but Leica will not state that it meets the U. S. Government calibration criteria.
Many shady claims are bring made on GIS systems that just don't hold water. I can see where such systems are great for initial planning, but when you are talking comps that will affect what someone gets paid for a service, you better be ready to spend a LOT of time in court.
It's bad science. The physics calculations just don't verify the claims.
Remember back to the early gps field units. We had a salesman tell us not only would this new gear give sub-meter accuracy, BUT he laid a dime on the ground and said that was the x/y accuracy we could expect and then he stood it on edge and told us that was the Z accuracy we could expect. Really???
Nothing has changed when it comes to new stuff sales claims. Not in over 1,00 years. That's why we have the OLD saying "Caveat Emptor".
We have used UAV's to great success with photogrammetry for certain applications (large civil ptojects) - consistently achieving an average of 40mm in verified area's when the site is stripped to bare earth.
We have determined that Lidar is a better option when there is long grass , foliage etc. There are circumstances where conventional RTK will be essential such as near vertical cuts/excavations. I think that UAV's could be considered the best solution for quarry volumes and the like
In my opinion with the correct verification procedure UAV's are indispensable in today's large scale volume surveys if one is not to fall behind the 8 ball. In saying that - we have had subcontractors that we have commissioned for UAV monthly volumes that have failed dismally when verified.
thanks, I really thought this is some kind of a special high gain antenna
Going back to Frank's original posting.
There is a real concern that button pushers will leap in, produce rubbish and then get the technique a bad name. Manufacturers rely on selling the things, something which is less likely if they explain that it takes months of hard work to get your head around how the systems best operate and even longer before you have some degree of certainty as to how each site needs to be tackled in order to get an acceptable result. The difficulties start when non-surveyors can't analyse the results they produce and recognise obvious problems.
It'll be a long, long while before "press and go" comes as a reliable SUA system. In the meantime surveyors need to do what they have always done best: think it through logically, check and then check again.
I think the lens calibration matter is a bit of a red herring - Photoscan matching is good, especially when the forward and side overlaps are high and the sheer weight of data seems to get the result. Any daft points are easily screened out once the initial cloud has been generated.
A4 size targets are about the minimum useful size from 400 ft.
