SURVEYING

While the component technologies of airborne laser mapping or lidar (light detection and ranging) have been in existence for years, recent refinements mean the technology is more cost efficient and accurate. Lidar surveys are completed quickly and they allow areas that are environmentally sensitive or difficult to access to be surveyed remotely.

This article reports on a definitive comparison between a lidar survey and a quality control ground survey conducted independently.

In the community of Angus, 15 kilometres southwest of Barrie, Ontario, about 1,700 square kilometres of surface water drainage converges from the Nottawasaga Valley watershed. A decrease in the gradient results in low river flow velocities and a thick, stable ice cover in the winter months. These factors, in conjunction with various obstructions, natural and man-made, often result in ice jams during spring break-up in Angus. At this point the Nottawasaga River meanders through the community and converges with the Pine and Mad Rivers, and with Bear Creek.

Greenland International Consulting, a professional consulting engineering firm of Toronto and Collingwood, was retained by the Nottawasaga Conservation Authority and the township of Essa to undertake a comprehensive topographic field survey, as part of an ongoing flood remediation study.

The company completed a levelling survey in November 2001, which proved that suspected discrepancies existed throughout the old mapping.

Greenland recommended that new topographic mapping should be prepared for the study area, and Lasermap Image Plus/GPR was contracted to do the mapping using the latest generation of lidar equipment and colour photography.

A lidar system consists of a laser which measures a distance from the unit to the ground, an inertial measurement unit (IMU) for measuring the attitude of the laser in relation to vertical, and a global positioning system (GPS) for an accurate location of the airborne position. The laser uses an accurately calibrated mirror to scan side to side, directing the laser pulse to collect elevation and land cover data over a swath as the aircraft flies forward. A sensor collects the reflected laser energy from treetops, roads, buildings and the terrain. The points are then interpreted using mathematical algorithms to provide model surfaces.

The colour photography was rectified and delivered as a digital orthophoto map, along with spot heights and one metre contours developed from the lidar data.

A field verification survey was then completed by Greenland to meet the requirements of the Ontario Ministry of Natural Resources. Lasermap was not informed that any field checking was to take place.

The map tile with the largest number of easily accessible, identifiable spot elevations and contours was chosen. This map included elevations located on the bank of the Nottawasaga River, as well as in wooded areas where traditionally it is more difficult to survey. Spot and contour elevations were also checked on other mapping tiles.

The ministry’s mapping specifications state spot elevations on mapping must be within 33% of a half metre or 0.17 metres, and contours must be within 50% of a half metre or 0.25 metres of their surveyed elevations, at least 90% of the time.

As lidar is an airborne technology, the overall error budget can be expected to be within 15 centimetres in open, hard surface areas (limited primarily by the accuracy of the GPS and inertial systems). By comparison, it is reasonable to expect a properly conducted ground survey to be accurate to three centimetres. However, a lidar survey can collect data points that are as close as one metre over the whole mapping area — a quantity of data that would be totally impractical, if not impossible, to acquire using ground survey methods.

Consequently, although the absolute accuracy of the lidar data compared to field survey methods is not as precise, the overall accuracy of the lidar model of the ground is more realistic. Further, while the errors in lidar data caused by the GPS and IMU in a worst case are cumulative, they can also be somewhat self-cancelling. Greenland was pleased to find that the Ontario Ministry of Natural Resources criteria were more than met, and indeed, the average difference between all verified field surveyed spot elevations and the lidar mapping elevations was 0.07 metre. The average difference between all verified surveyed contour locations and the contours produced from the lidar was 0.15 metre.

The analysis showed that lidar mapping meets Ministry requirements for mapping used in flood remediation projects, and is a technology that could be applied to many engineering mapping tasks.

Article supplied by Lasermap Image Plus/GPR Consultants of Quebec. Jim Hartman is senior project manager with Greenland International Consulting, jhartman.greenland@bellnet.ca. Robert Fowler is vice president of sales for Lasermap Image Plus/GPR Consultants, bobf@lasermap.com. The authors acknowledge the Nottawasaga Valley Conservation Authority and Township of Essa for their support in the preparation of this article.