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Calculating built-up area as a percentage of total surface area

I have a question regarding the calculation of built-up area as a percentage of the total surface area using TQL queries. Built-up area is in this case defined as the total area with a specific function (housing, offices, roads but also functions like grassland, etc.).

I use the following query/calculation to calculate the unbuilt area (Neighborhood 26 is the neighborhood/project area on which I want to perform the calculation):

  • SELECT_LANDSIZE_WHERE_NEIGHBORHOOD_IS_26 - SELECT_LOTSIZE_WHERE_NEIGHBORHOOD_IS_26

According to the TQL page on the wiki the definitions of LANDSIZE and LOTSIZE are as follows:

  • LANDSIZE: Amount of surface area. (m2)
  • LOTSIZE: Amount of surface area used by a construction. (m2)

My assumption would be that LANDSIZE and LOTSIZE would be equal if the whole area is covered with constructions. However, after filling the total area with a construction (in a test run or session), I still have a slight difference between LANDSIZE and LOTSIZE: my LANDSIZE is 78501 m², and my LOTSIZE results in 76255 m² (a 2.8% difference in area).

How can this difference be explained? There is some water in this area (±1000 m²), which could explain some of the difference between the two values, but still doesn't account for the full 2.8% difference.

Thanks in advance!

Comments

  • edited August 22

    Hi Linda,

    Your assumption is correct in that if you cover the whole neighborhood with constructions, the LANDSIZE = LOTSIZE + amount of water terrain (since it is not possible to build on water). However small differences (typically less than 1%) can occur due to rounding off issues and due to the the fact that a neighborhood can consist of multiple complex polygons (constructions) where in between very small gaps can occur where you can not build anything because it is too small to place a 3D object.

    However, 2.8% is not negligible. Therefore I think that maybe the whole neighborhood is not yet covered with constructions, even though it may look like it. Have you checked if there are multiple stakeholders in the project that own parts (maybe even slivers) of the neighborhood which only they can build something on?

    Tygron support team

  • Thanks Godelief! It looks like the difference was indeed caused by multiple land owners. I added ownership to my TQL query, and my LOTSIZE (+ water terrain) value is now equal to the total LANDSIZE.

  • Glad to hear that you found the issue and that it is solved now, Linda!

    Tygron support team

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