Using RTK Steering Data for Soil Erosion Control and Water Logging Prevention
by Tim Neale

Many farmers have purchased auto-steer for their tractors, but many don’t know that they have an added bonus with this equipment. Topographical (or elevation mapping) starts with the collection of elevation/height data from an RTK GPS system.  These highly accurate (less than 50mm vertical accuracy) GPS systems that are used to auto-steer agricultural machinery are variations of the same systems that are used by surveyors to collect spatial data.  Once the data is collected, we can analyse the data to produce contour, elevation, slope and aspect maps.

Elevation data can be collected by the GPS system whilst it is steering a tractor performing a farming operation.  Alternatively, if the farmer does not have RTK GPS, the data can be using an RTK GPS mounted in a quad bike or 4WD vehicle (photo below).

The elevation data is collected by driving the vehicle over the paddock at regular swath intervals; with the GPS unit logging data points every few metres.  Obviously, the narrower the swath width the better, as more data is collected to produce more detailed topographic maps of the landscape, which is an advantage of data collected from a tractor conducting an operation such as planting. 

On some systems, such as Auto-farm, you need to turn the logging feature on as it doesn’t happen automatically. Others, such as Trimble and John Deere, collect when you are doing coverage mapping, and can be easily exported. The best way for us to use the data is to copy and paste the complete data card and send via a DVD. For John Deere customers, if already have your data in JD Apex, you can do a batch export. A detailed guide on how to do this can be found at http://www.precisionagriculture.com.au/news-and-articles.php

Topographic farm maps are extremely useful for planning farm layouts that achieve the best possible compromise between operational efficiency and surface water management.  Planned farm layouts aim to maximize farm operational efficiency, control erosion, and minimise water logging.  The value of topographic information is considerably increased when the contours lines are laid over high-resolution aerial or satellite imagery, as the picture below shows.

New software such as Optisurface allows us to examine the data in 3D as well. This helps in the visualisation of the field. A video of this can be seen on YouTube (http://youtu.be/TSzq9NR-Nx0).

Another feature of Optisurface is that you can model runoff velocities and depths from a paddock with a simulated rainfall event. The runoff velocity preducitions are shown below left and satellite image on the right. You can see where the velocities are high (red), this correlates well with the image on the left showing the actual erosion scars.

Another example (500ha paddock) is shown below with the simulated runoff velocity (m/sec) on the left and runoff depth (m) is on the right. Note contour banks on right are full of water. In this paddock and additional contour bank is probably needed. This mapping is very useful to determine the erosion hotspots in the field, before it even rains.

If you are driving directly up and downslope with your operations, then you can also use the data collected from the autosteer to examine your contour bank spacing’s and capacities. The data below is the plot of elevation data in MS Excel. The small bumps are each contour bank. It is easy to see both the depth of the bank and the profile. The machine obviously needs to be logging very regularly to get this sort of information (i.e once a second).

With the recent flooding and erosion some people experienced extreme soil losses. Using data collected from before and after the erosion or flood, you can compare between the two. This is assuming the two maps are collected with the same tractor/sprayer, and the base station was not ‘averaged’ or moved between dates.

So if you have purchased a dual-frequency RTK “2cm” system from Trimble, Autofarm (GPS-Ag), John Deere, Beeline, Leica, or AgGuide, you could get a topographic map for your farm which could help with drainage, farm layout, as well as the positioning of contour banks.