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How We Use Pix4Dfields and our Drones for Variable Rate Spraying

Updated: Jan 2

Cost savings

One of the biggest reasons we love using drones to help farmers is the dramatic reduction in chemical and input costs we can help them achieve through targeted applications. When treating weeds, it is common to have isolated areas with high weed pressure, while the rest of the field may have low to moderate weed pressure. The drone's variable-rate spraying technology allows you to use the lowest effective rate in areas with low weed pressure and a higher rate in areas with high weed pressure. For maximum savings, farmers can choose to treat only the areas with high weed pressure.

In the example we use in this article, for a 24-acre field we treated to eliminate broadleaf weeds, the chemical cost alone to treat the whole field at the same rate would be $900. If we used a high rate on weed-infested areas and a low rate on the rest of the field, the chemical cost would be $635. If we exclusively treated the areas with high weed pressure, we could reduce that chemical cost down to $412.

This field was a relatively small example. As field sizes grow, the ability to unlock these savings increases.

This article will explain how we do weed detection and variable rate spraying in depth using Pix4Dfields and the DJI Agras T-40.


The detailed process of completing a mapping mission is beyond the scope of this article.

To do this analysis, you will need to collect a grid pattern of high-resolution images from a relatively low altitude above ground level (100–175 feet, if possible). This will give you the appropriate resolution to identify weed growth in a field.

Import images into Pix4D fields.

  • Open Pix4Dfields

  • Create a new mission.

  • Click Import Images and select the images from your mission.

  • Use accurate processing and the highest resolution available.

  • Click to start processing.

Build a field boundary and clip

  • Click the boundaries button.

  • Click to create points around the edge of the field. Since we plan to use this for a spray mission later, ensure the boundary is at least 15 feet inside any obstacles on the field edge (such as trees).

  • Mark obstacles within the field with the "add hole" tool. Be sure to extend your hole a few feet past the edges of the obstacle to avoid potential crashes.

  • Click the trim button below the boundary name on the right of the screen and trim your mapping layer to fit the boundary.

Option 1: operation tool freehand

  • Click on the operation tool at the top left of your application.

  • Do not pre-fill grid cells.

  • Use your field boundary to create the area for the operation tool.

  • Choose gallons/ac as the units for your prescription map.

  • I like to adjust the cell size to around 10–20 feet for simplicity and to roughly align with the drone's spray pattern. I then rotate the grid to align with my planned spray route direction.

  • From there, we will draw in our sprayed area over the locations with high weed pressure.

  • We will use the paint bucket fill tool and click in an area we did not draw to fill in the rest of the map with our second prescription rate if we plan to spray at higher and lower rates.

  • The operation tool on the right allows you to set the spray rate for each of your rate zones.

  • Notes: You can do multiple different rates across the map.

Option 2: Magic Tool

  • The Magic tool is an AI feature detection tool in Pix4D fields that classifies a grid based on the image features inside each cell. This tool is useful for weed identification and plant health analysis to identify problem areas without drawing them manually.

  • Open the magic tool and select grid size and orientation.

  • Select wanted and unwanted cells until the output represents the weed areas accurately.

  • Clean up manually to add or subtract any area that was missed.

  • Save as an annotation

  • In the annotations tab, click copy all and duplicate to boundaries.

  • Open the annotations tool.

  • Click Use existing annotations to pre-fill the grid.

  • Select magic output AND overall boundary.

  • Continue as before with the magic tool output filled in.


  • Export output as a VRA mission.

  • Drag and drop a file onto a microSD card.

  • Insert the card into the remote, select the file, and click import.

  • Click begin

  • Select the file icon and select the variable rate mission you just created.

  • Click Use and begin spraying.

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