Remote Sensing and agriculture always have been best friends. With the arrival of Sentinel-2 data, the friendship got even more stronger! In this article, we’re trying to figure out how we could use Sentinel-2 data to make an unsupervised crop parcel classification. With a reference image by side, we then determine the crop types. The study area is in the UTM Zone 31UFS (Belgium).
Our goal in this article is to identify different agricultural crop parcels using temporal satellite imageries by applying an unsupervised classification. Agricultural parcels look different depending on the season we’re looking at. Land is usually naked during winter and crops commence their growth in early Spring. The growth peak is around May, June or July for most of the cultures but some others are highest in August or September. Doing a classification on a unique scene will not be suitable as all bare lands will look the same (even if they later differentiate depending on the crop type).
The images on which we do the classification were taken at two different periods. Sentinel-2 scenes are composed of 13 different bands of different resolutions. The crop types, and all the land in general, thus have a different spectral signature. The more bands they are, the easier the differentiation between two crop types will be. Merging both scenes into one and unique document having a total of 20 bands, leaving by side the 60-m resolution bands 1-9-10, will make the differenciation even more relevant. Notice also that two bare land parcels looking the same at the same period are thus not considered as being of the same crop type when they have different spectral signature at another moment.
We decided to make a temporal unsupervised classification using satellite images on two different moments: One was taken the 21th of April 2016, the other the 26th of August 2016.
Here are the steps we followed. Note that I use ArcGIS for Desktop with the Spatial Analyst extension.
- Data acquisition (21/04/2016 and 26/08/2016)
- Band composition
- Unsupervised classification
- Crop identification using a reference document (official SPW document)
The Sentinel-2 data can be acquired using the Sentinels Scientific Data Hub or using the open Sentinel-2 on AWS service (preferred method). From the AWS service webpage, use the interactive data search application. Zoom to the area of interest, specify the data and the cloud coverage, and go on the tile data link page.
The dates are not chosen randomly: we need to have a minimum cloud cover percentage. The document of the 21st of April is the best one during the considered growing season. Fortunately, the last scene available (26th of August) is cloud free.
As said, we want, first, to create a unique scene composed of 20 bands. Using ArcGIS for Desktop, we use the Composite Bands tool. As input, we specify all the layers B02.jp2, B03.jp2, etc. but not B01.jp2, B09.jp2 and B11.jp2. These bands corresponds respectively to the aerosol band, water vapour and cirrus bands. We exclude them because their resolution is about 60m, which is the highest of all the bands considered.
The unsupervised classification is done straighforward. Activate the Spatial Analyst extension and add the Spatial Analyst toolbar to the menu.
Click on the Iso Cluster Unsupervised Classification feature and specify 35 as the number of output classes (remember, the scenes represent an area of 100x100km).
Here’s the raw result in greayscale.
We need now to clean up this result. This image contains a lot of speckles that we need to clean up. Also, there are lots of areas which are so small around other areas that we need to delete.
We run the Majority Filter that assign to isolated pixels the pixel of their more frequent neighbors.
And then a Boundary Clean operation.
The Region Group processing groups together pixels that have the same values. After this operation, we have the 35 classes cleaned up. Attention however as small areas such as isolated pixels, isolated and small pixel corridors, etc. can remain in the image. We now clean this up as we first need to define a threshold from which we consider removing the area
We use the Set Null operation where we consider that areas of less than 40 pixels are removed.
The last operation is Nibble. As input, we take into account the mask that we just define with the Set Null operation and the cleaned unsupervised image (coming out from the Boundary Clean operation).
We finally got our clean classification result! The last thing to do is to add a reference layer in order to identify the crops. The same areas on both the reference layer and our classification are supposed to be of the same type. Notice however that we discovered some differences and we see two main reasons:
- Our classification is based on 2016 imageries whereas the reference document is from 2015. It is possible that crop type change from one year to another
- Our technique to identify the same crop types is not optimal (similar spectral signatures from different crop type, too broad temporal resolution and not enough images taken into account (April – August), etc.)
As we are in Belgium and mostly in the Walloon Region, we can take a ready to use layer provided by the SPW (Service Public de Wallonie).
Here is our final result.
We’ve seen that with the two provided Sentinel-2 data using both 10 bands and ArcGIS for Desktop, we were able to run an unsupervised classification and to assign the detected zone to crop type using a reference image.