1.3.3 GCP Workflow

💡 GCP work is required for drone data-based surveying, earthwork volume calculation, etc. that require clear real-world reference values. "GCP work can be skipped if the absolute real-world coordinate system is not required, such as simple field monitoring. However, the following problems may occur if GCP is not applied.

⚠️ Problems when GCP is not applied

1. Height information (Z-axis data) may appear as an arbitrary value that does not match the field.(For example, the actual height of the field is 60m, but the height value that appears in the drone data is -110m.)
2. Error in (x,y,z) values that slightly distort the situation and location of the field from other dates. The error range is approximately 5cm to 2m
3. Reduced modeling quality → Increased measurement error value.

Things to check before GCP work

EPSG code

• The EPSG code (coordinate system) is used to measure the GCP points when surveying is required to successfully process the GCP work.
• If the EPSG code is different from the code used for GCP survey, it cannot be processed.

Measure 4 or more GCP points using a GPS surveying instrument

• At least 4 GCP settings are required to create an accurate model on the Meissa Platform.
• It is advantageous to arrange each GCP point in a shape that surrounds the site, but it should not go beyond the drone shooting range of the site.

GCP Example on Platform

Check if the GCP markers are clearly photographed in drone photos

• A point that is easily identifiable in drone photos is required. You can also use a separate marker near the site, or you can use the surrounding environment that is clearly identifiable if it is difficult to create a marker. (Manhole covers, road markings, etc.)
• If the GCP marker is not clearly visible in the photo, it cannot be used as a GCP point.

GCP Example

GCP Example

The more photos that look at the GCP point, the more accurate it is

• If the number of photos with GCP markings is small, errors are more likely to occur.

Check if the GCP file is in the correct format

• Check that the file is in the format that is uploaded to the 'Sample View'.

GCP File Upload

After completing the photo upload, click the "Next" button at the bottom right to proceed to the "GCP Correction" step and perform GCP correction.

Screen before proceeding with GCP correction

Upload GCP file and set EPSG code (coordinate system)

• Check if the GCP file is in the correct format, then proceed with the upload.
• GCP files must be uploaded as CSV or TXT files.
• Select the coordinate system used for GCP measurement, then add the GCP file.

Existing GCP Download

Existing GCP File Download

• If the GCP does not change, you can download and use the GCP by clicking "Download" in the upper right corner of the previous project.

How to create a new GCP

GCP csv example

GCP txt example

• If the GCP changes, you must create it as a CSV or TXT file as follows.
• The GCP name, X coordinate, Y coordinate, and Z height value must be entered in that order.

🚫 Warning: GCP upload is not possible if the EPSG code is incorrect, the GCP data has errors, or the coordinates are different from the site.

GCP file upload error screen

GCP Marking

GCP editing is performed as follows:

• Find all GCP markers in the photos after the survey, then click the mouse cursor at the exact location to add a marker.
• Click each GCP listed in the "GCP List" to mark the selected image.
• Mark all photos that contain GCP markers in the "GCP Image List". (At least 3 images per GCP)
• Use the "GCP Auto-Recommend" feature for faster marking.
  • Mark at least one image from the images listed in the "GCP Image List" and click the "GCP Auto-Recommend" button.

• • After a maximum of 2 minutes of analysis, GCP location recommendations are made for the remaining images.

• Markers marked in green indicate "Completed images" and markers marked in black indicate "Images that need to be marked" with the recommended feature applied.
• Once all photos with the GCP marker are processed, select the next GCP point from the "GCP List" and repeat the above steps. (4 or more GCPs are recommended)
• When marking is complete for all GCP points, click the "Complete Marking" button in the upper right corner to exit the GCP editor.

⚠️ GCP Use Precautions and Tips

1. Choose a location for the GCP point that will not be obscured by obstacles during drone shooting.
   1. For example, objects that can block GCP markers, such as walls, trees, and power lines that are more than 1 meter high.
2. Choose a location on a wide and flat surface for the GCP point.
3. The number of GCPs required may vary depending on the size of the site.
   1. The size of the site is relative, so you can contact the site manager to get information about the number.
4. It is advantageous to have the GCP points spaced as close together as possible and to arrange them in a shape that surrounds the site, but it is important to position them at the very edge of the site so that the number of photos that include the GCP markers is not too small.
5. In case the GCP point is lost, it is a good idea to acquire an additional 1 to 2 spare GCP points.
6. It may be useful to take a separate photo of the GCP point on site and keep it on file.
   1. For example, a picture of yourself standing with a measuring instrument so that the surroundings are visible.
7. It is a good idea to acquire GCPs regardless of the spacing of the GCPs in areas where the elevation changes significantly.
8. For ease of maintenance, it is a good idea to install it in a location within the site that is not easily lost.