Dr. Amanda Caudill
Since the recent news that the EU will soon be introducing regulations on certain commodities to curb illegal deforestation, everyone seems to be talking about “polygons”. So what’s a polygon, why are we hearing so much about them and how are they measured?
We’re here to unpack it all…
The what:
There is a landscape ecology theory that says everything within a landscape can be described by points, lines, and polygons. For example, a tree could be seen as a point; a road, a line (a connection between at least 2 points); and a polygon, a lake (closed loop between 3 or more points). Point, line, and polygon are the 3 formats for storing geographic location data in mapping software systems, like Geographic Information Systems (GIS).
Image Source: Colin Williams (NEON) https://www.earthdatascience.org/workshops/gis-open-source-python/intro-vector-data-python/
The why:
The EU will very soon be implementing regulations to prevent deforestation – EUDR. Geographic locations of coffee farms will be required as part of the due diligence for the directive to show that the farms are not in areas that have been deforested.
While we are awaiting the specific details to be formally published, reliable sources report that for farms less than 4 hectares in area, a single point of geographic coordinates is sufficient for location, and for farms greater than 4 hectares the perimeter around the farm will need to be shown, i.e., polygon data.
The how:
So how do you collect this farm polygon data? If there are distinct boundaries of a farm that can be determined from satellite imagery or maps, then the perimeter of the farm can be traced and geolocated. However, this is not the case for most coffee farms. For most of the farms, you need someone to tell you where their farm ends and another farm or property begins. It isn’t something that you can see on a map – so you must physically be on the farm and record coordinate points along the boundary line. ISEAL published a useful guidance doc on collecting polygon data that may be of interest if you want more info.
Handheld GPS units can be used in the field that will automatically record continuous points or tracks your path as you walk the perimeter of the farm. Several apps for smart phones are available that will do the same*. Alternatively, as you walk the perimeter, you could take multiple GPS points (aka vertices or waypoints) along the path and then join them to create a polygon. They have irregular shapes, so the more points you record along the perimeter, the more accurate your shape will be.
It may not be possible to walk the perimeter of a farm due to obstacles such as streams or rivers or steep slopes that are impassible. As mentioned before, a polygon is just a closed loop of points – so you could take GPS points along the perimeter, even not necessarily in sequential order, and then estimate the boundary line between the points. Although this is less accurate than walking the perimeter of the farm, it may be the best option in some cases.
*Here is a list of some available apps for collecting GPS data
Field Area Measure
GIS Mapper – Surveying App
GPS Area Calculator
GPS Tracker
Mergin Maps: QGIS in pocket
Planimeter
Agro Measure Map Pro
Google Maps (using the option of Google My Maps and drawing the farm perimeter on the satellite imagery), Google Earth mobile app (entering at least 3 reference points)
