20170203
19202000
1.0
FGDC CSDGM Metadata
FALSE
structures_poly_230
002
35547.197200
330581.911000
777773.437600
959729.762900
1
0.000
Projected
GCS_North_American_1983
NAD_1983_StatePlane_Massachusetts_Mainland_FIPS_2001
<ProjectedCoordinateSystem xsi:type='typens:ProjectedCoordinateSystem' xmlns:xsi='http://www.w3.org/2001/XMLSchema-instance' xmlns:xs='http://www.w3.org/2001/XMLSchema' xmlns:typens='http://www.esri.com/schemas/ArcGIS/10.4'><WKT>PROJCS["NAD_1983_StatePlane_Massachusetts_Mainland_FIPS_2001",GEOGCS["GCS_North_American_1983",DATUM["D_North_American_1983",SPHEROID["GRS_1980",6378137.0,298.257222101]],PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],PROJECTION["Lambert_Conformal_Conic"],PARAMETER["False_Easting",200000.0],PARAMETER["False_Northing",750000.0],PARAMETER["Central_Meridian",-71.5],PARAMETER["Standard_Parallel_1",41.71666666666667],PARAMETER["Standard_Parallel_2",42.68333333333333],PARAMETER["Latitude_Of_Origin",41.0],UNIT["Meter",1.0],AUTHORITY["EPSG",26986]]</WKT><XOrigin>-36530900</XOrigin><YOrigin>-28803200</YOrigin><XYScale>122610652.81195112</XYScale><ZOrigin>-100000</ZOrigin><ZScale>10000</ZScale><MOrigin>-100000</MOrigin><MScale>10000</MScale><XYTolerance>0.001</XYTolerance><ZTolerance>0.001</ZTolerance><MTolerance>0.001</MTolerance><HighPrecision>true</HighPrecision><WKID>26986</WKID><LatestWKID>26986</LatestWKID></ProjectedCoordinateSystem>
Linear Unit: Meter (1.000000)
20171002
12540900
20171002
12540900
FGDC
150000000
5000
Version 6.2 (Build 9200) ; Esri ArcGIS 10.4.1.5686
Building Structures (2-D, from Ortho Imagery)
Michael Trust
MassGIS
Sr. GIS Database Administrator
One Ashburton Place, Room 819
Boston
MA
02108
michael.trust@state.ma.us
USA
617-619-5615
2014-05-09T00:00:00
2017-05-22T00:00:00
002
-73.512718
-69.901714
42.888192
41.233024
1
Two-dimensional roofprints of buildings (structures) throughout Massachusetts, originally interpreted from 2011-2012 ortho imagery. The data are being updated for changes based on newer aerial imagery.
<DIV STYLE="text-align:Left;"><DIV><DIV><P><SPAN>This dataset consists of 2-dimensional roof outlines ("roofprints") for all buildings larger than 150 square feet, as interpreted by a contractor (Rolta) for the whole area of the Commonwealth using DigitalGlobe ortho images obtained in 2011 and 2012, supplemented with LiDAR (Light Detection And Ranging) data collected from 2002 to 2011 for the eastern half of the state. </SPAN></P><P><SPAN>The roofprints as delivered were enhanced by MassGIS using Normalized Digital Surface Models (NDSMs) derived from the same LiDAR data. Other layers were used, including the Level 3 Parcels, to aid in review, especially where LiDAR data were not available.</SPAN></P><P><SPAN>This feature class is being updated using ortho imagery captured in 2013, 2014 and 2015. The newer imagery allows MassGIS staff to remove, modify and add structures to keep up with more current ground conditions. Structures from the original compilation that are removed are stored in an "archive" feature class for edit tracking and historical purposes.</SPAN></P><P><SPAN STYLE="font-weight:bold;">Last updated on 5/22/2017.</SPAN></P><P><SPAN>In ArcSDE the layer is named STRUCTURES_POLY.</SPAN></P></DIV></DIV></DIV>
Rolta, MassGIS
umasspelham
<DIV STYLE="text-align:Left;"><DIV><DIV><P><SPAN>None</SPAN></P></DIV></DIV></DIV>
MassGIS maintains this layer. Data from municipal or other sources may replace features in this dataset as they become available.
