Suggested Software and Workflows

Question

I work at a utility that is trying to utilize a Matrice 300 rtk drone and a Zenmuse P1 camera to map change detection on a power canal. We would primarily be looking for erosion along the banks. There was a failure some years ago and we would like to proactively mitigate any future failures. I was curious if anyone had any experience doing similar work or has suggestions on the efficacy of photogrammetry vs Lidar in this use case, as well as what software was necessary post processing. Thanks!

Jonah_Hall · Answer

Hello Kevin! I haven't seen anyone comment on the lidar aspect much so I may try and add some information on that process. My colleague Jeremiah pointed out two considerations when it comes to leveraging lidar. The first being if there are any vegetation that could obscure the ground from the camera (grasses / shrubs) and the other being the cost.

If you have used or looked at photogrammetry software (Site Scan for ArcGIS, Drone2Map, Pix4D, etc) then you may have noticed that they offer the ability to export not only digital surface models (DSM) but digital terrain models (DTM) as well. Now while each software or software suite may have a different way of generating those surfaces one common way is for the photogrammetric derived point cloud to be classified and then have those surface derivatives generated. Now, when your dataset has dense vegetation that obscures the ground ideally the software will not classify any of that area as ground. This will not effect the quality of the DSM as that dataset is only reporting the top elevation at the location, but areas with no classified ground points will have their elevations interpolated for the DTM. As you can imagine the larger the area obscured, the more interpolation and potentially more errors introduced into the dataset.

One of the big advantages for lidar as a remote sensing dataset is the ability to penetrate vegetation and provide better coverage over these obscured areas. There are a host of other benefits that come with lidar as a data source but in terms of your application I want to simply highlight the penetration and z value precision. I do want to paint an accurate picture for this data source and say that with "great scanning power comes great processing". Generating raw lidar datasets will require additional steps compared to the streamlined workflows you may be use to with creating photogrammetry datasets post-flight. These processes are handled either by the lidar scanner manufacturer or from the sensor integrator you purchased the scanner.

One approach which was highlighted above by Derrick Westoby that I would like to emphasize is this idea of identify then or quantify. While it is dependent on the scale of your operation, a tip 'n cue approach which leverages both datasets might be another viable solution. Leveraging photogrammetry or UAS imagery inspections along canals at a higher frequency you may be able to locate areas of concern and then capture lidar over said areas. The inverse would be infrequent lidar captures and then intermediary imagery collects at set intervals or post-event to provide a more up to date information. With either case and really any approach I will also highly emphasize what my colleague Coty said about the necessity of ground control for any form of comparison analysis.

Best of luck and I am always happy to discuss more!

Jonah

CodyBinkelheim · Answer

There's no mention above regarding ground control.  If you're seeking to compare datasets from different days, it will be critical that you have good 3D control points distributed across the project area.  Since your area of interest is a linear feature, make sure you have control near the ends and multiple points along the canal.  "How many?" depends on the length captured in a single flight but I'd suggest a minimum of 10, using 6-7 for control and withholding 3-4 as check points.  Of course, more is better.  You'll want the accuracy of your control to be ~3 times greater than what you're seeking to measure, so if you want to detect changes of ~10 cm, aim for control accurate to 4-5 cm.   Last, I would recommend you survey permanent features (concrete corners) so you can use the same control points for multiple dates (and you can go back and measure control *after* the flight has been completed - e.g. if you decide you need more control points).  Temporary control markers are certainly usable but it may be visually confusing if you have temporary control markers in different locations on different dates.

KevinKepp_Cowlitz · Answer

I appreciate all of the replies, lots to learn. Thanks!