This is for internal testers at the moment. English Tutorial is coming soon.

Summary

  • Who should read this article?
  • How to insert GCP in Altizure?
  • Attention
  • What is a Preliminary GCP Report and how to read it?
  • Difference between Preliminary GCP Calibration and Complete GCP Calibration

Who should read this article?

You must have the basic knowledge of surveying the ground control point (GCP) in order to use this feature. In some countries or regions, license is required in surveying ground control points. Please follow the local laws and regulations. At the moment, the testing is by invitation only. If you want to get an invitation, please email us at support@altizure.com. We’ll send you one as soon as we can provide more seats for testing.


How to insert GCP in Altizure?

  1. While creating a project, you need to check the box of Ground Control Point.
    create-enablegcp.png

  2. Follow the instructions on the website, upload your photos and create a project.

  3. Wait for the Camera Poses (SFM) Task to finish, and add the ground control points by following the steps below.
    SFM finished-2.png

  4. Evaluate the SFM Result and make sure that there’s no huge error. This is important because it would be useless to add GCPs when you have an erroneous SFM result. However, if you do get a wrong result, you can update some images and rerun the SFM step. Or, you can report it to our Support Team by using Report Problem.

  5. If the SFM result is good, you can move on to the next step. Click the purple icon and open the GCP Viewer.
    Step 1 & 2-3.png

  6. Here in this step, you can only see the camera poses with an orthophoto if you add GCPs for the first time in this project (just as the picture shown below).

    Click Import on the top-left panel to upload GCP data.

    But before moving on, let's first understand the functions of Export. Literally speaking, it means to export the coordinates data that you have already marked on the map in .json file. You can use this function when you want to keep a record.

    After you finish marking the GCP positions in at least one photo of a Ground Control Point (which we'll talk about later), our machine will regard this point as "real" and record its position. So, when you Export the coordinate data, the .json file you export will include the locations of this GCP.

    If you do the same thing for all the GCPs, then you'll have all the coordinates in the exported .json file.
    import button-3-1.png

  7. There are generally two ways to import data.

    The first is to import it in .txt format, while the second is .json format.

    If you are importing the coordinates into this project for the first time, you can only choose to import them in .txt file.

    For .json format, as explained above, it is just another option for you to import data.
    popup-1.png

  8. After you upload the GCP information, a top-left panel with several crosshair icons will appear. Each icon represents a GCP. You can click it and start to mark the GCPs in the orthophoto.
    crosshair-1.png

  9. After approximately marking at least three GCPs in the photo, you can click Calibrate Coordinate and then Confirm it. By doing so, the machine will automatically mark the rest of the coordinates you import. This means that based on the GCPs which you already marked on the orthophoto, the machine can roughly predict other GCP positions.
    click calibrate-2-1.png
    click confirm-3-1.png
    2 types of GCPs-1.png
    Please notice: This is just a preliminary marking that helps you roughly locate the GCP positions in the map. These positions may shift a few meters away from the right spots. You need to adjust them into their exact locations in the following steps.

  10. Select a GCP you want to adjust. The photos that the drone shot around this spot, will appear at the bottom of the screen.
    select a photo-1.png

  11. Click any photo, and you can see a red crosshair in it. This is the GCP position either predicted by the machine or previously marked by you in the preliminary marking.
    Zoom in the photo and mark your GCP in the photo precisely.

    Click Save. Or otherwise, the machine will not record the changes you make.

    Do the same thing in 5-7 photos for each GCP so that you can triangulate an accurate 3D ground control point.
    select a photo 2-1.png
    select a photo 3-1.png
    select a photo 4-1.png
    Tip: There’s an Eye icon on the bottom-left of the photo. That is the toggle of a green crosshair that represents the original GCP position in the preliminary marking. Click it, and you can switch on/off the green crosshair.
    select a photo 5-1.png

  12. To assess the GCP calibration quality of your project, you can select Use for Validation and label some GCPs as checkpoints. In processing, these checkpoints will neither be used for georeferencing nor for optimization. But by comparing the checkpoint’s 3D position with the coordinate you input, the machine will have a better assessment of the project accuracy. So, you still need to mark 5-7 photos for each checkpoint.

