USING UNMANNED AERIAL VEHICLE (DRONE/FLYCAM) TECHNOLOGY IN SURVEY WORK OF PORTCOAST 1. Capturing aerial images by using Drone Taking images by drones is currently applied in many fields especially in topographic survey and construction supervision. The advantages of drone imagery are much more than those of traditional aerial image taking technology. Drones can fly at many different attitudes. Taking photo from under 300 meter attitude will get aerial images with high resolution (from 12cm to 3cm per pixel). Photos being taken by drone are not covered by cloud. Satellite images do not have this advantage. Drone image processing algorithm is always developed and updated supporting orthomosaic image generation with high quality (up to 5cm per pixel) conforming to topographic mapping standards 2. DJI Phantom 4 DJI Phantom 4 is one of newest drone products being recently introduced by DJI. Phantom 4 is integrated with a 20 megapixel camera and satellite positioning systems (GPS and GLONASS) increasing spatial resolution and accuracy of aerial photos. Figure: DJI Phantom 4 DJI Phantom 4 takes off vertically and is suitable for taking off at complicated terrain. DJI Phantom 4 velocity is about 15 m/s improving productivity of work. Each device can survey about 300 ha per day in good weather conditions. 3. Commmercial softwares There are many drone photo processing software programs. Each software has its own advantages. Typical software programs are Pix4D from the US, Agisoft Photoscan from Russia and 3DSurvey from Slovenkia. In addition, there is a software named Drone Deploy supporting online and real time orthomosaic image during taking photo process.
Table: Typical drone photo processing softwares No Software Function Advantages Disadvantages 1 Pix4D Orthomosaic image 3D model Elevation model Multi spectrum processing Measuring and computing Fly through animation video exporting Easy to use Variety camera model support Good technical support High price Water surface is hard to process and creating orthomosaic image 2 Agisoft Photoscan Orthomosaic image 3D model Elevation model Multi spectrum processing Measuring and computing Variety camera model support Good coordinates system support Orthomosaic image algorithm is good to process water surface. High prices Bad technical support Non professional technical support Without updates 3 Drone Deploy Orthomosaic image 3D model Elevation model Multi spectrum processing Measuring and computing 4 3D Survey Orthomosaic image 3D model Elevation model Multi spectrum processing Measuring and computing Easy to use Good technical support Real time process Simple interface Variety camera model support Low price Limited customize option Slow processing speed Hard to use Slow processing speed
4. Introducing Pix4D Pix4D is recently one of advanced Drone image processing software. Pix4D provides two main functions which are orthomosaic image and digital elevation model with high accuracy. In addition, Pix4D is able to generate visualized 3D models and render fly through animation videos which are very suitable in inspecting and construction supervising at site. Pix4D software is currently used by many organizations and companies specializing in land surveying, construction in order to improve productivity and word effect. Figure: Pix4D software interface 5. Using drone and Pix4D 5.1. Input data Drone mapping requires 3 main input data groups. Drone images Ground control points for geo referencing Topographic maps for quality check Plan images are photos being taken by drone vertically. The number of drone images can vary from dozen to thousand photos depending on the scope and size of projects. Ground control points are collected by using GPS receivers. Topographic maps are established by using RTK or total station method. Topographic maps must have suitable measured point density with high accuracy in order to ensure high quality of orthomosaic images.
Figure: Drone images Figure: Using a GPS receiver to collect the ground control point Figure: Mapping data for checking
5.2. Capturing images by using drone Photo taking by drones can be deployed manually or automatically. Before the drone is deployed to take images, the boundary of the project must be defined in order to establish fly lines. Boundary of fly lines must cover the whole project area. In series of photos taken by drones need a specific overlap rate. Overlap shooting rate is range from 50% to 85% depending on the scopes and accuracy requirement of the project. The high overlap rate gives better results with high accuracy but it causes large input data set and takes more time for processing. 5.3. Establishing ground control points Horizontal and vertical control points are established by static GPS measuring method. Ground control points should be placed evenly in the project area. Ground control points are marked by ground target (white papers with a dark circle in the center). When deploying drones, ground targets will be displayed on photos serving geo referencing and improving accuracy of the result. 5.4. Image processing with Pix4D Drone mapper processing has 3 main steps: Step 1: Initial process When drone image data set is imported to Pix4D, the software will display geolocation of each photo. Pix4D calculates point cloud from the data set which are necessary for 2D and 3D. Figure: Input data set Step 2: Using Ground Control Points After the initial process has been done, horizontal and vertical control points are imported to the software. GCPs are marked on correspondence images for geo referencing and optimizing point cloud.
Figure: Importing horizontal and vertical control points Step 3: Orthomosaic and 3D model After geo referencing and optimizing steps, the software will do the rest of the process. The software interpolates Mesh from point cloud and then generates orthormosaic images or 3D model. Figure: The result of orthomosaic image Figure: Virtual model (3D) of Ca Na in Ninh Thuan Province
Figure: Virtual Model (3D) of a port project in Kampot, Campodia A topographic map is overlaid over the orthomosaic image to examine the accuracy. 0.30m Figure: The horizontal difference between the orthomosaic image and the result of total station method (Ca Na port project) 0.11m
Figure: The horizontal difference between the orthomosaic image and the result of total station method (Rach Gia port, Kien Giang province) 0.20m Figure: The horizontal difference between the orthomosaic image and the result of RTK method (Kampot port project, Campodia) 6. Conclusion Using drone technology and processing software provides a lot of advantages, improves productivity and optimizes land survey work. Advantages of drone mapping method include: Saving work labor and time. Improving efficiency and quality of land survey work. Orthomosaic images have high spatial resolution and accuracy. Drone image acquisition does not depend on satellite ephemerides. Meeting the need in many fields.