In this article, we would like to continue our publications on the possibility of using the DJI PHANTOM 4 PRO additionally upgraded with accurate GNSS receiver for surveying and mapping, as an example of a practical project for aerial survey of a quarry near Tortosa city in Spain.
I would like to note at once that all the processes of aerial photography and data processing were carried out jointly with local representative responsible for surveying and volume calculating for this quarry.
It didn’t take a lot of time to prepare our drone to work after transportation. It was enough to get a quadrocopter from the transport case, install a removable GNSS antenna and turn on the power of the drone (Fig. 1). Installed on the point with known coordinates Topcon GR3 GNSS receiver was used as a base station.
The flight mission was planned at an altitude of 120 meters from the bottom of the quarry with 80% overlap of images. The total area of the survey was 2 hectares. The relief of the quarry is cut enough, administrative buildings and equipment are placed on the territory. On the slopes and peaks of the mountains there are a lot of trees and shrubs (Pic. 2).
The whole process of aerial photography took about 15 minutes.
Preliminary processing was carried out on the-site using a field notebook. We carried out Post-processing of GNSS measurements and photogrammetric processing with using the coordinates of the centers of images determined with a centimeter accuracy by the on-board GNSS receiver (Pic. 3). As a result points cloud (Pic. 4, 5,6) orthomosaic (Pic. 9 ). and DTM were created (Pic.8)
To assess the quality of data we used ground control points provided by the customer. The errors in plan and height were up to 8 cm and 15 cm, respectively. After focal distance calibrating with using at least one of the reference points, the accuracy in height was improved up to 8 cm.
Finally digital terrain model was created by automatic classification of the points cloud (Pic 7,8)
It should be noted that the resulting 3D model with 5 cm resolution corresponds to LIDAR data, and it is possible to classify vegetation, buildings, structures and other technological objects and create a detailed and accurate 3D terrain model.
The results of this projects fully meet the requirements of the customer and showed, on a practical example, the following advantages of unmanned aerial survey technology before using traditional methods:
— high accuracy and details
— high efficiency and productivity
— high percentage of automation of the processing
— low cost
— short terms of project implementation