Setting Variables for the Eligible Area Analysis Job

The Eligible Area job allows you to create a polygon that meets specific land cover-based selection criteria. Unlike most analyses, this job generates a new geofile. The eligible area job is available for two datasets – you can choose between Land Cover (Dynamic World) or Forest Cover (Hansen).

Here’s a breakdown of the selection criteria and tips on setting each variable.

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1. Years to Analyze 

Here, you can choose which years the analysis should consider to compute the eligible area. The eligible area must meet your criteria in every selected year. If a pixel only meets the criteria in some years, it won’t be considered eligible. 

The default setting for the Forest Cover (Hansen) dataset is that the last 10 years will be analyzed, for Land Cover (Dynamic World) the last 8 years as this dataset is only available from 2016 onwards.

2. Selected classes

This is the most important selection criterion that needs to be defined as a minimum input to start the analysis. Here, you define which land cover classes are eligible. You can choose multiple classes, and all other classes will be considered non-eligible.

3. Selected Classes to Buffer and Exclude

All classes that are not chosen in the second step are automatically deemed as non-eligible. This variable allows you to choose the classes that are non-eligible and that you want to buffer. The buffer can be selected in step 6.

For example, you might want to buffer water bodies or buildings to exclude nearby areas from eligibility.

4. Minimum Plot Size

As the eligible area can consist of various smaller plots, this variable defines the minimum size for each of the plots that your eligible area consists of. For instance, setting it to 5000 square meters means only plots larger than 0.5 hectares will be considered. The larger the value, the easier the result is to handle, as the number of small, isolated polygons will be reduced.

5. Minimum Hole Size 

This variable controls how large non-eligible "holes" within an eligible polygon can be before they are excluded from the eligible area. Any holes larger than the minimum hole size are treated as non-eligible, any holes smaller than the minimum hole size will be ignored and treated as part of the eligible area.

For example, in a large area of 500,000 hectares, a small <0.5-hectare hole might not impact your analysis, so you could set the hole size to 5000 square meters (0.5 hectares). However, if your area of interest is only 5 hectares, a 0.5-hectare hole could be quite significant, meaning you'd want to set the hole size smaller for it to be treated as non-eligible.

While you can set the hole size to 0 (the most conservative option), this may result in overly complex polygons, increasing file size and reducing processing efficiency. Finding the right balance between accuracy and practicality is key.

6. Buffer Area Around Excluded Areas

This defines the size of the buffer (in meters) around excluded classes selected in step 3.

For example, if you buffer "Water" with a 100-meter buffer, all areas within 100 meters of any water body will be deemed ineligible as well. Buffer size is highly dependent on your project’s requirements.

You can read more about the concept of buffers in GIS in this blog post.

7. Simplification Tolerance 

Simplification tolerance is the allowable distance (in meters) by which points in a shape can be moved during simplification. The larger this value, the easier the resulting shape is to handle. To learn more about what simplification tolerance is, check out this blog post: Simplification Tolerance: How to Simplify Complex Shapes.

What tolerance you should use depends on the overall size of your project. Does it really matter if the outline of the eligible area shifts by 1 meter? 5 meters? In most cases, probably not. Moving some points might make the eligible area bigger, while moving others could make it smaller.

Based on our experience, a 10-meter simplification tolerance, tested on several areas of around 100,000 hectares, resulted in a difference in eligible area of about 0.5%. While this isn’t a strict recommendation, it shows that if the tolerance isn’t set too high, the impact on the results is usually quite small.

8. Threshold (only applicable to Forest Cover [Hansen])

The Hansen Canopy Cover Threshold is a value used to define what qualifies as a forest based on the percentage of tree canopy cover. Typically, the default threshold is set at 30% canopy density. This means that a 30 x 30-meter pixel is classified as forest if at least 30% of the area is covered by tree canopy. 

You can adjust this threshold depending on the region you're analyzing or the project requirements, as lowering the threshold is useful in areas with sparse tree cover, while increasing it may exclude more areas from being classified as forests. However, we recommend keeping the default of 30% unless you have a specific reason to change it.

For more details, check out Case Study I in this blog post: The Ultimate Guide to Assessing Forest Cover and Land Eligibility for Forest Carbon Projects with Hansen and Dynamic World.

9. Focus Prompt

This affects the AI-generated report text for this analysis job. You can include information such as project intention, name, and start date, and this will be reflected in the chapter of the final report. It is totally optional.

Written by Delphine-Marie Zacharias 🧡