What are the uses (public and private industry) of building footprints?
I'm trying to figure out how polygon building footprints can be used. i.e. are they used for determining tax liabilities, etc...
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There are a number of places where footprints can come in very handy
Taxation: As @Mapperz said, taxation is one area. The percentage of property that is built on is sometimes used as a tax criterion.
Planning: Knowing where structures already exist on property can help in the planning process due to applied setbacks and minimum distances from houses and property lines. Another example would be planning the location of a water line or a sewer line that needs to cross property on an easement. If the footprint exists, the location constraints become more immediately obvious.
Public Works/Public Safety: Knowing where structures are located can show where access to the rear of property is constrained. This can be important for public safety, if there is a fire, can a fire truck get to the back of a property, or is there not room between two structures. From the public works perspective, if there is a facility running on an easement across a piece of property, knowing where vehicles can get in to be able to effect repairs is critical.
In some ways, these mirror the public sector uses. In a lot of cases, the public sector is creating data layers and then finds a benefit in exposing them to the public. Here are some possible reasons why.
Planning: If someone wants to put in a new building, they can perform initial research to find where existing structures are that may constrain their plans.
Visualization/Simulation: If someone is trying to create a 3d model or fly-through of an area, doing extrusion on the building footprints vs the entire property, will give you a more representative simulation.
Analysis: Line of sight analysis for future development. If you are designing a building having footprints will allows you to analyze more accurately what will or won't be visible. This could also be useful for property valuation for sales purposes.
Suitability analysis: Example: A solar panel contractor is evaluating an area for marketing purposes. They take building footprints to see whether the majority of the residential structures are oriented in a way that makes them more suitable for a solar panel emplacement. If there is not a high percentage, it is potentially not worth marketing in an area.
These are just a few ideas, but hopefully they give you some direction.
LiDAR can be used to make 3-D representations of buildings (as well as building footprints). These 3-D buildings can be used in viewsheds to model if a building will block a certain aesthetic feature. They can also be used to model shadows.
Example: You wouldn't want to build a new city park in an area that's perpetually in a building's shadow.