Is it possible to find the minimum and maximum elevations for either the point cloud as a whole? Or even better, a specific category of a classified point cloud. I need to know the minimum altitude in order to help set the base contour elevations and intervals to meet a mapping standard.
Hi Gordon,
In Pix4D v2.1.43 (64-bit), the only way that I’ve found minima-maxima heights, be they the altitude of the camera, or the elevation of a single point, is by manually selecting it in the rayCloud viewer and examining its Properties and then its computed position. I have been unable to locate a discrete file in any of the project’s folders which contains a (human) readable listing of computed positions for each camera or point.
In other software; e.g., Global Mapper, the point cloud exported from Pix4D can be opened and then its minima-maxima elevation revealed from the Overlay Control Center, Metadata…, Metadata tab. Note, however, this won’t include data related to the camera, just the computed points.
Your question also speaks to the general need for surveyed ground control points in a known terrestrial datum. Depending on the camera used, the EXIF data in each image may or may not contain a value for altitude, and most likely, any value given will relate to a barometrically derived height above grade at the point of the drone’s take off position. This is very different than being related to a known terrestrial vertical datum.
In the absence of surveyed ground control points, Pix4D will use only the EXIF data. This could produce some unwanted surprises as we see see in the example below - note the minus sign:
The problem seen in this example above was not caused by Pix4D, but from erroneous EXIF data - see Altitude Issue When Surveying with the DJI Inspire for further discussion as to the cause.
Introducing surveyed ground control points, Pix4D is able to compensate for even these huge errors of altitude as shown in the screen capture at the top of this reply. And regardless of the magnitude in barometrically derived errors, using surveyed ground control points allows your project to be spatially relevant to a known vertical datum.
See this related article:
What does the Output Params Folder contain?
https://support.pix4d.com/hc/en-us/articles/202977149#gsc.tab=0