# Automating polygon “useable area” check in QGIS? [closed]

I need to run a query that will identify all property remainders (available land for construction purposes) that can accommodate a 40m² building footprint (must be square).

I have used imagery classification to create building footprints for existing structures on properties and have clipped them from the land parcels for calculating area of undeveloped land within each property boundary.

I can't assume that each remainder measuring >40m² can accommodate the construction of a 40m² building because the calculation will include any sliver of land that wasn't part of the existing building area, for example, the gap between house and boundary, small front garden area etc.

I need to do this check for approximately 20,000 property boundaries, so automation is key and I have little to no coding experience.

• This is a very challenging task, esp. without coding experience. This gis.stackexchange.com/questions/166230/… might give you some ideas – FelixIP Nov 4 '16 at 2:40
• I was wondering if there is a function that can create polylines connecting the corners of the existing building footprints to the nearest property boundary edge, then splitting the polygons along these lines and doing a select by size query, to return all segments that would meet the size criteria. – Brendon Miles Watkiss Nov 4 '16 at 10:46
• Since you're looking for squares, this may be easier using a raster approach, so I'd try Hornbydd's answer from the other question. Too bad 40 isn't a perfect square. – lynxlynxlynx Nov 6 '16 at 15:06
• I'm definitely going to give that approach a go. The solutions suggested by Hornbydd are genius, thanks @FelixIP for the link. – Brendon Miles Watkiss Nov 6 '16 at 15:34
• You can use the the ratio of perimeter and area to exclude slivers. For example in case of 40 m2 square (~6.3m side), the perimeter / area = 0.63, for a 40 m2 circle perimeter / area = 0.56, 8m x 5m rectangle perimeter / area = 0.65, 40m x 1m rectangle perimeter / area = 2.05. – Zoltan Nov 6 '16 at 17:34