Farm Report #6
The Great Laser Plane Adventure

I planted all of my farm to cotton this year, so I had no field tillage to do during the summer. From mid-July until late August, I left my John Deere 4430 tractor with my neighbors, the Teicheiras, so they could use it for a few chores such as ditching, disking, and drag scraping. Because I was finished irrigating, I needed the tractor back so I could fill in the drain ditch and smooth down the row ends in preparation for defoliation spraying.

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As I was wandering around looking for one of the Teicheiras to discuss the prospect of moving the tractor back to my farm, I saw Robert (the son) and Gilbert Robles (the very same Gilbert who drove their cotton picker in my cotton last year) out in a bare field which was evenly marked with a grid of white paper bags placed exactly every hundred feet in both the East-West and North-South axes. This is the Fancy-Dan plural of axis, and has nothing to do with Lizzy Borden. I drove into the field to see what they were doing. They were using the laser sender unit they had purchased last year, along with a new receiver unit which was attached to a surveyor's stick, to survey the elevations of the field. They were doing this to get the numbers needed to calculate the best way to use their laser-plane land leveler to level the field. Well, they weren't exactly going to "level" the field. They were going to "flatten" the field, smoothing out the variations in the fall of the land to improve the irrigation performance.

Before I go on, I probably should try to explain how this laser plane thing works. The setup consists of two units. The sender (above) is a laser which is mounted about 12 feet high on a platform. It rapidly rotates, sending the laser light in a circle like a lighthouse does, except the light is a laser, so it remains in a very narrow beam. The mounting has an automatic leveler built into it, so when it's set to all zeros, the laser's circle of light is perfectly level. The sender has two adjustments which allow the operator to set the two axes to different slopes, or falls, or whatever they're called. The adjustments are calibrated in feet per 100 feet, or meters per 100 meters. It has a practical accuracy of about a quarter of an inch per 100 feet.

The other unit is the receiver (above), which detects the laser. It is mounted on the earth mover. A complicated (meaning: I don't really know how it works, but it does) set of controls allows the laser receiver to control the height of the bucket on the earth mover. The operator can adjust the settings on the receiver, and he can override the receiver when he needs to pick up a bucketful of earth and transport it to another section of the field.

Once the operator has done a preliminary field survey establishing the average fall and side fall, (Note to professional surveyors: this is strictly an amateur effort, and I don't even know the proper terminology. I don't know whether it's properly called "fall," or "slope," or some other term. I've taken to using "fall" for the slope along the furrow axis, and "side fall" for the slope across the furrow axis.) he can then set the laser controls and flatten out the field, removing all the small hills and valleys which degrade the irrigation efficiency.

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Then I made my first mistake. Instead of seeing that Robert and Gilbert were busy doing something important, and leaving them the hell alone, I rode along in their pickup, talking with Robert as Gilbert ran the stick and read out the elevations for Robert to write down.

The tractor question was quickly settled- they would be bringing it back in a day or two. I assured Robert that I was in no great hurry, and that any time they wanted to bring the tractor back was just fine. Then I asked him how he was planning to use the numbers Gilbert was giving him. I foolishly opined that this land leveling thing just cried out for a computer solution. It should be simple to put the raw elevation numbers into a spreadsheet, calculate the average elevation, fall and side fall, and then compute exactly how to set the laser to do the job. Hah!

The year before, when Robert used the laser for the first time, he employed a "trial-and-error" method for setting the laser and leveling a small field. That worked, after a fashion, but now the field to be leveled was very large, and methodological improvements were definitely in order. So we agreed that Robert would e-mail me the raw numbers, and I would see what I could do with them.

People who do laser leveling on a commercial scale use some fairly advanced equipment to accomplish what we were setting out to do. GPS receivers automate the process of locating the survey points. Dedicated computer equipment and programs do the "cut and fill" calculations automatically, printing out the results for the operator. But these things cost a lot of money, and the Teicheiras had already spent quite enough, thank you, on their gadgets. We would figure this thing out ourselves.