Michael Trust
MassGIS
Sr. GIS Database Administrator
One Ashburton Place, Room 819
Boston
MA
02108
michael.trust@state.ma.us
USA
617-619-5615
Michael Trust
Michael Trust
MassGIS
Sr. GIS Database Administrator
One Ashburton Place, Room 819
Boston
MA
02108
michael.trust@state.ma.us
USA
617-619-5615
Michael Trust
Shapefile
ArcGIS 10
MassGIS
One Ashburton Place, Room 1601
Boston
MA
02108
USA
617-619-5611
Shapefile
ArcGIS 10
Visit http://www.mass.gov/itd/structures.html
0.000
dataset
EPSG
2.1(3.0.1)
Vector
Simple
FALSE
0
FALSE
FALSE
0
structures_poly_230
Feature Class
0
A collection of geographic features with the same geometry type (such as point, line, or polygon), the same attributes, and the same spatial reference.
ESRI
FID
FID
OID
4
0
0
Internal feature number.
Esri
Sequential unique whole numbers that are automatically generated.
STRUCT_ID
STRUCT_ID
String
20
0
0
Unique polygon identifier, based on X,Y centroid coordinate of the feature in NAD83 Mass. State Plane meters
MassGIS
Unique values calculated for each feature.
SOURCE
SOURCE
String
100
0
0
Polygon source. All polygons from the original compilation are coded "ROLTA". Polygons digitized by MassGIS are coded "MAGIS". Other codes (in data delivered by the town of Dedham) include "PLANIMETRY", "SCANNED_PLAN", and "HEADSUP_DIG". "MAGIS_R" indicates a structure was found by MassGIS during review using newer imagery but the structure was also found in the imagery used by Rolta.
MassGIS
MAGIS
Polygons digitized by MassGIS are coded
MassGIS
SOURCETYPE
SOURCETYPE
String
20
0
0
Type of feature. Current codes are "ROOFPRINT", "ROOFPRINT_SHIFTED", and "FOOTPRINT".
MassGIS
ROOFPRINT
Roofprint
MassGIS
ROOFPRINT_SHIFTED
Roofprint shifted to correct for building lean
MassGIS
FOOTPRINT
Building footprint
MassGIS
SOURCEDATE
SOURCEDATE
Integer
10
10
0
Date (year) of source data used to create the structure polygon. Coded "20110000" or "20120000". See http://www.mass.gov/anf/docs/itd/services/massgis/dg-2011-2012-index.pdf. The eight-digit format is to allow for more accurately recording the date as local datasets are used to update the statewide coverage.
MassGIS
Various dates in YYYYMMDD format.
SOURCEDATA
SOURCEDATA
String
50
0
0
Indicates what imagery was used as a source for digitizing structure polygons
MassGIS
DG Worldview2 8-band satellite
DigitalGlobe Worldview2 8-band satellite
MassGIS
USGS 2013 30 CM AERIAL IMAGERY
USGS 2013 30cm ortho imagery
MassGIS
MOVED
MOVED
String
1
0
0
Indicates with "Y" or "N" whether or not the polygon was shifted to account for building lean in source imagery.
MassGIS
Y
Yes, polygon was shifted
MassGIS
N
No, polygon was not shifted
MassGIS
AREA_SQ_FT
AREA_SQ_FT
Double
19
0
0
Area of the structure polygon in square feet, calculated with the "Calculate Geometry" tool in ArcGIS software.
MassGIS
Various values calculated with the "Calculate Geometry" tool in ArcGIS software.
TOWN_ID
TOWN_ID
Integer
5
5
0
Identifier (1-351) for the city/town in which the structure is located.
MassGIS
1
351
TOWN_ID2
TOWN_ID2
Integer
5
5
0
Second identifier (1-351) for the city/town in which the structure is located, if the structure falls within two municipalities.
MassGIS
1
351
TOWN_ID3
TOWN_ID3
Integer
5
5
0
Third identifier (1-351) for the city/town in which the structure is located, if the structure falls within three municipalities.
MassGIS
1
351
LOCAL_ID
LOCAL_ID
String
25
0
0
Identifier used by local entity. Currently not used as no local data are included in this layer.
MassGIS
Unique values from non-MassGIS source.
