What is a Land Surveyor?

Paul Shaw, GeoVision

Many readers will be familiar with the traditional image of a surveyor measuring the land with a theodolite from which maps are produced for engineers and architects. In fact the profession has gone through a revolution in the last 20 years. A land surveyor is now called a Geospatial or Geomatics Engineer and the profession encompasses not only survey and levelling but many other skills. These include Remote Sensing – the use of lasers, radar, cameras etc to capture geographic information remotely; Geographic Information Systems – the processing and management of geographic information; 3D visualization – the presentation of geographic information in 3 dimensions; Hydrography, measurement of sea depths and sea bed details. This editorial will focus on four Remote Sensing technologies that are particularly relevant to readers of this magazine. These are LIDAR, Synthetic Aperture Radar, High Resolution Satellites and Hyper-spectral sensors.

LIDAR or light detection and ranging, is a ground or air based laser scanning technology that measures a reflection to just about anything, including particles in the air as well as to sea beds and to land. It sends out a swathe of laser light, the intensity of points and accuracy dependent on the distance measured, but accuracies in the centimetre range are possible. Air based LIDAR has been used to map the ground and sea bed to help assess the impact of floods, for route planning and monitoring of road and railway embankments. Ground based LIDAR can measure pollutants like carbon monoxide, ozone, and benzene - the transmitted wavelength is adapted to the properties of the pollutants of interest. The time of the received signals gives an indication of the presence and quantity of the pollutant. LIDAR is also used to get wind speed and direction at different altitudes up to 35 km above the ground. LIDAR measures velocity by determining the apparent change of frequency of the return signal from particles carried by the wind. Close range laser scanning is used to map complex 3d areas like buildings, accident scenes or oil-rigs that would normally be difficult to model. The points are converted into planes or simply transferred into a software package like AutoCAD to enable design or as-built measurements. (See example left)

Synthetic aperture radar or SAR is an air or space borne system that uses radar to create imagery and heights. Whilst LIDAR is ground based or used in helicopters and turbo-props, airborne SAR uses jet planes or satellites. This means very large areas can be covered quickly. The airborne system gives accuracies of up to 0.5m, satellite SAR’s around 25m. Another advantage of SAR over LIDAR is its ability to penetrate clouds and rain, haze or smoke. Because it is not weather dependent it is ideal as a monitoring tool, especially where regular comparisons are required. SAR is used for monitoring geo-thermal activities like volcanoes and monitoring the effects of urban and agricultural development on sensitive ecosystems such as the Rhine River valley. Analysis of the phase difference of SAR imagery taken to the same area at different times with the same radar has shown that very small changes in the earth’s surface can be detected. This technique, known as Interferometric Synthetic Aperture Radar or InSAR, gives centimetre range accuracy. It can be used to monitor subsidence caused by mineral or water abstraction, earthquakes, landslides etc. For instance, oil and gas companies could use this technology to detect ground movement caused through oil or gas abstraction. (See example right)

High resolution satellite imagery has a ground resolution of up to 0.6m and multi-spectral capability, normally at 4 different wave bands. It is possible to angle the sensor as it passes over an area to get stereo images. This allows digital terrain models to be generated up to 2m height accuracy. The sensors cover the same ground regularly, one at almost daily intervals. The combination of high resolution imagery and unrestricted access has many practical applications. Buildings, roads, tracks, foliage, even vehicles can be clearly seen. The imagery could be used for monitoring land changes, flooding etc and mapping up to 1.5000 scale. The imagery detail can be digitised to create a vector layer. It is possible to view any imagery in 3D from different perspectives as long as you have a digital terrain model. Once a 3D model is available it is possible to create fly-throughs. The route of the pipeline, road or railway can be selected and a series of images generated in movie format. The elevation above the ground, speed and direction are all variables. The fly-through is useful for route analysis as well as presentations to the public to show environmental impact. The design can be super-imposed on the imagery to make the route seem as realistic as possible. SUMED in Egypt have used high resolution imagery to map their pipelines. (See example left)

Hyper-spectral sensors have multi-wavelength capability. All elements have their own absorption characteristics - knowing these wavelengths helps identify the element. CASI, or Compact Airborne Spectrographic Imager is an example of such a sensor. The system can measure up to 288 wavebands covering the visible and near infra-red regions of the spectrum. Spatial resolution can be up to one metre dependent on the flying altitude. It has water penetration capability. It is possible to detect suspended sediment plumes caused through coastal erosion – estimates can then be made of changes in coastal morphology. You can select a wavelength to identify different vegetation types, oil flows and pollution sources like outflows. Monitoring of coral has been of particular relevance over the last few years, both due to coral disease and global warming. (See example right)

Whilst the above 4 remote sensing technologies are very useful as stand-alone systems it is possible to add value by combining these different technologies. The restrictions of one may be overcome by using another. For instance one could merge High Resolution satellite imagery with LIDAR heights to get 0.6m horizontal accuracy and 0.1m vertical accuracy. Another example relevant to the oil and gas industry combines an inertial navigation system with magnetic flux leakage. This system, called an Inertial Mapping Unit (IMU), is used for pipeline monitoring. The IMU can measure the position of pipeline assets like valves, welds, supports and defects like corrosion, dents and metal loss within and attached to the pipe. This system was used by the Egyptian Natural Gas company (GASCO) to map their pipeline network. The IMU travels 14km along the pipe in one hour so it is possible to cover many kilometres in one launch. The value added is not only accurate position of these defects and assets but also that a maintenance crew can drive straight to the defect using GPS to carry out repairs. (See example above)

This editorial has highlighted some of the available remote sensing technologies and their practical applications. There are many others and new ones are being introduced all the time. Many of these technologies are global in nature and not subject to military restrictions. They are also cost effective. This is good news for the countries with little existing mapping. In fact there is such a wide range of technologies and technology combinations now that the land surveyor’s true skill lies in selecting the appropriate system for a given task or problem. The next time you think of a land surveyor you will hopefully realize there is a lot more to his skills than just measuring angles and distances with a theodolite.