Monitoring of a construction site

Kassel, one of Germanys most dynamic and growing cities, is facing several large construction sites to get ahead of the demanding tasks. Especially the growing need for residential property and industrial estate in and around the cities shapes every planning process the municipality faces. With an approximated construction volume of more than 70 million euro and an area covering 136 hectares, the exploitation of the business park Langes Feld ranks high in the biggest construction projects in the area. Planning to sell the first areas close to the motorway A49, efficient time management was needed. Using Leica Geosystems Unmanned Aerial Vehicle (UAV) solution to get a clear view on the sites progress helped to stay on schedule.

Precision from the beginning

To coordinate 40 construction workers and 18 earthmoving machines on site, perfect planning and a steady stream of information is necessary to get a clear view over current tasks. A typical task of surveyors to support the construction management is the providing of precise geo-data and scrutiny of moved masses. Using the collected data, economic and environmental alternatives can be shown to decision makers to increase sustainability and efficiency.

High resolution digital cameras attached to UAVs is becoming more and more a credible alternative in surveying and mapping to traditional terrestrial methods. Using photogrammetry, for many a formerly unloved field of education, allows you to take up large areas of interest within a very short span, so it is getting more into the focus of engineers. Highly detailed point clouds, orthophotos and digital elevation models (DEMs) are possible results that can be created with Leica Geosystems UAVs. With additional software solutions, there is even the possibility to create high precise stereophonic pictures.

Airborne support

In addition to the traditional ways of surveying the construction site, an airborne long-term monitoring was done with an UAV. With the aim to support the conventional surveying services in geo data collection, particularly to control and document the moved masses, qualitative and quantitative proofing and inventory control were the main tasks done with the Leica Geosystems UAV solution. Highest accuracy, fast availability of the collected data and increased safety go hand in hand with reducing costs and surveying a site without any interruption of the ongoing workflows. Additional challenges were vegetation, machinery and working materials in the receiving area. These are also recorded by the UAV sensor technology and must be edited during the evaluation.

Autonomous flights under steady supervision

Special software was used to perform flight planning before the first flight since comparable data were generated and the flight plan did not change over the period. The user is supported by the use of map material and camera-databases for best photogrammetric flight planning. The requirements on accuracy and cost as well as the required repetitive recording of the construction field required an optimal use of the available technology and compliance with standardised workflows. Before any flight was done, ground control points (GCP) had to be measured to precisely adjust the collected data. The use of a RTK-capable GNSS receiver on the UAV for high-precision direct georeferencing of image data, for the project meant a reduction of expense to a minimum and a clear time saving. In the present case, only one GCP was used to eliminate possible residual errors in the camera calibration. The detailed preparation and the standardised workflows with the UAV ensured a small time effort for the actual flights. With only two flights, a partial area of the total construction project of 15 ha could be surveyed in a very short time. The high-resolution photos with a centimeter-accurate georeferencing were fed into Agisoft PhotoScan for evaluation. Within only three hours after the flight, the customers were able to get the results of the flights in the form of a georeferenced point cloud as well as an orthophoto for further evaluation. The point clouds were further processed with Leica Infinity office software after the first analysis of the data quality.

Reducing costs by increasing value of the information

By using Leica Geosystems UAV, it was possible to carry out a complete and accurate condition survey of the construction project every week without entering the site and to move around in the potential danger area and disturbing the ongoing processes.

In addition to a complete and 3D documentation of the construction process, the point clouds with 100 points per square metre were also used to document mass movements and to perform an accuracy check of the earthworks carried out by heavy machines. Disturbing vegetation as well as machines and material can be manually removed from the calculation within a short time. High-resolution orthophotos with a resolution up to 1 cm per pixel were used for areas, lengths and batch determinations and represent an excellent overview.

The repetitive recording at the site will take place until the end of construction. With more than 50 planned recordings or with the UAV, more than 10,000 individual photos are recorded and processed. These data alone will require terabyte data storage.

Supporting the officials with the collected data, the smooth processing and the speed of available data, the UAV proves itself as a valuable and important technique to survey the site during every construction phase.