How to Correct for Wrong Image Altitude without GCP

Error

It can happen that the model is not accurately geolocalized because its vertical coordinate is shifted.The planar georeference is however correct.


Description

The model and exported outputs show a wrong altitude. The error is introduced by the vertical coordinates of the images written in the geotags. Using Ground Control Points (GCPs) would precisely adjust the model altitude but alternative solutions exist when no GCP are available.


Cause

In most cases, GPS inaccuracy of the drone or the camera explains the vertical shift. Depending on the brand and model it can reach up to 100-meter difference.


Solution

There are two ways to approximately place the model at the correct altitude if no GCP was measured on the field:

 

Editing the image EXIF

 
Important: This solution is applicable if the image vertical coordinate is constant during the data acquisition, e.g. grid horizontal flight.

The true absolute altitude of the images is assumed to be known given the flight height and the geolocation of the drone operator.

1. Check the Vertical Coordinate System of the images. It should be consistent with the altitude value that is used to correct the vertical shift.

1.1. On the menu bar, click Project > GCP/MTP Manager...
1.2. In the section GCP Coordinate System, click Edit...
1.3. Check the box Advanced Coordinate Options.
1.5. In the section Vertical Coordinate System, select the appropriate option.

2. On the menu bar, click Project > Image Properties Editor...
3. In the section Image Geolocation, click To File...
4. On the pop-up Export Image Geolocation, check the selected option for the field Coordinates Order and click Browse... to choose a location and a name (.txt export format by default). Click Save.
5. Click OK.
6. Open the file with a spreadsheet application.
7. Edit the altitude column with the corrected value.
8. Save and export the edited file in the .csv format.
9. On the menu bar, click Project > Image Properties Editor...
10. In the section Image Geolocation, click From File...
11. On the pop-up Select Geolocation File, select the appropriate File Format and click Browse... to choose the file to be imported. Click Open.
12. Click OK.
13. Run step 1. Initial Processing for the first time or on the menu bar, click Process > Reoptimize to adjust the results.

 

Importing points from a Web Map Service

This solution works in any case. It is suggested to get for instance 5 points from a Web Map Service server over the area (e.g. Daft Logic). These points will be used as GCPs. For more information: 202560149.

1. Run step 1. Initial Processing.
2. Define the appropriate GCP Coordinate System. For more information: 202560029.

 
Information: The Daft Logic website takes the coordinates from Google Maps (WGS 84 ellipsoid) and refers to the mean sea level (MSL) egm96 to estimate the vertical coordinate.

2.1. On the menu bar, click Project > GCP/MTP Manager...
2.2. In the section GCP Coordinate System, click Edit...
2.3. In the section Coordinate System Definition, select Know Coordinate System [m] and WGS 84 from the list (ellipsoid icon).
2.4. Check the box Advanced Coordinate Options.
2.5. In the section Vertical Coordinate System, select MSL and egm96 from the drop-down list.

3. Import the points using the rayCloud and mark them on some images. For more information: 202560349.

3.1. On the right sidebar, in the field Type, select 3D GCP from the drop-down list.
3.2. On the right sidebar, manually enter the coordinates taken from the website.

4. On the menu bar, click Process > Reoptimize.

 

 

Was this article helpful?
0 out of 0 found this helpful


Powered by Zendesk