As I began this project, I had two problems: I knew next to nothing about the theory and practice of land leveling, beyond the obvious; and I had never used a spreadsheet to do this kind of work. Both problems would give me trouble, but the second would be much worse than the first.

Robert's e-mail containing the raw elevation data arrived the next night. A few cells were missing, because either those squares contained buildings or they were too high to use the same setting on the sender unit. I began by calculating the average elevation for the field and the average falls and side falls. I assumed that the average elevation would be a valuable thing to know.

I decided early on that the calculations would require three steps, done on three sheets. The first, of course, used the original data to calculate the average elevation, fall, and side fall. The second sheet was the target sheet, used to add the average falls and side falls to some benchmark location. This was a simple arithmetic problem. The third sheet was the cuts and fills sheet, used to calculate the cuts and fills. Here, I just subtracted the target data from the original data, cell to cell, using the copy and paste functions. It all went very quickly. Again, this was a simple spreadsheet process. Then I used the program's chart functions to build charts of the original field and the cuts and fills. The first chart was to help the operator visualize the existing field. It was a full 3-D chart. The second chart was also a 3-D chart, but I set it up to be a direct overhead chart, showing only the areas to be cut and the areas to be filled, without any 3-D effects.

In doing the first sheet, using the raw data, I was prevented by circumstances from making any big mistakes. After all, I was just adding and dividing. But the second sheet was a different story. I got the idea that the existing average elevation was a useful thing to know and that this information would help me choose a benchmark to anchor the fall and side fall additions and subtractions. This benchmark was also the location in the field used to set the receiver unit to match the sender unit. The third sheet was, again, simple arithmetic, and the methods used prevented any mistakes.

The problem I immediately encountered was a troubling one, to say the least. When I used the cell on the original data sheet nearest in elevation to the average as a benchmark, the total cuts and fills did not match up. They weren't even close. I was fairly certain that my method for finding the total cuts and fills was correct, so I was stuck with the seeming paradox- the average elevation refused to yield a suitable benchmark.

At this time, an "expert" from the manufacturer of the laser equipment paid a visit to Robert. When queried about the "average elevation" problem, he assured us that the average elevation was, indeed, the right place to set our benchmark. But the fact remained that I could not get the numbers to work using that setup.

I was perplexed about this average elevation difficulty for about two days, until I realized what the truth of the matter was. The average elevation had nothing to do with the proper benchmark. In fact, if the average elevation turned out to be also the correct benchmark, the field was probably already flat. No, the benchmark almost certainly was at some other elevation. Not only that, but there would be many benchmarks, at variously elevations, wherever the target sheet intersected the original sheet.

"Of course!" I said to myself as I slapped myself on the side of the head, "How could it be otherwise?" My breakthrough came just in time. Bob (the father) was operating the earth mover, and he was unhappy with the original land leveling plan. It was accurate, but it was resulting in far too much earth moving to suit him. He wanted more numbers, different numbers, and above all, less earth moving. I could see that recalculating the project, dividing the field into smaller sections, and figuring in different falls and side falls, would result in less earth moving. Unfortunately, that would also quickly overload my ability to redo the spreadsheets. I would need better methods, and my new understanding of the benchmark problem offered the needed relief.

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As soon as this obvious insight about the benchmark question came to me, I also realized that my method for finding the benchmark was all wrong. In fact, there was no way to find the benchmark first and then use it to define the target sheet. No, the right way was to use just the opposite process. Instead of going to some proposed benchmark and then adding and subtracting the falls and side falls from that benchmark, I set the lowest corner to some arbitrary low number. Then I added all the falls and side falls to that number by using separate control cells to contain the fall and side fall numbers. Changing the elevation of the lowest corner raised or lowered the entire target sheet. I could redo the target sheet instantly. I could alter the falls and side falls by changing the numbers in the control cells. Then the process of matching the target sheet with the original sheet to get the desired cuts and fills was simply a matter of feeding numbers into the lowest corner's cell. The benchmarks were found by looking on the cuts and fills sheet for those cells closest to zero. Sometimes zeros appeared, but anything within a hundredth of a foot or so would be close enough. It would be easy!