    Tip: It would be better to select the GCPs in the centre of your orthophoto as checkpoints, rather than the ones near the corners or edges.
    use for validation-1.png

  13. Notice: Please make sure that you have at least 3 GCPs other than the ones you chose as checkpoints.

  14. Close the GCP Viewer and click the button Preliminary GCP Calibration to allow the machine to run a preliminary calibration.
    run preliminary-1.png

  15. Check the Preliminary GCP Calibration Report and see if there’s any huge error. In case of that, please double check the photos and see if all the GCPs are marked in the right position. Rerun the Preliminary GCP Calibration and repeat the above steps until you get a satisfactory result.
    download report-1.png

  16. To make it easier for you to revise the GCP positions, we recommend you to click Export and save the GCP data after marking all the GCPs. This allows you to save some time when you want to revise the positions next time.

  17. Click Complete GCP Calibration when you get a good result, and click Next Step to continue.
    complete and next step-1.png


Attention

  1. In order to better triangulate the 3D position of a GCP, you are recommended to mark the GCP from different angles.
  2. The GCP should be distinct and clear enough on the aerial photo, so that it’s easy to locate the GCP position on the photo.
  3. You need to mark the GCP positions in 5-7 photos for each GCP. For each photo, the GCP position should be at the center of the photo. An example is shown below. It would be better if the GCP position is within the red rectangle area in the photo below.
    faq_gcp-16-1.png

What is a Preliminary GCP Report and how to read it?

It sounds a bit complicated, but actually it’s as simple as that. To understand it, just imagine that building a 3D model is like putting all the photos in their right positions corresponding to a place on the 3D model. So, in the SFM stage, the machine will predict the position of each photo. But this is just a preliminary estimation and the machine is not 100% sure about it. However, by adding GCPs in the reconstruction process, the machine now has something to refer to. Based on these GCPs, the machine can slightly adjust the positions it previously predicted.

After you marked all the GCPs in photos and started a preliminary calibration, our software would try to fit your GCPs into the camera poses (SFM) result without making dramatic changes to each camera pose. The software will then generate a Preliminary GCP Report, telling you the fitting deviation that the SFM result might have based on the GCPs as you have marked.

However, some people may wonder, since Preliminary GCP Calibration doesn’t make any adjustments to each camera pose, what’s the point of doing it? The main purpose is to help measure the accuracy of the GCP positions that the user has marked in photos. In this way, the user can identify the errors and make necessary corrections in time.

  • Space Error: (Unit: meter) The distance between the 3D coordinate triangulated from the GCP positions that you have marked on 2D photos, and the absolute 3D coordinate of this GCP. Please notice that the distance here is in 3D.

  • Ground Error: (Unit: meter) The distance between the projected point of the 3D coordinate triangulated from the GCP positions that you have marked on 2D photos, and the projected point of the absolute 3D coordinate of this GCP.

  • Height Error: (Unit: meter) The distance between the height of the 3D coordinate triangulated from the GCP positions that you have marked on 2D photos, and the height of the absolute 3D coordinate of this GCP.

  • Reprojection Error (Unit: pixel) The GCP positions that you have marked on 2D photos, will triangulate a 3D coordinate. This coordinate will then be reprojected back to each photo of this GCP. The Reprojection Error is the distance between the reprojected point on a photo and the GCP position that you have marked on this photo.

Below is a Preliminary GCP Report with no big errors:
GCP report example.png


Possible Reasons for the Errors in GCP Report:

  • Height Error is big:
  1. You didn't mark the GCP position on the oblique photos of the control points.
  2. The GCP coordinates you have imported might be wrong at the first place.
  3. The GCP positions in different photos of this control point are not totally identical to each other.
  4. The GCP position that you have marked on the photos of this control point, is not the same as its actual location.
  5. The GCP location is at or near the edge of the model. This could accumulate the errors in 3D reconstruction, and hence, lead to big Height Error.
  • Ground Error is big:
  1. The GCP position that you have marked on the photos of this control point, is not the same as its actual location.
  2. The GCP positions in different photos of this control point are not totally identical to each other.
  • Reprojection Error is big:

    The GCP positions in different photos of this control point are not totally identical to each other.

  • Ground Error is small, while Height Error is big:

    When you import the GCP coordinates, you flip the X, Y coordinates.

If you can't figure out the reasons, please use the Report Problem function on the project page. Our technicians will help look into it.


Difference between Preliminary GCP Calibration and Complete GCP Calibration

In Preliminary GCP Calibration, the machine will not make any real changes to camera poses or the position of each photo. But in Complete GCP Calibration, the machine will make actual adjustments to the camera poses.

Below is an excel report for Complete GCP Calibration with no big errors.
complete-gcp-cal.png