Photogrammetry is the science of making measurements from photographs, especially for recovering the exact positions of surface points. Unmanned aerial systems (UAS) use photogrammetry by geotagging (logging a precise x, y and z coordinate) images taken in flight and then using special software to process the imagery into 3D models. Multiple images are taken at regular intervals and the overlap between images allows the software to calculate mathematically elevations and positions.
These models are accurate from 1 cm to 1 meter. High accuracy photogrammetry is dependent on Ground Control Points or RTK (Real Time Kinematics) geotags on the images. Project cost is directly related to the desired accuracy and level of detail. The desired mapping accuracy determines the mission altitude and sensor.
LiDAR is a surveying technology that measures distance by illuminating a target with laser light. Lidar exists as an acronym of Light Detection And Ranging. LiDAR sensors are now small enough that they can be carried on UAS. The laser scanner on the UAS emits up to 100,000 pulses of light per second to the ground and measures how long it takes each pulse to reflect back to the unit.
Specialized software is also used to process this data into point clouds. These point clouds are filtered by a skilled analyst to remove non ground points. Ground based LiDAR systems are accurate to the mm however air based LiDAR are accurate from 1 cm to 30 cm.
The greatest benefit to end users of LiDAR is its ability to penetrate dense forest canopies for high accuracy bare earth models and the ability to cover very large areas quickly using manned aviation. The benefits of photogrammetry is the export of orthophotos and lowered cost.
To compensate for the ground penetration limitations, photogrammetrists use classification algorithms to identify ground objects and remove these objects from the point cloud. This system has proven very useful and greatly improved the value of photogrammetry projects.
The bottom line is that each system has benefits and disadvantages.
When considering flood mapping, LiDAR is excellent for mapping under the canopy to a high degree of precision. Photogrammetry can provide high accuracy planimetrics (mapping of structures, roads and buildings). LiDAR and photogrammetry are simply different systems for creating LAS files. Each has its own strengths and capabilities and the choice between the two should be based on the technical requirements of the project.
Both photogrammetry and LiDAR have significant advantages of over ground based survey techniques. Specifically in areas with difficult access, high traffic, or disaster areas, these systems can significantly improve worker safety by eliminating the need for an on site presence.