When MWH Geo-Surveys International was employed to conduct a micro-gravity survey of an Omani mine, the company’s senseFly eBee drone was the only solution its team trusted to produce the elevation model the client required. (Read/download the full project story.)
According to Kevin MacNabb, a founding partner and owner of MWH Geo-Surveys, the conventional options for obtaining a high-resolution digital elevation model (DEM) of the site were limited.
“The public SRTM DEM that is currently available for the Al Ram site, with its resolution of 90 m, was highly inaccurate, particularly within the altered mine landscape,” he says. “Alternatively, creating a DEM using reflector-less laser sightings would have been very time consuming, labour intensive and subject to laser errors. A manned aerial photogrammetric survey or a satellite derived elevation model were also potential alternatives, but these would have cost well beyond the project’s budget.”
Creating a DEM using reflector-less laser sightings would have been very time consuming, labour intensive and subject to laser errors
As for existing ground survey data, MacNabb says that, to his knowledge, the site has never been surveyed using traditional instruments.“The mining operation involves tracing and excavating chromite veins, so a site survey isn’t a necessity,” he notes.
Thus MacNabb turned to MWH’s senseFly eBee drone, which the company purchased for exactly this type of data collection. “The drone allows us to create cost-effective, high-resolution DEMs and orthophotos, in support of geophysical surveys like that at Al Ram, solving a technical problem very effectively,” he says.
To create the site DEM required, MacNabb’s team flew 11 eBee mapping flights over three days, aerially surveying an area of 6.5 square kilometres. “The topography ranged from 590 to 780 metres above sea level, plus we had gusty winds of 8 metres per second and daytime temperatures of over 38º C were normal,” MacNabb explains. “The drone worked effectively throughout.”
These flights were followed by several days of data processing in the office. The team used Postflight Terra 3D to transform the project’s 2,398 images into the required DEM. This elevation model was then orthorectified by identifying pre-surveyed ground control targets inside Postflight Terra 3D’s rayCloud Editor.
The DEM that MWH Geo-Surveys generated was determined to have an average absolute vertical accuracy of 10 cm; calculated by comparing the eBee’s data to more than 10,000 surveyed RTK GNSS positions, recorded at the gravity survey’s sites.
“An effective micro-gravity survey at such a rugged and highly irregular site wouldn’t have been possible without an extraordinarily accurate elevation model,” MacNabb says. “With the drone’s model, we were able to compute terrain corrections to the gravity field with an unprecedented level of accuracy. This in turn allowed our subsequent gravity model to map subsurface changes much more precisely, including those caused by small, thin chromite veins.”
With the drone’s model, we were able to compute terrain corrections to the gravity field with an unprecedented level of accuracy