Measuring Land Settlement - GCP, RTK, Lidar?


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Looking at a potential application where I need to measure the settlement of land over time.  The land is not covered by much vegetation so photogrammetry is an option.  However obviously I need at least one ground control point outside of the subject area that is on stable land (such as a survey monument).  I'm looking for relative elevation accuracy within 3 inches which can be achieved by flying pretty low (Say 0.5 inch/pixel GSD).  It would seem to me, with a ground control point on can "anchor" each subsequent survey to,  there's no practical significant benefit to using RTK or Lidar?

 

Edited by Jazee
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On 8/29/2018 at 8:39 AM, Jazee said:

there's no practical significant benefit to using RTK or Lidar?

The vertical axis is the least accurate axis, but 3” should be achievable depending on which analytical software your using.  We us ContextCapture, overpriced but works well for the type of work we do.  Pix4D or Correlator3D should work well for NADER, if accuracy over a large area is important I wouldn’t trust the likes of DroneDeploy.  

How much settlement do you anticipate over what period of time?  3” sounds like a lot of settlement.  If it’s less you may have to incorporate more than just photogrammetry.

We usually blend both aerial and terrestrial photogrammetry with terrestrial LiDAR and ground control points (GIS) data over six to ten acre scans.  The key is in aligning all of this in the same coordinate system.  As a rule of thumb we use four GCP per acre depending on the structure we’re scanning.  We generally survey the LiDAR drops and simply use the GCP as tie points.  If we don’t use LiDAR then we survey the GCP’s on the boundary and use the rest as tie points.  

Regarding RTK, we use our own INS system, the only RTK we thought worked for a reasonable price was LOKI but they are now almost exclusively DJI and we’re prohibited from using DJI (or any Chinese manufactured UAV).  But depending on the accuracy and certification or level of trust necessary your probably going to need an additional method of aligning and confirming the accuracy of the scan.  

When I started this reply I thought this should be pretty straight forward, but the more I thought about it the more complicated it became, it really depends on how accurate, over what area that will help determine how many variables you have to account for to get an accurate scan.  

Also, because of the clearances we have to maintain from equipment we usually fly 140-200’ so we use a 43MP camera.  

Good luck.  Let us know what you decide and how it goes.

 

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Great answer by Av8Chuck. My thoughts were pretty much the same in that it really depends on the size of the project and the desired accuracy. The Z axis is always the hardest to get consistent, repeatable measurements of, so knowing all the inherent limitations is very important. The thing with the GCP(s) is that it is more than an anchor for the Z measurement. They also are crucial in ensuring that warping of the image it kept to a minimum. So to me it seems like you will definitely need more than just the one GCP or you won't really be able to compare flight to flight.

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