Kowai River photo survey

Kowai_wideViewMappazzo recently completed an aerial photo survey study for the Selwyn District Council. Data generated during the mission provide useful data for planning of civil works and ongoing management of the Malvern Hills water race intake on the Kowai River.

The Selwyn District Council was in the process of design for modifications to a section of their water race along the Kowai River near Springfield in Canterbury. After completing a traditional ground survey they were interested in running a trial UAV (Photo Survey) to examine additional details, obtain a high resolution orthophoto and provide a real life comparison of drone based photogrammetry techniques to ground based survey.

Mappazzo used Clarka to undertake a low altitude flight over the area and used the existing survey to georeference the orthophoto and 3D photo survey model. An optimum flight path was identified and programmed prior to the flight (Figure 1) so that Clarka could fly the required course and collect images autonomously. This approach allowed the Mappazzo’s team to maintain a focus on monitoring flight progress and responding to site conditions, and provides optimum efficiency and coverage for the flight.

Clarka covered the surveyed area (approx. 5ha) in a single 10 minute flight capturing a total of 121 images for use in the photo survey. The efficiency of the flight meant that loads of battery capacity remained and the team used some of this to capture video footage of the river and demonstrate some other capabilities.

The resulting data was processed by two different methods to examine the different products that could be generated (at different processing cost). Brief descriptions of the products generated for this project are below.

Orthophoto and Elevation Model

To obtain a georeferenced orthophoto Mappazzo offers a low cost alternative to traditional processing, harnessing the power of “the cloud” to provide an entry level product suitable for most environmental and planning applications. The cost of processing by this method is absorbed into our fees and included in the price of flights.

The orthophoto and elevation model can be hosted online for easy viewing and is available in electronic form for use in GIS or CAD applications. A little additional processing of this data can easily produce contours or GIS features. This product was prepared for the Kowai site to provide an example of base level processing. The low flight altitude and availability of ground control (survey) points resulted in a very high quality product by this method, with pixel resolution of 1.6cm and very good alignment to surveyed features.

An overlay of the traditional survey and the orthophoto (Figure 2) illustrates how well this product can align to known features when processed with ground control.

The output of this initial processing can be viewed online through the following links:

Detailed photo survey processing

More intensive (local) processing was undertaken by Mappazzo to illustrate the full capability of the photo survey method and provide the full range of data outputs available from the survey flight.

The raw output from this processing is an extensive dataset of photo survey “points” (referred to as a “point cloud) that provide a full 3D reconstruction of the site. The point cloud is then processed further to characterise features (ground, vegetation, buildings etc) and create more accurate models of the ground surface and structures.

The point cloud was loaded into 3D design software, used to create a 3D surface model and compared against key cross-sections obtained by the traditional survey method. The results of this comparison proved to be very good with only minor variances between the ground survey and photo survey. View the image via the link on the left to see a cross-section through one of the key earthworks alignments.

There were obvious benefits of obtaining the photo survey for the site as it provided a complete record of the site with wider more detailed coverage of the terrain and major features than the traditional survey method. The orthophoto and 3D models will be used by Selwyn Council and their consultants to identify additional design options and ground features offsite. Noted constraints of the photo survey were the inability to target “covered” objects (such as drain inverts and features covered by dense tree canopy) and the need for ground survey of small objects.

For more information about this project or to enquire about obtaining an orthophoto or photo survey for your site please contact us.