Open data and Metaldehyde risk mapping [update]

This is an updated post based on this blog post from 2014. The work presented here is still a proof of concept to demonstrate the utility of open data. If you’d like to work with Geoger to develop this further, please get in touch.

The idea

Metaldehyde is a pesticide found primarily in slug pellets and associated products, and is a critical pollutant of water courses. I wanted to know which open datasets could be used to create workable maps of potential risk areas. This blog post reports on the successful implementation of an initial method and discusses some options for ongoing development.

The data

In the initial study in 2014 Landsat-5 satellite imagery was obtained for an area covering Hampshire, UK. The crop of interest for was oil seed rape and the simplest way to recognise this is to use imagery captured when the crop is in bright yellow flower in late April / early May.  The image used was captured on 30 April 2011 and a subset was created (see Figure 1 in the gallery below) relating to four Ordnance Survey OpenData OS Terrain 50 tiles. OS Terrain 50 is an elevation dataset provided at 50 m resolution (Figure 3 – black relates to low elevations and white to higher values). Other OpenData layers were also downloaded to provide water body information and wider basemap context (Figure 2 – roads, watercourses, woodland and buildings are plotted on top of the satellite imagery). Open aerial imagery and elevation data were obtained for the corresponding area from the Hampshire Hub. The aerial data were collected during the summer of 2013.

The method

Open source GIS software was used for all the data processing in this trial. The .asc elevation files were stitched together into a mosaic, as were the true colour aerial photographs. Oil seed rape fields were identified in the Landsat imagery using a simple classification scheme to keep the processing quick and easy (Figure 4). The waterbody layer was buffered and areas of ‘risky’ slope were calculated from the 50 m OS Terrain data (Figure 5). These were then overlain and areas of overlap used to highlight potential risk areas (Figure 6). The images in the gallery show some of these processing steps – click on an image to see a larger version.

Once the areas of potential risk had been identified for an area on the Landsat image, the aerial photography was loaded for one of the risk hotspots (Figure 7). Orange areas show moderate potential risk and red areas show highest potential risk. The field boundaries for the oil seed rape fields identified in the satellite imagery were digitised from the aerial imagery and the analysis was re-run using the finer resolution elevation information. This then shows within field risk areas, allowing pesticide application to be better monitored and controlled (Figure 8).

Developing the method

The 2014 work proved that open datasets could be used as the source of potential risk model parameters.The results are different from many existing studies as they show within field risk areas, as opposed to just classifying a field as ‘at risk’ or not. As the method stands, the satellite imagery is the only dataset that would require updating on an annual basis. This would be to obtain crop details and update maps of risk for a specific growing season, based on which fields are growing high risk crops.

Accessibility to open satellite data is improving all the time. The images below are of the 2011 Landsat image and another Landsat image collected in 2013, both subset to the area used in this study. The oil-seed rape fields are once again highlighted by their yellow flowers, and the two images demonstrate the change in planting patterns. Although issues of cloud cover can never guarantee data collection over an area, the increased number of satellites in orbit is bringing repeat times for data collection over an area down to near daily. This makes for a cost effective method of monitoring annual risk of metaldehyde run-off locations.


Additional information needs to be added to the model to improve the methodology, and the results would need to be validated prior to the method being operationalised but it has been demonstrated that year-on-year risk mapping is now feasible for specific pollutants.

Other information

Many of the parameter values used in this work were obtained from other studies. The following presentations are worth reading if you are interested in this subject:

  • Yorkshire Water catchment management document.
  • Anglian Water, Affinity Water and Essex and Suffolk Water farming document.

Also have a look at these websites: