Sort out the parameters such as overlap, sensor size and focal length in oblique photography.

? With the rapid development of drones, the tilt photography industry has ushered in a new wave, and more and more people use drones to engage in data collection in the surveying and mapping industry. In the process of data acquisition, various problems are encountered, which leads to the flight data not reaching the standard and the model reconstruction cannot be completed. Here, according to my own contact, I briefly sort out the parameters such as overlap, sensor, focal length, flight speed, photo interval and their relationships in the process of oblique shooting. Please correct me if there is anything wrong or wrong.

Much of this article is quoted from "Why is 3D reconstruction so bad? In Smart3D series 2. -Explore those neglected photo requirements and skills. "

Many 3D model reconstruction softwares need to input camera sensor parameters and focal length. Usually, the input is the long dimension of the sensor.

The sensor is a fixed parameter of the camera and has nothing to do with the drone. Although many UAVs have their own cameras, the sensor parameters are only related to the cameras. So as long as you know the camera model, you can know the sensor size. Many friends don't know how to get this parameter value, so here is the search method directly.

2. 1. 1 General situation:

Take Sony ar7 camera as an example.

Direct Baidu search Sony ar7,

Click to enter.

It can be seen here that the long side of the sensor is 35.8 mm.

2. 1.2 special case

There are also some camera manufacturers who do not explicitly give the sensor size, but generally give the type and diagonal size, which can be converted here.

Take the DJI drone Genie 3 as an example.

Baidu elf 3

For the sensor, even if the size is not clearly given, it is generally the conventional size of the camera. Here is a picture from Baidu.

It can be seen here that the long dimension of the sensor of 1/2.3 is 6.16mm.

2.2 focal length

I don't want to say more about this. I got drunk without knowing the focal length. . .

However, generally, the film without special treatment has saved the focal length parameters. You can directly right-click the image to view the properties, in which the focal length is recorded in mm in detail.

Note that the focal length parameter is 35mm equivalent focal length. Please select a value corresponding to the focal length attribute.

How to ensure overlap in aerial photography? What should the overlap be?

According to different aerial users, the degree of overlap is different. If it is only for fast image mosaic, the horizontal overlap is generally above 60%, and the routing (vertical) overlap is above 70%. If it is used for 3D reconstruction, it is suggested that the horizontal overlap is above 70% and the route (vertical) overlap is above 80%. The above values are empirical, unofficial and for reference only.

How to ensure the coincidence degree? Many flight control softwares are automatic, and the route can be automatically planned only by inputting camera parameters, flying height and overlap. So how did these routes come from? How to design the route by yourself and how to ensure that the overlap meets the requirements?

Here is actually the pinhole imaging principle learned in junior high school. Assume that the camera is perpendicular to the long-side flight direction and the route spacing is X. The schematic diagram is as follows:

According to the above conditions, the lateral overlap is calculated by the length of the sensor here.

d/ccd = h/len

The overlapping length is = x+(len/2-x) * 2 = len-x.

Horizontal overlap = (len-x)/len= 1-x/len

It shouldn't be difficult to solve this equation, should it? X= lateral overlap *h/d*ccd

It is the same along the flight direction except for the short side size of the sensor.

The above process calculates the camera distance, lateral distance a and heading distance b.

What speed should it really fly? What is the photo interval? How does automatic planning software control taking photos?

In general, in order to speed up the flight and shorten the operation time, the camera has the fastest speed on the premise of ensuring the stability of the camera quality.

B= flight speed * photo interval

All the above calculations are based on the specified flying altitude. How to determine the flying height? This is related to the accuracy of the model. What kind of precision model is needed and what camera is used determines the most suitable altitude for flying.

The first is the determination of photo resolution.

What's the resolution of this photo?

The same principle is pinhole imaging, and the long-side dimension ccd of the sensor corresponds to the long-side pixel number wx pixel of the photo.

d/ccd = h/len

Shooting resolution (m/ pixel) =ccd/d*h/w

Formula: The accuracy of oblique photography model = three times the resolution of orthographic projection of the same project.

What is the model accuracy in oblique photography and the resolution/scale in surveying and mapping, so as to obtain more contents related to resolution and scale?

In oblique photography, it is often said that my model is accurate to several centimeters, and the data I fly is a model with 5cm accuracy. How is this 5cm measured?

The model accuracy of oblique photography is generally three times the resolution of photos and three times the ground resolution of orthophoto images generated by photos. If the resolution of the generated orthographic image is 2cm/ pixel, the model accuracy is basically 5- 10cm.

Formula: The accuracy of oblique photography model = three times the resolution of orthographic projection of the same project.

The above contents are based on the software I have come into contact with in my work and my own experience in the company's software development process, and I have no attitude of praise or criticism for each software and related references.

Welcome more discussion.