Pin-Hole Technologies, UK
The purpose of the survey is purely to assess the encroachment of any building/construction work on the gas pipes. Therefore, the images will not be used for any purpose other than visual analysis, so no detailed measurements will be made. The images do not have to be georectified (this is the process by which the image is processed to have x and y co-ordinates); consequently several amendments may be made to the survey criteria
The purpose of this survey is purely to assess the encroachment of any buildings/construction work on the gas pipes. The criterion for this project includes capturing vertical seamless geo-referenced aerial imagery of 1700km of gas pipeline at = 50cm GSD. The photo-centres for each image captured are to be plotted and mapped in relation to the pipeline. All images are to be annotated displaying their unique ID. This project has proven Pin-Hole Technologies experience and ability with corridor surveys, Pin-Hole were heavily involved with issues associated with flight planning, image capture and logistics, and were responsible for all processing of the images and GIS related work. The following is an overview of the considerations that the company had to take into account, and the issues were overcome to ensure that the project was completed effectively.
Aerial Survey Planning
The purpose of this survey is purely to assess the encroachment of any buildings/construction work on the gas pipes. The images will not be used for any purpose other than visual analysis, so no detailed measurements will be made. This means that the images do not have to be georectified (this is the process by which the image is processed to have x and y co-ordinates), consequently several amendments may be made to the survey criteria. The amount of overlap in the images may be reduced, although this is not a particularly good idea, as in keeping this at 20 – 30% overlap results in seamless imagery. However, in a survey that involves georectification, when the area of interest turn at an angle of 5° or more, the plane should perform a complete U turn, in order to ensure that all of the imagery is captured at an angle as close to nadir as possible. This is not the case in this scenario, as the imagery did not need to be georectified, so we were able to capture changes in direction of the pipelines of greater than 5° and still capture imagery that was suitable for the purpose of assessment. This section of the paper will by-pass the regular considerations of weather and airspace restrictions and concentrate on the problems directly related to a single flight-line corridor survey.
criteria for imaging
As imagery is not being used for photogrammetry or geo-rectification; constraints of 5° roll and <3° pitch do not need to be adhered to for this particular project. This allows the pilot to make low degree turns of up to 5° between photo-centres without significant implications to the imagery. In the situation where the bend in the pipeline exceeds 5° a full turn has to be made as altitude decreases and affects the scale of the photography significantly. A full turn as opposed to a regular survey-U or S turn allows the aircraft to line up for the next section of pipeline. If the aircraft exceeds 5° and image capture continues then the distance of the photo-centre from its target location will to far from nadir for successful image capture. This is before inaccuracies of the dGPS are taken into account. The imagery also becomes high angle oblique due to aircraft roll.
For this type of survey other twin-engine aircraft used for larger area block aerial surveys would find it difficult to manoeuvre to this degree at such a slow cruise. The Partenevia 68B lends itself to this type of survey as is provides enough room for pilot, navigator and camera operator along with equipment in relative comfort for medium size surveys with a range up to 1700km. The Partenevia 68B has the stability to operate well as a survey aircraft. For this corridor style survey the Partenevia has proved cost effective in comparison to using a helicopter. Altitude during this project will vary from 2500 to 4000 ft depending on flying restrictions within the area. The Partenevia has a service ceiling of 20,000ft. For this particular survey the Partenevia 68B is choice over the Single engine Cessna 205 or 206 as they have a range with standard fuel and reserves of around 870km. There is one single engine, fixed wing aircraft that has the technical capability to be used as an aerial survey vehicle to similar economy, this being the Pulsar XP. The Pulsar has a max range of 1400 miles (2240Km), and max speed of 165mph, but has very limited cabin room for equipment. Once converted for survey use, the co-pilot would have to act as the camera operator, and the equipment would have to be set up in the luggage compartment.