NYC Building Footprints Part II

This post is a follow-up to the previous building footprint post. It expands upon some topics, and covers some new areas. And as with most everything, a bit of background is necessary to understand where we have come from and, in some cases, why things are the way they are. Progress is made incrementally. The current state of NYC geospatial data has improved immensely but certainly further improvements are warranted.

Change
Although NYC is largely a ‘built’ city, construction activity is continually taking place. As such, building footprint edits are made to account for these changes in the non-digital world and differences will be seen from extract to extract. Additionally, as errors and omissions are encountered in the data, corrections are made. The building footprints is a dynamic data set, extracted quarterly and we hope to move to a continuous update stream in the near future. Nonetheless, change will still need to be handled. More on that to come in the next year.

In the case of demolished buildings, these building geometries are archived and provided as a separate historical buildings file on the NYC Open Data portal.

BIN
The Building Identification Number (BIN) provides a unique identifier for the buildings to which they are assigned. Not every building within the building footprints database has been assigned a BIN. For those building not yet assigned a BIN or where a BIN has yet to be inserted into the building footprints, a placeholder is inserted. These placeholders have been referred to as ‘million’ BINs. They are identified by a borough code plus six zeros.

The borough codes are as follows:
Manhattan = 1
Brooklyn = 2
Bronx = 3
Queens = 4
Staten Island = 5

BINs are assigned by the Department of City Planning (DCP). BINs originated from the Property Address Directory (PAD), one of the data sources of Geosupport. PAD predated the building footprints; therefore PAD relied on other sources to define buildings. With the advent of the building footprints, many more buildings needed to be assigned a BIN. This work is ongoing. As DCP assigns BINs, they are provided to DoITT and inserted into the corresponding building footprints. At present there are only 27,792 ‘million’ BINs remaining in the December 2014 building footprint extract. That represents 2.5% of the 1,082,483 building footprints. The majority of these are detached garages or minor buildings on lots. This number will continue to decrease until we reach complete coverage.

BBL
For all tax lots, except condominiums (condos), there is a single representative BBL across all City agencies. Condos are the exception due to the fact that each individual unit (i.e., apartment) within a condo building has its own BBL. Therefore, condos have multiple BBLs per tax lot. It is my understanding that the Billing BBL was created by The Department of Finance (DOF) as a way of representing a condo’s management entity for the purpose of correspondence and record keeping. Billing BBLs always have 75 as the first two digits in the block portion of the BBL (e.g., 7501.). Unfortunately there does not seem to be agreement across all City agencies, or even within an agency, on a unique BBL for condo lots.

DOF uses the Billing BBL for RPAD and the Base BBL (also referred to as the FKA [Formerly Known As]) for the Digital Tax Map and ACRIS. DCP uses the billing BBL for MapPluto.

The building footprints use the Billing BBL. The building footprints carry the BBL as a means of providing a way of associating buildings to tax lots. Since the BBLs are managed outside of the building footprints, the BBLs are synchronized periodically. Due to the different update frequency of MapPluto and the building footprints, inconsistencies can be present. In the December 2014 extract there were 5,199 BBL mismatches representing 0.4% of the total.

There are also cases where buildings do not fall within an official tax lot. For these, DCP assigns a ‘dummy’ lot number of 9999. An example is the Subway station at 96th and Broadway (BIN 1089286, BBL 10124399990). These ‘dummy’ lots are in PAD but do not exist in MapPluto.

A reminder to always read the metadata. To borrow from the Ancient Greek aphorism “know thyself”, know thy data. In addition to improving the data, we look to continually improve the metadata.

Finally, to the data editors that work in relative obscurity at DoITT, DCP and DOF I say thank you for a job well done. To all I wish you a Happy Holidays. Till next year…

NYC Building Footprints

I have seen and received quite a number of emails, and have even seen applications that confuse MapPluto for a building data set. To clarify what the building footprint’s represent as well as to remove any confusion between the two very different data sets, I decided to write this post.

MapPluto
MapPluto is a compilation of City agency data at the tax lot (aka parcel) level produced and distributed by the Department of City Planning. A tax lot defines the basic unit of land ownership. Much has been written about MapPluto, so I do not intend to cover this data set in detail. However, it is important to understand that a tax lot can encompass multiple buildings.

A NYC tax lot uses Borough Block and Lot (BBL) as a unique parcel identifier. DCP compiles a variety of City data sets at the parcel level into MapPluto. One of the main data sources is the Department of Finance’s (DOF) Real Property and Assessment Data (RPAD). One of the attributes in RPAD is Number of floors, which is included in MapPluto as NumFloors. DCP defines this column in the metadata as being for “…the primary building on the tax lot, the number of full and partial stories starting from the ground floor.” This is due to the fact, as previously stated, that there can be multiple buildings on a tax lot. Since only one value is possible, DCP elected to go with the number of floors of the ‘primary’ building.

An example of a tax lot with multiple buildings is the community of Breezy Point, Queens. Originally a gated community of summer bungalows that are now permanent homes, Breezy Point spans 12 tax lots and encompasses 3,017 buildings. one of the parcels (BBL 4163400050) includes 424 buildings and has a value of 1 for the number of floors. Although the houses in Breezy Point are of similar housing stock, the number of floors is for the ‘primary’ building and thus not an exact figure. Another common example are NYC Housing Authority (NYCHA) developments. Although buildings within a development are often the same number of floors, this is not always the case.

In general, it is the responsibility of the person working with the data to read the metadata to get an understanding of the data and its limitations and constraints. There are cases where values are estimated, imputed or no longer actively maintained. In the case of MapPluto, building height applies to only one of potentially many buildings on a tax lot. This is not an error but a limitation of the data.

Building Footprints
Building footprints represent the ground-level perimeter outline of a building (i.e., footprint) greater than or equal to 400 square feet and greater than or equal to 12 feet in height unless they were previously captured and have a Building Identification Number (BIN). The purpose for the size and height constraint is to prevent the capture of non-buildings (e.g., containers, tents), which we have seen in the past. The specifications to which built features are captured and which are not can be found in the metadata.

The building footprints include ground and roof height elevations. These values are in feet and are derived photogrammetrically using stereo imagery, LiDAR and a TIN model.

There are cases where there will be no value in these columns. The reason for this is how the building footprints are maintained. To understand this, we need to revisit the past.

The building footprints were first captured as part of the first NYC Planimetrics in 1997 based on 1996 imagery. The NYC Planimetrics came to be called NYCMap. An excellent article on this effort can be found in the New Yorker (unfortunately a subscription is required to access the full article). In the beginning there was no plan for the periodic update of the planimetrics. Since 2006 the planimetrics have been updated on a four-year cycle.

Utilizing the Department of Buildings (DOB) permit data (new construction, major alterations and demolitions), it was determined that the building footprints could be updated on a more frequent basis. Since 2004 the buildings have been updated regularly and since the NYC Open Data Portal launched have been updated on a quarterly basis. These updates are done on-screen using heads up digitizing. Since these updates are not done photogrammetrically elevations values are not available and thus not in the database. With each planimetric update, buildings that are digitized on-screen are replaced with photogrammetrically-captured buildings and elevation values are assigned.

Lastly the Building Identification Numbers (BIN) assigned by DCP are inserted into the corresponding building footprint. The BIN is the unique identifier used by City agencies to identify buildings. Many agencies utilize the BIN to associate additional data to a building. BIN is returned by Geoclient API geocoding service provided by DoITT.