What Is Digital Photogrammetry?

Digital photogrammetry refers to a measurement method based on the basic principles of photogrammetry, using computer technology to extract the geometric and physical information expressed by a digital object. ^[1]

The development of digital photogrammetry originates from the practice of photogrammetry automation, that is, the use of related technologies to achieve true automatic mapping. Photogrammetry automation is the ideal pursued by photogrammetry workers for many years. The earliest studies involving photogrammetric automation date back to 1930, but have not been implemented. Until 1950, the first automated photogrammetric mapping device was developed by the American Engineering Corps Research and Development Laboratory in cooperation with Bausch and Lomb Optical Instrument Company. At that time, changes in the grayscale on the image were converted into electrical signals, which were automated using electronic technology. This effort has been developed over many years and has been implemented on optical projection, mechanical or analytical instruments, such as B8-Stereomat, Topocart, etc. There are also some automatic photogrammetry systems that specifically use CRT scanning, such as UNAMACE, GPM systems. At the same time, photogrammetric workers are also trying to convert the electrical signal converted from the grayscale of the image into a digital signal (that is, a digital image), and then an electronic computer implements the automated process of photogrammetry. In the United States in the early 1960s, the DAMC system developed successfully was such an all-digital automatic mapping system. It uses the STK-1 precision stereo coordinate system produced by Swiss Wild Company to digitize the image, and then uses an IBM7094 electronic computer to realize photogrammetric automation. Professor Wang Zhizhuo of Wuhan University of Surveying and Mapping Technology put forward the idea and plan for the development of a fully digital automated mapping system in 1978, and completed the fully digital automated mapping software system WUDAMS in 1985. The photogrammetry automation was also implemented digitally. Therefore, digital photogrammetry is an inevitable product of photogrammetry automation.

With the development of computer technology and its applications and

There are two main views on the definition of digital photogrammetry in the world.

One is that digital photogrammetry is based on the basic principles of digital imaging and photogrammetry.

1. Computer Aided Mapping

Computer aided mapping

The process of digital photogrammetric processing generally includes the following 6 steps.

Step 1: Digitalize the image

The purpose of image digitization conversion is to digitize aerial or aerospace photographic images into digital images, and express them as a two-dimensional pixel gray matrix.

Step 2: Orientation of the digital image

Digital image orientation includes internal orientation, relative orientation, and absolute orientation of the entire digital image to determine related parameters.

Orientation: Determine the relationship between the scanning coordinate system and the image plane coordinate system.

Relative orientation: The image coordinates are used to automatically determine the image coordinates of the relative orientation points in the stereo digital image, and the relative orientation parameters are calculated using analytical photogrammetry relative orientation.

Absolute orientation: Use known image coordinates and internal orientation parameters to calculate the coordinates of control points in one digital image, and use image matching algorithms to automatically determine their coordinates in another digital image.

Step 3: Digital Image Processing

Digital image processing includes arranging and transforming the digital image pixels according to the scanning coordinate system to the epipolar direction, and enhancing and extracting features of the image.

Step 4: Building a Digital Ground Model

Establishing a digital ground model includes one-dimensional image matching along the epipolar line, calculating model coordinates of the points, and establishing a digital ground model with image gray values.

Step 5: Generate Digital Contours

According to the regular grid DEM, a certain interpolation algorithm is used to generate digital contour lines. The specific process is to first track discrete points of the contours at a predetermined contour interval in the DEM, and then smoothly encrypt to form digital contour data.

Step 6: Generate Ortho Digital Images

Ortho digital image: The original digital image is corrected into an ortho image using digital ortho-projection (digital differential correction) technology. If digital contour lines are combined with digital orthoimages, ortho-digital images with contours are generated. ^[4]