Vegetation Measurement and Monitoring

13.5 Photogrammetric Techniques

Learning Guide

Photogrammetry is the science of making measurements from photographs. The American Society for Photogrammetry and Remote Sensing defined photogrammetry as “the art, science, and technology of obtaining reliable information about physical objects and the environment through processes of recording, measuring and interpreting photographic images and patterns of recorded radiant electromagnetic energy and other phenomena.”

 

Photogrammetric techniques allow us to take simple photographs of the ground or an object and create 2-dimensional (i.e., maps) or 3-dimensional (i.e., elevation surfaces) models with known geometric properties. In other words, photogrammetry allows us to merge together multiple photographs and assign them a scale. This allows for measurements from the maps or models to be translated into real-world measurements.

 

With the availbility of drones, it is now relatively easy to acquire digital aerial images for creating high-resolution remote sensing maps and 3D models.

Image from 10.13140/RG.2.2.31356.95365

 

Photogrammetry has been a science for almost as long as cameras have been used for taking pictures. While historically photogrammetry was performed by highly-trained analysts using printed-out photographs, most photogrammetric techniques are now done electronically. Digital Photogrammetry. refers to the photogrammetric analysis of digital images using specialized photogrammetric software.

 

Photogrammetric techniques identify objects common to multiple photographs and then estimate the location and angle of the camera where each photograph was taken. It is then possible to create a 3-dimensional representation of the objects in the photographs, and to precisely stitch the photographs together into a seamless map. Example here is a 3D point cloud model of a potted plant created from 16 photographs taken from a cell phone.

 

Photogrammetric Applications in Natural Resource Monitoring

Photogrammety has many applications in natural resource montioring, and with the increasing availability and decreasing cost of drones, photogrammetric analysis has seen a resurgence recently. Below are some examples of photogrammetric analyses for monitoring.

 

Gillan et al. (2017) used images from a fixed-wing (airplane) drone to map soil surface elevation in the Chihuahuan Desert outside of Las Cruces, NM. A digital photogrammetric technique called Structure from Motion was used to create 3D models of each plot to characterize the ground elevation. Using repeated flights of the same area, Gillan et al. were able to show the amount of soil erosion and deposition that was occurring on the landscape. Drone-based measurements matched well with ground measurements. Photogrammetric measurements from drone images makes it possible to measure soil erosion – an indicator that would be almost impossible to measure with field methods across landscapes in this amount of detail. Images from Gillan et al. (2017), http://dx.doi.org/10.3390/rs9050437.

 

Cunliffe et al. (2016) developed a workflow for estimating biomass and carbon stocks of rangeland vegetation using drone imagery and Structure from Motion photogrammetric techniques. Their process included development of both 3D plant canopy height models and classification of the drone imagery into vegetation types. Images from Cunliffe et al. (2016), http://dx.doi.org/10.1016/j.rse.2016.05.019.

 

Fraser et al. (2016) used drone imagery and Structure from Motion photogrammetry to estimate height of woody vegetation and to classify vegetatiaon types in a tundra ecosystem in northwestern Canada. Images from Fraser et al. (2016), http://dx.doi.org/10.1139/as-2016-0008.