Step 4: Match project and control plots

Now you’ve created your project plots and control plots in the previous step, we need to match the plots in the two datasets so that they can be compared.

Earth Blox template: VM0047 Step 4: Match project and control plots

This template matches project plots with control plots using a 'k-nearest neighbours' approach, as specified in the VM0047 documentation. This template produces a dataset containing all valid control plots, and includes both matched and unmatched control plots.

Upload your Project plots and Control plots

Start by uploading your project plots dataset and candidate control plots dataset to your Assets. 

Learn more: Uploading a feature collection to Assets

Add your Project plots and Control plots to the workflows

Replace the example project plots dataset in the workflows with your own.

1. In the workflow using Example project plots, click ‘View or change dataset’
2. Under the Assets tab, find your Project plots dataset
3. Uncheck ‘Use recommended blocks’ 

Repeat this for the workflow using Example control plots.

The Filter by attribute blocks ensure that only control plots with valid stocking index measurements are included in the matching.

Learn more: Filter by attribute block

The Match sample plots block matches one or more control plots to each project plot. You can customise how many are matched – this is the ‘k’ in ‘k nearest neighbours. 

If there’s noise in the data (more likely if you are using an optical index), a higher k value can even it out. However, a high quality dataset like Chloris Geospatial: Aboveground Biomass Stock and Change (30m) will have low noise, so a relatively low k value is acceptable.

The workflow outputs a dataset of both matched and unmatched control plots using the Export dataset block, and also show the results on the Dashboard using the Create table block. 

Matched control plots will include:

1. The ID of the project plot to which they are matched (matched_uid)
2. The distance metric describing how close the match is between the control plot and the project plot to which it is matched (distance)
3. A weight metric describing how close the match is between the control plot and the project plot to which it is matched (the sum of the weights of all the control plots matched to a given project plot is 1).

? ‘Distance’ refers to the similarity of characteristics (the closer the distance, the more similar the characteristics) rather than physical distance.