Imagery Processing
Center for Geospatial Analytics at North Carolina State University
Photogrammetric process
Photogrammetric process
Data processing
UAS data
What do we get after the flight mission?
Digital imagery
- usually on the camera SD card
- can be geotagged (depends on camera)
- Camera lens location is “written into” each photo’s EXIF file
- this is not necessarily the case…
Flight log
- Onboard Inertial Measurement Unit (IMU) accurately measures the orientation of airborne sensors,
- Information is logged into a text file (flight log),
- Contains elements of exterior orientation (EO, more later in the lecture)
GCP coordinates
- Measured by GPS coordinates of the panels set before the flight
- Photo ID points (distinguishable ground features) can be surveyed later on
- It is important to know the GCPs coordinate system (spatial reference system)
Spatial reference system
- Defines how the two-dimensional, projected map in your GIS is related to real places on the earth
- It is crucial to know what is your data reference system!
- There are global map projections, but most map projections are created and optimized to project smaller areas of the earth’s surface
- There are two different types of coordinate reference systems: Geographic Coordinate Systems and Projected Coordinate Systems
- Spatial reference list (EPSG codes for coordinate reference systems)
UAS data processing outputs
What do we get after processing the data?
Orthophoto
- Aerial imagery geometrically corrected (“orthorectified”) such that the scale is uniform
- Raster: consists of red, green, and blue bands
Digital Surface Model
- DEM/DTM - Digital Elevation Model / Digital Terrain Model
- Representation of a terrain’s elevation
- Bare-earth raster grid
- DSM - Digital Surface Model
- Representation of a visible surface
- Captures the natural and built features on the Earth’s surface
Pointcloud
- Representation of the external surface of an object
- Set of vertices in a three-dimensional coordinate system
- Vector or raster?
- Dale Lutz once said, “point cloud is a badly behaved raster”
Digital imagery
Structure from Motion (SfM)
- Range imaging technique
- Process of estimating 3D structures from 2D image sequences
- May be coupled with local motion signals
Exterior orientation (EO)
EO = Position and orientation in the object space
6 elements necessary for any photogrammetric processing:
- X, Y, and Z of the exposure station position (latitude, longitude, and altitude of the camera)
- Angular orientation: ω, φ, and κ (yaw, pitch, and roll)
Flight log
- Log file contains elements of exterior orientation that are measured by onboard Inertial Measurement Unit (IMU) and written into a text file
- Sometimes (most DJI products) exterior orientation parameters are saved in photos’ EXIF file
- Log contains information about the location of the camera, not the location of the depicted object - more info in this section of lecture 3
Interior orientation
- In the past: camera calibration report
- Now: Self-calibration (auto-calibration) is the process of determining intrinsic camera parameters directly from uncalibrated images
- Can be automatically derived using Structure from Motion (SfM) methods
Ground Control Points
- GCP - Target in the project area with known 3 coordinates (X, Y, Z or lat, long, alt)
- For more information about placing targets and importance of GCPs see this section of lecture 3
- For more information about processing the data with GCPs see intro to the assignment
Processing options
