Photogrammetry is the science and technology of obtaining reliable information about physical objects and the environment through the process of recording, measuring, and interpreting photographic images.

It is widely used in various fields such as asset inspection, geography, geology, architecture, engineering, forestry, urban planning. 

Here's a short summary of the process

1. Image Acquisition: The process begins with capturing overlapping photographs of the object or area of interest from different vantage points. I utilise a drone to acquire these images. Sometimes we supplement the images with ground based cameras. Most drones are programmed to fly a specific pattern to acquire the images and this can be repeated as many times as required, so that progress can be measured. Flight times are dictated by the size of the area to be surveyed as well as the accuracy required to create a realistic model.
2. Image Processing: The acquired images are then processed using specialised photogrammetric software to analyse and identify common points or features in the overlapping images, which are used to establish correspondences between the images. The higher the resolution of the output will dictate the number of overlapping points, which will then be used to program the drone to acquire the relevant images. Low resolution models can be compiled from 20 photos and high-resolution images may require several hundred images. 
3. Triangulation: After uploading the data to the software system and using the identified common points the software performs triangulation to determine the 3D coordinates of these points in space. This back solves the position of the camera at the time each image was taken and then determining the location of the common points relative to these camera positions. The camera records specified meta data, which is stored alongside the image. This will include GPS co-ordinates and camera direction and angle of deflection. The drone is usually stationary when the image is taken.
4. Surface Reconstruction: Once the 3D coordinates of points are calculated, photogrammetric techniques are used to reconstruct the surface geometry of the object or area being studied. This involves creating a digital elevation model (DEM) or a 3D mesh representing the surface. Once a model is created in the software, it can be exported to other software systems for further processing. The processing time is dependant on the accuracy and size of the model. 
5. Data Analysis and Visualization: The resulting 3D model can be further analysed and visualized to extract various types of information for representation on a screen or export to another software system. For example, CAD. The model can also be used to calculate volumes, measure distances  and create topographic maps. Maps can be periodically recreated to allow for comparatives to previous versions. For example determining the volume of materials dug from a quarry, by calculating the volume of the mine and comparing the results.

We use different types of photogrammetry techniques, including:

• Aerial Photogrammetry: Using photographs taken from drones to capture areas of interest from above,
• Terrestrial Photogrammetry: Utilizing ground-based photographs, typically for surveying buildings, monuments, or other structures.

Photogrammetry has become increasingly advanced and sophisticated with the advance higher resolution cameras, machine learning, and LiDAR (Light Detection and Ranging), which enhance the accuracy and efficiency of the process. These models can assist various industries by providing precise and detailed spatial information for analysis, planning, measuring progress or providing historical perspectives.