Hi there guys and gals. I’m rather new to this forum, so please do excuse me if the question that I ask has already been asked or dealt with. I have been asked by a surveyor to do a survey of a large field, however, the field has potentially got knee high grass/scrub on it. When the surveyor does a traditional survey, they obviously take the readings from “ground” level with their equipment. Am I right in saying that photogrammetry will more than likely capture the surface area/detail of the elevated surface (i.e. top of scrub/grass)? If this is the case, for me to get an accurate “ground level” survey would it help to use GCP’s at ground level? Any advice or tips for me, on how to get accurate data for this type of scenario, would be very much appreciated. Thanks to all in advance.
The problem is the RGB camera takes what it sees and we analyze what it provides us, so the computer would likely be analyzing the grass and not the ground. Your GCP’s would give you the ground levels at the points of the GCP’s, but your elevations of the rest of the area would likely be indicating the grass level.
thank you for your reply. This is exactly what my thoughts were. Is there a workaround that you might know of, or how would you go about doing something like this? Kind of defeats the object of being able to assist surveyors to help them cut down on their survey times!!?
Short of going to a LiDAR based solution, they only thing that I can think of is to mow the grass short if that’s a reasonable option.
Ok thanks guys for the useful information, its very much appreciated. Lidar - too expensive for me, and I am definitely not pushing a lawnmower around 21Hectares (ha ha ha).
6 posts were split to a new topic: Emlid Discussion
I’m renewing an old topic in hope that someone might have more useful information than I did last year.
I have a potential 100 Ac, survey grade, mapping project that would consist of 90% tall grass (hay). It will be mowed to a height of 3 to 5" prior to the drone flight (if I take the job). The site is very flat with general elevation variation of no more than a couple of feet (excluding field ditches, etc.)
GCP’s would be established in at typical spacing and all features not suitable for drone survey would be surveyed using traditional methods. Deliverables would be othrophoto and probably DXF contours generated by DD software with contours cleaned up manually in Civil 3D. I could export point cloud but I do not have RCP subscription so it would have to be LAS (not sure what could be used for post processing LAS file). I could also export DEM but do have not experience post processing that type of file.
My question is if anyone has experience with elevation data that I can expect with short grass and if there is a workflow that can provide ground elevation (assuming DTM reflects the top of grass) for this type of situation.
Any info would be greatly appreciated,
Good morning @StevenB! Having the field freshly mown will be a big benefit to your survey. First question I have is if you are flying with any PPK/RTK GPS? Just food for thought that may come into play later in the discussion.
Bringing a contour map into Civil 3D is fine, but with the fact that DroneDeploy only allows us to download down to a 1ft map it may not be suitable for you requirements. It will be more accurate than the “pretty” contour maps that we normally see from engineering and the contours are a more direct recreation of the point cloud, but any steps or grade breaks of less than 12in will be deformed. This is where your terrestrial survey comes into play. You will want to create your 3D breaklines in the contour map before making a surface. The other option is that if the surveyed area is a complete polygon you can merge that to the overall drone surface.
QGIS has a plugin for LAS editing and would then allow you to create sub-foot contours, but editing the point cloud directly is still the preferred method for this kind of operation.
I would highly recommend Carlson Precision 3D if you are providing services for your business or to external clients. It is a much more powerful editing tool than Pix4D or Recap. You can collaborate XML or TIN surfaces between it and Civil 3D or Carlson Civil. You can also work with Google Earth Surfaces for conceptual and rough grading, draping Google Earth images and your drone orthomosaic map. The most powerful features are the point cloud filtering methods. You define grid cells and the program isolates the ground points and then removes points above those and outliers automatically according to the min/max/slope parameters you set. Once the point cloud has been corrected you can then export the corrected surface (not the contours) in either Civil CAD programs and adjust the plane elevation using checkpoints (not GCP’s) and other terrestrial topo you did.
Carlson P3D Topo Menu. More options are available when the point cloud is loaded and the surface is created.
One more. Here is the Recap Civil 3D workflow. Not near as streamlined and from my experience, not as accurate because of the limited filtering.
- From Insert ribbon, click “Attach” button
- Browse to RCP file and click “Open” button to import into Civil 3D,
- In the next dialog box, click OK and specify insertion point
- Zoom to see the point cloud
- Select Point Cloud with cursor so that contextual ribbon displays.
- Click “Scan Colors” icon.
- Select the drop-down list arrow and choose “Classification”. (You see your point cloud change color which verifies that your point cloud contains point classifications).
- On the command line, type MAPCREATEPCSURFACE
- In the drawing, select the inserted point cloud and hit Enter
- In the Create Surface from Point Cloud dialog box set as desired (creates TIF file)
- Click Filter on the lower left of the dialog box. In the next dialog box, select filter “Ground”, and OK. (filters all points out except the ground classified points when creating your DEM file).
- Tiff file has ground points.
- Create a Civil 3D surface and import the DEM file you created as a surface definition.
Note : To filter out all information, other than ground classified points, you need to turn this ReCap point cloud into a DEM file using the Map 3D command.
Thanks for detailed info Michael. I have worked a little with the Recap workflow when we had a trial subscription to RCP point clouds. Ultimately, we could not justify the extra cost. I am a PLS but have worked for years in the site engineering. Most of my mapping projects have been for construction sites (bare earth, progress monitoring, as-built confirmation, etc.) so tweaking point clouds for DTM’s on un-improved sites is not in my wheel-house.
I work with lots of surveyor’s and some are beginning to recognize the value of drone for certain applications and so are asking me to fly sites as a sub for their surveying projects. I will check to see if they use Carlson software and go from there. So do you think the filtering tools in Carlson would be able to differentiate bare ground point from vegetation with that small of a difference (say 3" mowing height)? I have looked at Virtual Surveyor and sounds like some of the same tools you are describing but have not given it a trial yet. Sound like Carlson is better solution. Is Precision 3D a stand alone product?
I found the Recap flow lacking because we do all of our work in a survey workflow and Recap does not translate well back to survey. Carlson was originally built for survey so all of their solutions are easily backwards compatible.
The filtering tools will pickup as small as an elevation difference as you want. It becomes a balancing act of how much you can remove without decimating the cloud too much. The project I am working on right now has 62M points and I was able to decimate it down to 12M while maintaining that there were no gaps larger than 3ft with grade breaks preserved. Most of the points are within 2in of each other.
This is a 25ft cell.
That said, you can use the Carlson P3D Topo data in Civil 3D seamlessly. It is a standalone product and a one-time purchase of $1750. You can get on annual maintenance for upgrades, but I am running 2018 (from mid-2017) and have not seen any great things that would lead me to upgrade.
I have heard good things about VirtualSurveyor and some of its capabilities may suit your scenario better, but it is a subscription at $150/month. I do not see where it exports native surface files, just contours.