Surveying levees with GPS technology uses a triangle of instruments. A base station, vehicle and satellites create real time data regarding elevation in a field.
GPS technology moves information through the triangle 34,000 miles round trip per second. It increases efficiency by eliminating problems such as dust and wind, which interferes with the laser and base station system shown in last week’s post.
The base station is the constant. It doesn’t move once it is placed in the field.
According to NASA, “a group of more than 20 satellites make up the Global Positioning System, or GPS. If you have a GPS receiver, these satellites can help figure out your exact location … A satellite orbits Earth when its speed is balanced by the pull of Earth's gravity.”[i]
The satellites in space coordinate with the vehicle moving around the field.
Before any survey lines are dug, the surveyor will ride the outer perimeter of the field in a vehicle to note the high and low points using a screen showing the GPS data gathered in real time. Starting at the lowest or highest point of a field, the surveyor or farmer will set a benchmark elevation level at the lowest or highest elevation. To keep an even flood across the field, the surveyor looks for a change in elevation of two-tenths of a foot.
Once the surveyor has the information from the GPS program of where the levees should go based on the elevation of the field, he uses the digger attached behind the vehicle to dig the surveyed lines the farmer will follow to pull levees.
GPS technology uses similar indication lights, much like the laser and base station. The three green lights indicate the surveyor is following the correct path to place the levee.
As the elevation of the land changes, the surveyor will turn right or left to keep the lights green. If the red indicator lights are flashing a couple spots to the left or right, GPS technology is accurate enough straying slightly from the path is still efficient for placing surveyed lines.
Farmers need the levees to be in the correct place for proper flooding, but they also need to be able to access the field efficiently during harvest. Placing levees too close together make it difficult for the farmer to maneuver equipment into the fields once the harvest season begins, which can lower efficiency.
For example, if the elevation of the field is increasing, the GPS would tell the surveyor to place levees closer together to accommodate the steeper grade. The surveyor, knowing the farmer will need to fit a combine within two levees during harvest, will adjust the elevation from two-tenths of a foot to three-tenths or
four-tenths of a foot. This gives the farmer more room to work, and it still ensure the rice will be evenly flooded.
Stay tuned for next week’s post to learn more about levee gates.
[i] “What Is a Satellite?,” NASA, Feb. 12, 2009, http://www.nasa.gov/audience/forstudents/5-8/features/nasa-knows/what-is-a-satellite-58.html.