Measuring Volumes of Rock

In late July 2006, my wife Genevieve and I were on vacation at the Michigan Dunes when we got the call. Together we own G-1 Aerial Surveys Inc. in Sycamore, Illinois. The prospective client wanted the impossible; the 600-acre Elmer Larson Quarry near our town needed stockpile inventory data. It was a Thursday afternoon, and they wanted numbers for their site by the following Monday. I knew there was no way to deliver such figures in this small timeframe. So I decided to tell the prospective client the truth. We could possibly fl y over their site so that its stockpile status could be preserved in aerial photos, but it would take at least two to three weeks to get accurate numbers for their stockpile quantities.

Our firm did not even have the needed software to calculate such values. To facilitate this, we hired a subcontractor civil engineering firm to calculate our figures using AutoDesk Land Desktop. To rush the project, we compiled stereo model data of the stockpiles day and night.

Normally, the aerial survey map method involves collecting planimetric data first and vertical data last. But we worked the job backwards, collecting vertical data for stockpile analysis first. We fed data to our subcontractor for calculations as fast as we could collect it. It took 23 days to finish the stockpile calculations and provide the quarry with figures they could work with, the best we could do under such rush conditions. Even though we couldn't provide data in just one weekend like the client wanted, they still were happy with our work.

They informed us we would be recalculating their site six months in the future, which was last December. I wanted to use the time between July and December to find ways to speed up the stockpile calculations and site mapping process. Fortunately, we had been looking to replace our AutoCAD 2006 with AutoDesk 2007 Civil 3D. Little did I know how much of an advanced new tool it would prove.

I wanted to try two test evolutions to evaluate Civil 3D 2007 before committing to use it on a full production project. First, I wanted to use its surface building functions to recalculate our original stockpile quantities and compare them to those from our subcontractor using AutoCAD Land Desktop. I was pleased with the results. Not only did we get figures using Civil 3D 2007 within 95 percent on our first try, but we calculated them in under 12 hours. This was much better than the 37 hours it took AutoCAD Land Desktop to do this work. Then we hired a professional trainer from AutoDesk to determine why we were still five percent off the original figures. It turned out we needed to create our surface boundaries in a slightly different way, and once we did, our calculations were exactly the same as those produced by Land Desktop.

Ready for the Next Step

This was encouraging, as I now felt confident that Civil 3D 2007 was ready for production work. Not only could we depend on it for faster results but we could now do volume calculations in house and keep more of our project's profit for ourselves—a win-win situation in any company owner's eyes. Next, I wanted to test the software on one more process before we committed to use it for our December 2006 stockpile analysis.

As part of the July 2006 fly over, I included stereo coverage of not only the client's stockpiles but also their limestone pit. I proposed that they authorize additional funds so we could perform "reverse volumes" of their pit to compare to new pit numbers for the December 2006 flyover. This would allow them to use the differential of pit volumes between the two flyovers as an audit of their inventory figures for their product out the gate and product in stockpiles. Little did I know we could calculate much more than just this differential from creating pit surface volumes. Once we had produced surfaces of their pit, I could see new ways to calculate useful information from this new data set.

Elmer Larson Quarry uses rock from the pit in segments grouped by depth called lifts. Different levels of limestone from the quarry are useful for different aggregate products. With this in mind, we segmented the entire pit surface into six lift levels, each corresponding to a particular layer of rock product. With this, we could compare each lift to the level above it and produce separate lift volume figures so the client could know how much rock had been removed from each lift. Next, we created two separated surfaces for each lift, one representing all the rock as it stood on that level and the second with all the "rock in play" removed from that level. That allowed us to create a differential for each level that produced the volume of "rock in play." This allowed the client to know how much extra inventory was standing in the pit ready to be processed by their primary plant. These figures represented their inventory in process. The last figures we calculated for each lift were the reserve inventories. To do this, we project ed a surface underground for each lift to estimate how much reserve volume they could expect from each aggregate type for their projected next six months of production.

With these processes now in place, we were ready to take on our December 2006 flyover. The time requirements were still tight, but with these new tools and methods at our disposal, I was confident we could meet our commitments. The client wanted the site flown as close to December 28, 2006 as possible. They needed stock pile figures by no later than January 10. This gave us only a two-week window to perform and deliver accurate results. This was complicated even more by weather and film processing considerations. It takes on average three to five days after a job is flown to develop and scan film for use in our Summit Evolution Soft Copy Stereo Instrument system. To make matters worse, in Northern Illinois, we see an average of only three good flying days in December.

Tight Conditions

I got our quarry client to agree to a three-week window from December 7 through the 28th for flying the site. We were lucky on December 24, as our flying contractor Alterra Surveys in Wisconsin got a day of good weather. So we had film in the can on Christmas Eve, the best Christmas present I have gotten in many years! But even with the extra four days, I knew this would be a tight time line.

Fortunately, AutoDesk 2007 Civil 3D performed flawlessly. We received our scanned data flightlines on January 2, 2007. I knew it would still take a tremendous effort to meet our time requirement for stockpile data. But now that we were also calculating our own stockpile volumes, I could push myself and my staff into an around-the-clock effort.

To add to the pressure, the original July 2006 project contained only 50 stockpile areas, and now the site contained 61 stockpiles. Our efforts bore fruit, and we met our time requirement, as our client had 60 of the 61 stockpile volumes in their hands by January 10, 2007.

Within the next few days, not only did we provide the missing stockpile, but we added an additional five dozen "waste piles" to help make up the difference of missing aggregate across the site. These additional calculations would not have been possible using a subcontractor, as the costs would have been too high. By doing this in house, we helped our client close the gaps in their audit. We also surfaced the new levels in the pit so they could be compared to those from the July 2006 flyover. This also allowed our client to shore up their figures and provide a better audit of their site.

Now that we had met their time requirements for stockpile volume data, we could move on to other calculations for their site. One of the best items we have produced so far from this is a 3D drawing that visually compares the pit surface between the July 2006 and December 2006 flyovers. We have also calculated both "rock in play" figures and reserve volumes for the pit. Now we are finishing the topo of the entire site, including for the first time the additional 600 acres of farm land surrounding the quarry site.

When we first started work for the rock quarry, it was not entirely clear to them what comprehensive topographic mapping could do for their operations. We think they know now. Even now, we are working with them to find new ways for this map data, AutoDesk 2007 Civil 3D, and our experience with mapping and engineering to work for them. I look forward to many years of providing quality services to this client. We hope to add civil engineering and land surveying services to our products either by joining forces with a land survey/civil engineering firm or purchasing one. If we can do as a good a job for other clients as we have for the Elmer Larson Quarry, we will have a rich and fruitful future.

About the Author

Phillip Mol is chief photogrammetrist for G-1 Aerial Surveys, an aerial survey and programming company in Sycamore, Illinois. After graduating from the Naval Nuclear Propulsion School, he went to work for Inland Aerial Surveys in Riverside, California. He later started his own consulting business and helped California firms through their transition to digital mapping methods. He also spent several years working at Hammon Jenson and Wallen in Oakland, California helping perfect the first digital orthophoto mapping process. In 2006, he founded G-1 Aerial Surveys.

» Back to our May 2007 Issue

Website design and hosting provided by 270net Technologies in Frederick, Maryland.