Seemless Transition

Southern Resources Mapping Corp. (SRMC) has provided aerial photography and derived digital terrain models of the Red Hills Mine in Mississippi since 2003. Since then, they have expanded aerial mapping into every phase of coal mining by upgrading to digital imaging.

By Kevin P. Corbley

North American Coal Corp. opened the Red Hills Mine near Ackerman, Mississippi, in 1998 through its subsidiary Mississippi Lignite Mining Co. The mine now produces 3.5 million tons of lignite coal each year. According to SRMC’s president, John Matthews, who holds a degree in Mining Engineering, North American Coal and its affiliates are the most sophisticated users of aerial mapping he’s ever seen in the coal mining business.

“What Red Hills Mine is doing with mapping is really unique,” Matthews said.

For the past nine years, SRMC has flown the entire 5,000-acre surface mine site every four to six weeks and now also captures lower-altitude photos over its coal stockpiles on a monthly schedule. The frequency of mapping at the mine is a necessity to keep up with the rapid pace of mining activities and the constant reporting requirements of federal, state and local agencies, most notably the Mississippi Department of Environmental Quality.

“Digital Terrain Models (DTMs) are particularly critical to their operations,” said Matthews. “We usually provide them with orthoimages and DTMs sufficient to create five-foot contours over the entire mine, two-foot contours over the high wall [excavation boundary], and one-foot contours of the stockpiles.”
Mississippi Lignite Mining moves an average of 10 acres of land each month to extract the lignite, which lies below the surface in six discrete seams varying in thickness from two to six feet. The first phase of extraction involves removing the soil overburden with a 40-cubic yard electric shovel to expose the top coal layer. From there, successive seams of coal and inter-burden are excavated either with bulldozers or a dragline, depending on the depth of the material.

No ground is disturbed until an excavation plan is carefully designed, explained Travis Johnson, Red Hills senior mining engineer. The orthoimagery provided by SRMC is used to plan pit layouts, and the accompanying DTMs are input into the Carlson Mining Software application to run cut-and-fill calculations, which determine the precise volume of soil that must be removed. The software is used to generate an excavation design.

“Putting our designs on a blank piece of paper can be hard to comprehend… so we overlay our designs on top of the orthos to show our operators where they will be working,” said Johnson.

An important aspect of modern surface mining is that reclamation progresses just as fast as excavation. As soil is separated from the lignite layers on the front end of the operation, 200-ton dump trucks are carrying these materials to the previously mined pit. The trucks deposit their loads so that bulldozers can spread the soil out and gradually build the surface back up to its approximate original contour in compliance with environmental reclamation requirements.

“We have a topographic grid [derived from pre-mining orthophotography] that shows us what our final grade is going to be,” said Johnson. “Every time we receive new DTMs from Southern Resources, we calculate the difference between the two and determine the volumes required to fill an area to final grade.”

On both sides of the mine, the engineering team makes its cut-and-fill calculations with the DTMs to determine how much ground has to be excavated or replaced. Engineers wirelessly transmit those data points as design files to the electric shovel, dragline, and bulldozers in the field. Each is equipped with an onboard computer that runs an application called Carlson Grade that uses GPS to precisely guide the Earth movers as they build up or remove soil in accordance with their daily assignment.

“The accuracy of the DTM is important,” said Johnson, especially on the reclamation side because the terrain elevation measurements from the final grade are used to generate reports to the regulatory agencies.

The use of aerial mapping information doesn’t end there. As the coal is extracted, it is loaded onto the dump trucks for a short trip to a hopper or a coal stockpile. After the coal is dumped into the hopper, the coal runs through a crusher to reduce it to the correct size. Once the coal is reduced to the correct size, a conveyor carries the coal to a neighboring power plant where it is burned to generate electricity. Near the end of each month, the mapping company images the stockpiles at low altitude, generates DTMs for one-foot contours, and calculates the volume of lignite. Mississippi Lignite Mine uses this information to assess its inventory and determine royalty payments to land owners.
 