Then two new questions raised their ugly heads. The first was: how much earth would Bob need to move? Okay, this was fairly simple. I would just go through the cuts and fills sheet, deleting all the fills. Then I could sum the cells, getting the total cuts. It was a manual process, and took a little extra time, but it was no big deal, right?

The second question was the killer. What was the compaction factor? Yes, laser plane fans, the matter of compaction arose. What is compaction? Well, it turns out that when you move earth and put it somewhere else, it gets compacted. You actually need to cut more earth than you plan on filling. That is to say- the cuts need to be more than the fills. How much more? That depends mostly on the soil type. It can range from only about ten percent to as much as forty percent. How do you know what compaction factor to use? Experience and trial-and-error. If you level your field, and you end up with extra earth, your compaction factor was too high. If you end up with unfilled low places, your compaction factor was too low. That is the dreaded compaction factor.

"Okay," I thought to myself, "this is not a problem. Solving for the compaction factor will automate the process of finding the total cuts." Understand, I had been working on the land leveling problem for about five days already, and I was getting a little soft-headed. You'll have to imagine the scenario. On any given day, I was discussing the earth moving with Bob, the computing aspects with Robert and the benchmark anomaly with any poor victim who would listen. I was rewriting and refiguring several workbooks and then driving back and forth between my computer in town and the field, about six miles out in the country, two or three times per day. The stacks of printouts were piling up in the tractor cab and Bob was getting a little more dubious about the whole thing every day. In fact, we were all learning as we were going along, as quickly as we could. Bob was learning how to run the earth mover, I was learning how to do the calculations, and Robert was learning just how lucky he was to not be directly involved in the whole damned mess.

Anyway, let's get back to the computer problem. I decided that I could easily calculate the compaction factor and the total cuts. All I needed to do was to use the SUMIF function in the spreadsheet program. That is to say, I needed to set up the spreadsheet to sum all the cuts while ignoring all the fills, and then to sum all the fills while ignoring all the cuts. This exactly fit the SUMIF function. I could divide the cuts by the fills to get the compaction factor. Then I made my next BIG MISTAKE.

I studied the spreadsheet program's help section, trying to figure out the SUMIF function. The examples given seemed to indicate that I would need two ranges of cells: one for the "range" and another for the "sum_range." The "criteria" would determine which corresponding cells would be summed. So I copied the cuts and fills sheet to another sheet I called the "Compaction Ratio" sheet. This is where I did the SUMIF functions. And it worked.

Well, it almost worked . When I tried to load any previously saved spreadsheet workbook with this latest improvement, the program would lock up. Everytime. Nada. Zip. Zilch. I was unhappy, to say the least. Bob was asking for more schemes and permutations every day. I finally had the spreadsheets set up to do the job correctly, but since I couldn't reload any existing workbooks, I had to redo them from scratch every time. I had done so many of them by then that recreating them was a snap. But still, I couldn't do them quickly enough to keep up with the progress of the actual land leveling job.

Finally, in desperation I decided to ignore the help file and try to use the same range of cells for both the range and the sum_range, thinking that using two separate sheets for the SUMIF function might be what was stressing either my computer, or the program itself, beyond its capabilities. After all, we were doing a lot of calculations here. Up to 500 cells added, subtracted, compared, divided, added and subtracted again, compared again, referenced from sheet to sheet- I was beginning to have trouble keeping track of all the different steps myself. But using the same sheet for both the range and the sum_range did the trick. All of a sudden the workbooks would work. I could save one and then reload it. I could load several of them at one time. Life was grand.

The computerized land leveler problem was solved just in time to reaffirm the final calculations needed to do the actual land leveling. Well, at least we'll be ready for the next field. Yeah, right.

Have a comment? Send to rsturge@inreach.com

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