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Precision Q&A – for Feb. 9, 2009


If a farm has a lot of hills, does a GPS guidance and mapping system need some extra technology compared to a system for a farm that is flat? What is this technology and how much does it cost?


The answer comes from Pam Haegeman, GIS technician with Mazer Group in Brandon, Man. She is also the precision farming specialist with Manitoba Zero Till Research Association. Here is her response:

Terrain compensation technology allows a GPS guidance or steering system to adjust its location when dealing with location shifts due to rolling or rough terrain. When a vehicle is on a side slope, the GPS location directly below the antenna shifts down the slope causing the GPS location to move down the slope as it travels. In that case, the guidance system needs to compensate because the drawbar is no longer lined up vertically with the GPS receiver. (See first image).

Most GPS systems use what are called gyroscopes (gyros) and/or accelerometers. Accelerometers measure the angle of gravitational pull towards the centre of the earth. This determines what slope you are at. Gyros measure how fast the slope angle is changing. It is a common myth in the farming industry that gyros have moving parts and can get “stuck.” In almost all agriculture applications, a “solid state” gyro is used, meaning it has no moving parts.

A GPS system can have gyros or accelerometers or a combination of the two. When both exist working together, the system works to not only compensate for rolling terrain, but also to eliminate errors due to “bumpy” fields and “crabbing.” These combination systems can tell the difference between a “bump” and a slope. If a system has only accelerometers, it can only measure gradual changes in slope and will not compensate for quick changes such as “bumpy” fields. If there is only a gyro present, it will be limited in the reaction to changing angles of the slope.

Another method of terrain compensation is using two GPS elevation measurements and comparing them to each other to correct the slope. To use this method properly, you must be using a GPS system with sub-inch accuracy.

It is important to note that the firmware in your GPS unit also makes a big difference in how well the unit works. Depending on how the terrain information is processed within the unit could mean the difference between straight or “wiggly” lines. It is also imperative to properly calibrate gyros and accelerometers before use.

The three axes (dimensions)

Precision Q&A

of movement on a tractor that the technology is used for are roll, pitch, and yaw. Roll is the axis going from the front of the vehicle to the back and measures the side to side rolls of the vehicle. This is the axis we are concerned with when it comes to side hills. Pitch is the axis going from the right to left through the engine of the tractor. It measures the forward and backward pitch of the tractor as it travels up and over hills. The yaw axis goes from the top of the tractor to the ground and measures the steering motion of the tractor side to side. This is the axis that we are concerned with when is comes to “crabbing.”

The more gyro-accelerometer pairs a GPS unit uses, the more corrections can be made, hence a smoother, straighter line. Most units on the market today have at least two pairs, usually assisting with the roll and yaw errors. The most you can have is three pairs of gyro-accelerometers, which cover all three dimensions of movement.

Some GPS companies have made terrain compensation mandatory in all units. Location of the gyro-accelerometers is also important. Some companies put them in the antennas on the top of the cab. Other companies mount them to the floor or wall in the cab. It is very important to mount sensor unit firmly. Any movement contradicting your tractors movement will cause the steering system to react improperly. Mounting these units with Velcro or duct tape is not recommended. A tip for cab-mounted units is to mount them securely to the rubber floor mat so the mat absorbs some of the vibrations and helps to smooth operation. You also need to mount the unit at right angles to the tractor so the gyroscopes line up with the tractor’s axes.

Different companies have their own ideas of what is the best way to get the most out of terrain compensation. Trimble EZ-Steer has two gyros and two accelerometers (T2) located in the controller module, which you mount on the floor or on the back wall of the cab. Trimble’s Autopilot hydraulic steering system has three gyros and three accelerometers (T3) that are also in the controller module (Nav 2). All of Trimble’s steering units automatically come with terrain compensation.

The old John Deere antenna (Gen 2) has an option to mount a terrain compensator (TCM) under the antenna on the cab roof. It contains one gyro and one accelerometer that correct any roll of the machine. The newer John Deere antennas have integrated terrain compensation (iTC) which has the TCM built in. John Deere uses steering sensors to compensate for the yaw directions.

Outback’s eDriveTC module contains two gyros that measure the speed of the tilt in the yaw and roll axes but contains no accelerometers. This module is mounted securely to the floor of the cab.

Raven has a couple options. You can use their yaw sensor (steering sensor), which has one gyro and add their roll sensor (TM1) which also has one gyro to cover two axes. Raven also has a three-gyro option (GPS Enhancer), which currently doesn’t have any accelerometers.

Mid-Tech also has a separate module you mount solidly to the floor of the cab called the Tilt Compensation Module. It uses an accelerometer to determine the slope the vehicle is on.

Terrain compensation is usually incorporated in a GPS unit, therefore included in the system’s price. Add on units can range from $300 to $3,000 depending on how many components and axes it works on and the quality of the components.

If you have a very flat field, you will still benefit from terrain compensation. It works with the GPS firmware to smooth out errors. Many GPS systems out there today offer the option to turn off the terrain compensation. A good experiment to convince anyone of the performance difference with terrain compensation is the turn it off for a couple hours so the farmer can see the difference it makes.



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