Making the Transition


Until early this year, Southern Resources had acquired aerial photography for Red Hills Mine using a Zeiss RMK-A 15/23 film camera. But in late 2011, the mapping firm decided the time had come to add digital imaging to its service offerings. SRMC saw the digital camera as a way to expand its existing business into new markets as well as satisfy current clients who increasingly were requesting digital collection.

Taking into account key considerations such as price, size, and weight, the firm focused on medium-format digital cameras in its purchase decision. But even with the search narrowed to just those systems, SRMC had to carefully evaluate the available medium-format products to ensure the firm would be able to continue offering its traditional line of mapping products, especially DTMs.

“People often don’t think of medium-format sensors as topographic mapping cameras,” said Matthews. “Due to the narrower footprint of the medium format, some of these cameras don’t have a base-to-height ratio sufficient for extraction of accurate elevation points in stereo pairs.”

SRMC ultimately purchased the Leica RCD30, which is a 60-megapixel medium-format camera offering multispectral imaging capabilities in the red, green, blue and near-infrared portions of the spectrum. Most importantly, the digital camera is marketed as a photogrammetric mapping system. Matthews looked forward to getting the new camera in the air to confirm that it could deliver topographic map products of the same quality as film.

He didn’t have to wait long. In early 2012, a mechanical problem grounded the aircraft SRMC used to fly its film camera, which prompted the firm to dispatch the newly installed digital system for a regularly scheduled Red Hills collect. The coal mine operator would be among the first to receive deliverables from the firm’s RCD30. Southern Resources was pleased to find the medium-format system easily met—and likely could have exceeded—its expectations for the DTMs.

“The only operational change we made [versus film] was to increase our forward overlap from 60 to 80 percent and side lap from 20 to 30 percent,” said Matthews. “This gives us enough separation between image centers to achieve the base-height ratio with sufficient vertical exaggeration for stereo-extraction of elevation points.”

During these missions, SRMC realized the forward motion compensation technology of the RCD30 enabled them to fly lower without blurring pixels than was possible with film. Matthews is confident the firm could have delivered DTMs capable of half-foot contours, if they had been requested. Topographic mapping at that level of detail had not been possible with SRMC’s film cameras.

For its first digital acquisitions at Red Hills, the aerial firm flew at about the same altitudes always used for film collection—8,000’ for generation of five-foot contours, 3,400’ for two-foot contours and 2,500’ for one-foot products. Processed about 25 percent faster than film, all of these topographic products met SRMC’s quality control inspections. The real test, however, would be the satisfaction of the mine operator when it received the orthoimagery and DTMs.

For Mississippi Lignite Mining, the transition to the digitally derived DTMs was seamless. The digital models met the same specifications as the previous ones, and the mine operator generated contours for its various applications just as it had in the past without making any special processing accommodations. This was the result SRMC was hoping for with regard to the DTMs.

The reaction to the orthoimages was another story and in a good way. Due to the visual nature of the imagery, the superior clarity and sharpness of the digital images jumped out at the mining engineers, and they were impressed with the added information content. Mississippi Lignite Mining’s Johnson explained the more detailed orthoimages are helping the technicians in the office and operators in the field make better decisions.

“We can see details [that weren’t as clear in the film photos] on the images, such as landmarks, that can be related to the operator before they begin a project,” said Johnson. “An operator can look at a design file in the office, and when they actually get to the same landmark they referenced off the image in the office, they know it is the one where they are working. It allows operators to visualize where they will be working before they step onto a piece of equipment.”

North American Coal plans to duplicate its sophisticated applications of aerial imagery and derived topographic information at a proposed new lignite mine in Mississippi where planning is already underway. The operator has contracted with Southern Resources Mapping to perform the aerial acquisitions over the new site.

Kevin P. Corbley is a business consultant based in Denver, Colorado, and can be reached at www.corbleycommunications.com.
 

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