Drones sometimes record inaccurate flight heights in the EXIF tags of the images causing products to be built at the wrong elevation. This includes 3D point clouds and meshes as well as 2D DSM/DTM images and contours lines. To correct the images for use in Drone2Map for ArcGIS 1.2 it is necessary to create a file with more accurate altitudes which can be imported into Drone2Map for ArcGIS 1.2.
The steps to correct image altitudes are detailed below. (Drone2Map for ArcGIS 1.3 will contain functionality to Adjust Image Altitudes automatically):
To adjust image altitudes, you will need three things:
- A CSV File containing the image name, lat, long, alt fields
- The Relative Altitude (height above ground) at which the drone was flown
- The Orthometric Elevation on the ground above which the drone was flown
Step #1: Creating the CSV file:
- Create a new Drone2Map for ArcGIS project
- Select the “3D Mapping Template”
- Change the “Vertical Reference” to “No Conversion”
- Add the images to the project
- Click “OK”
- Open the “Image Properties” dialog from the ribbon
- Click the” Image Options” dropdown
- Click “Export All”
- Click “Save”
Step #2: Finding the Relative Altitude:
You will need the altitude you specified in the Mission Planning application to fly the drone at. If you remember this altitude you can proceed to the next step, if not you will need to retrieve the relative altitude from the EXIF tags of one of the project images or some other file output by the Mission Planning application (Note: Not all drones record the relative altitude).
You will only need the Relative Altitude of a single image, usually just the first image in the project (taken once the drone reached the proper altitude of course).
- Retrieving the Relative Altitude from the EXIF tags:
- You will need software which reads the EXIF tags of an image. We will leave it to you to search the internet and find a suitable application, however we have used Exiftool without any issues in the past (https://sourceforge.net/projects/exiftool/)
- Look for the tag containing the Relative Altitude. For example with Exiftool:
About : DJI Meta Data
Format : image/jpeg
Absolute Altitude : +230.33
Relative Altitude : +27.10
Other locations of Relative Altitude:
Some Mission Planning applications write additional files that may contain image information. Look in all project and text files output by the Mission Planning application.
Step #3: Getting the Orthometric Elevation:
You will only need the Orthometric Elevation of the point at which a single image was taken, usually just the first image in the project (taken once the drone reached the proper altitude of course). Since drone project areas are relatively small this elevation can be applied to the entire project to successfully produce products representing the proper elevation.
We will be adding a Ground Control Point (GCP) to the Drone2Map for ArcGIS project temporarily to get the Orthometric Elevation. This GCP can then be deleted.
- Open the Drone2Map for ArcGIS project you created in Step #1
- Click “Manage GCPs” in the ribbon
- In the “Manage GCPs” window click “From Map”
- Click on the first image point in the map (you can close the “Image Viewer” if it opens)
- Record the “Elevation ” of the GCP added to the table in the “Manage GCPs” window
- Click “Clear” (Remove All GCPs)
Step #4: Correcting the Image Altitude values:
- Open the CSV File you created in Step #1
- Change the “altitude” column for all images to the sum of Relative Altitude + Orthometric Elevation
- Save and close the CSV file
Step #5: Applying the corrected Image Altitude values to the project:
- Open the Drone2Map for ArcGIS project you created in Step #1
- Open the “Image Properties” dialog from the ribbon
- Click the” Image Options” dropdown
- Click “Set All GPS Source”
- Browse to the CSV file you created in Step #1
- Click “OK” in the “Set GPS Source” dialog
- Click “OK” in the “Image Properties” dialog
Step #6: Run the project:
The project will run and produce products that reflect the proper Orthometric Elevations and will hence display properly on an Orthometric basemap/surface such as those in Drone2Map for ArcGIS and ArcGIS Online.
