IMAGE AND POINT CLOUD NOT IN THE SAME LOCATION

PIX4Dcloud
1

Morning All

We are having a problem with the coordination of our images online.

I am processing with the below CRS:

image
image1035×609 40.8 KB

Once processed I am uploading to pix4d cloud. When I id a point on the point cloud it is coming it at the correct location, but when I id a point on the ortho image it seems to be out on one axis by at times a few meters.

Has anyone got any ideas as to why this is?? Should I be processing in another coordinate system for the UK?

PS I only have this problem when uploading to the cloud. When I open the data in global mapper or qgis it seems to be coordinated completely fine.

Thanks for reading!!

Mat

2

TLDR; To ensure that PIX4Dcloud places a marker at the correct absolute elevation when you use the orthomosaic as your canvas, the project must also have a digital surface model (DSM).

Hi Mat,

Thanks for sharing the information and the screenshot of the Select the GCP coordinate reference system menu. At first glance, nothing looks out of the ordinary, so you’re on the right track.

Of the nine PIX4Dcloud projects that were created either by you or a colleague within the last 30 days, the following three projects have the potential to exhibit the unexpected behavior you described, i.e., a point, marker, or annotation on the orthomosaic is up to several meters away from the corresponding point or location in the point cloud:

  1. 1695745
  2. 1695785
  3. 1695605

Of those three projects, I suspect you are concerned about the first two projects in that list, i.e., 1695745 and 1695785.

If you have a different project(s) in mind, please send us the name, ID, or share link of the project you are concerned about.

If I’ve found the projects you are concerned about, i.e., 1695745 and 1695785, let’s continue.

At the time I opened both of those projects, each contained one point marker annotation at the center of a spray-painted white cross, which I trust was added by either you or a colleague. To test the horizontal alignment of the orthomosaic and point cloud, a second point marker annotation was added to the center of the same white cross in the point cloud.

The good news is that according to those markers, the orthomosaic and point cloud of both projects are aligned along the horizontal axes, where the difference between the Easting and Northing of the orthomosaic-based marker and point cloud-based marker for project:

  1. 1695745 was 0.01 and 0.003 meters, respectively.
  2. 1695785 was 0.007 and 0.02 meters, respectively.

This leaves us with the third and final axis, Z. Unlike the two horizontal axes, there was a significant difference in the absolute elevation of the orthomosaic-based and point-cloud-based markers in both projects, where the difference for project:

  1. 1695745 was approximately 5.2 meters
  2. 1695785 was 5.6 meters.

If it’s the difference between the elevation of the marker and the corresponding point in the dense point cloud, once again, the good news is there is only one reason that explains the difference in elevation. Since neither of the two projects has a digital surface model (DSM), PIX4Dcloud can’t compute the elevation of the project area at that location when you add a marker to the orthomosaic, so it sets the elevation of the marker to 0 meters.

This brings us to the two ways you can ensure markers are placed at the correct elevation from now on:

  1. If you prefer to add markers to the orthomosaic, add a DSM to the project along with the point cloud and orthomosaic. That way, when you use the orthomosaic to add a marker, PIX4Dcloud will compute the elevation at that location according to the DSM. You’re already familiar with the process of uploading reconstructions to PIX4Dcloud, but in case anyone else reading this is not, more information about that process is at How to upload or replace results to PIX4Dcloud
    .
  2. If you don’t want to upload a DSM to a project, use the point cloud as your canvas while adding markers. In that case, PIX4Dcloud will snap the marker to the point in the point cloud that is closest to your cursor so that the marker is located at the exact same horizontal and vertical position as the corresponding point.

Before you go through the trouble of uploading a DSM to the two projects we’ve focused on, you can use the third project I mentioned at the beginning of this message, i.e., 1695605 to test how PIX4Dcloud computes the elevation of an orthomosaic-based marker when the project includes a DSM. To do so, add a marker to the orthomosaic, switch to the project’s 3D view, and compare the absolute elevation of the marker to the corresponding points in the point cloud.

Please note that you will probably see a difference between the elevation of an orthomosaic-based marker and point cloud for the following reasons:

  1. Orthomosaic-based markers are not snapped to any point in the point cloud. Instead, the horizontal position corresponds with the orthomosaic and the elevation corresponds with the underlying DSM.
  2. The elevation of the project area according to a point cloud and its corresponding DSM will always exhibit some difference, with the understanding that the difference is expected to be insignificant.

If you’ve gotten this far, but you still haven’t gotten the answer to your question, please share the following information:

  1. The name, ID, or share link of the project or projects you are concerned about.
  2. Which axis, i.e., horizontal or vertical, exhibits the misalignment?
  3. A screenshot of the misalignment; preferably one that includes the entire screen and not just a zoomed-in portion of the screen.

On the other hand, if you have all the information you need to get back on track, please let us know.

Andrew