Minimale hoeveelheid groen selecteren

evy@urbansync.nl

Beste Tygron,

Ik zou graag de aaneengesloten stukken groen van 1ha op mijn kaart selecteren. Ik heb hiervoor het Vondelpark in Amsterdam geselecteerd en de volgende uitleg gebruikt "how to compute the presence of a minimum amount of green" (https://previewsupport.tygron.com/wiki/Howtocomputethepresenceofaminimumamountofgreen).

Het vondelpark is 470.000 vierkante meter groot, waarvan 16ha groen. Op mijn kaart is echter niets te zien. Ik heb het Vondelpark geselecteerd om zeker een aaneengesloten stuk van 1ha op mijn kaart te kunnen selecteren. Ik zou dus graag vragen hoe ik mijn probleem kan oplossen.

Alvast bedankt!

Rudolf

Hi Evy,

the methodology works as follows:

• In an Average Overlay: For any given cell a circle is drawn around it with radius R.
• In that same Average Overlay: A fraction is calculated, specifically a fraction of how much of that circle has the desired property.
• In a Combo Overlay: that fraction is multipled by the area of the circle, so that the actual surface area with the desired property is computed.
• Finally, in that same Combo Overlay, if that surface area meets your threshold, the cell is marked as meeting your criterium.

What I see in your screenshot is that the formula is made to assume a circle with a radius of 25. That circle's surface area will compute to 1963,5 m². You will never be able to find 160.000m² (16ha) in that circle, or even 10.000m² (1ha)

To rectify this mathematically, you should increase the size of the circle. This means changing the "25" in both the Average Overlay and the Combo Overlay.

For 1 ha: A radius of 60m is just large enough to meet the minimal size requirement, and would produce results if it was almost entirely green. I get a few small hits in the Vondelpark

For 16 ha: A radius of 250m is just large enough to meet the minimal size requirement, and would produce results if it was almost entirely green. In practice for the Vondelpark I get a hit when working with a radius of about 400m.

However, with these kinds of radii the amount of seperate green you're including in the results also becomes very large and so the calculation/results become less robust and significant. In addition, the Vondelpark is large and may not have data everywhere on its open fields, which may lead to "missing" green in your calculation. Since you're dealing with a known location, I would recommend a less dynamic approach:

• Adding an Area highlighting the Vondelpark, with Attribute VONDELPARK=1
• Making that Area into a grid using an Averaging Overlay set to a distance 0, and aimed at the attribute VONDELPARK.
• Then either using that Overlay as your input for the presence of significant green, or use a Combo Overlay to add that grene to whatever other sources of green you would like to take into consideration.

Regards!

evy@urbansync.nl

Hi Rudolf,

Thank you for the answer. It clarifies why in my calculation nothing is shown at a size of 1 hectare. 

Evy