SHAPE
Shape
Geometry
0
0
0
Feature geometry.
ESRI
Coordinates defining the features.
ARCHIVED
ARCHIVED
String
1
0
0
Yes/No code used by MassGIS for editing. Not included in shapefile downloads.
MassGIS
Y
Polygons may be coded "Y" during editing are deleted and then added to the in-house STRUCTURES_POLY_ARCHIVED feature class.
MassGIS
ARCHIVEDAT
ARCHIVEDAT
Integer
10
10
0
EDIT_DATE
EDIT_DATE
String
8
0
0
EDIT_BY
EDIT_BY
String
12
0
0
COMMENTS
COMMENTS
String
150
0
0
SHAPE_AREA
SHAPE_AREA
Double
19
0
0
Area of feature in internal units squared.
Esri
Positive real numbers that are automatically generated.
SHAPE_LEN
SHAPE_LEN
Double
19
0
0
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Michael Trust
MassGIS
Sr. GIS Database Administrator
mailing and physical address
One Ashburton Place, Room 819
Boston
MA
02108
USA
617-619-5615
michael.trust@state.ma.us
Building Structures (2-D, from Ortho Imagery)
Rolta, MassGIS
20170203
Buildings
Structures
Roofprints
None
Footprints
Commonwealth of Massachusetts
This dataset consists of 2-dimensional roof outlines ("roofprints") for all buildings larger than 150 square feet, as interpreted by a contractor (Rolta) for the whole area of the Commonwealth using DigitalGlobe ortho images obtained in 2011 and 2012, supplemented with LiDAR (Light Detection And Ranging) data collected from 2002 to 2011 for the eastern half of the state. The roofprints as delivered were enhanced by MassGIS using Normalized Digital Surface Models (NDSMs) derived from the same LiDAR data. Other layers were used, including the Level 3 Parcels, to aid in review, especially where LiDAR data were not available.This feature class is being updated using ortho imagery captured in 2013-2015. Last updated on 1/8/2016. In ArcSDE the layer is named STRUCTURES_POLY.
Two-dimensional roofprints of buildings (structures) throughout Massachusetts, interpreted from 2011-2012 ortho imagery.
20170203
publication date
Unknown
None
None
Rolta, MassGIS
20170203
http://www.mass.gov/itd/structures
MassGIS Web Documentation
Michael Trust
MassGIS
Sr. GIS Database Administrator
mailing and physical address
One Ashburton Place, Room 819
Boston
MA
02108
USA
617-619-5615
michael.trust@state.ma.us
MassGIS
mailing and physical address
One Ashburton Place, Room 819
Boston
MA
02108
USA
617-619-5611
Visit http://www.mass.gov/itd/structures.
Rolta
ORIGINAL ROOFPRINT PRODUCTION:
Rolta created the polygons based on the 2011 and 2012 DigitalGlobe Ortho images, the latest available orthos at the time, using LiDAR as a supplement to determine the shape of structures that were difficult to distinguish in the orthos. The data were saved in ESRI shapefile format and delivered to MassGIS for QA review, then processed to create the final deliverable.
Criteria Used for Creating a Roofprint
The following is a summary of the guidelines used in creating roofprints (taken from the Request for Response for this project):
A roofprint is a map polygon, with real world coordinates, representing the perimeter outline as it appears in aerial imagery of every structure or portion of a structure which has a roof. Roofprints shall be mapped for all structures equal to or larger than 150 square feet including the following:
Residential, commercial, and industrial structures (including roof over porches and decks)
Trailer homes and offices
Mobile homes
Garages, sheds, and other isolated structures
Additionally:
Features that do not have a roof covering usable areas, such as an open deck, the top surface of an electrical transmission or cell tower base, platforms for utility equipment, or other structures which do not have a usable “interior” or covered volume, shall not be interpreted for mapping. Also, vehicles, including truck trailer that are parked with or without a tractor attached, boats, airplanes, etc. should not be mapped. However, as noted above, trailers with any kind of residential or business use such as temporary classrooms, construction site field offices and the like must be captured. Greenhouses were generally considered not structures, unless attached to a roofed structure. Roofed dugouts of sufficient size were also included as structures. Tanks and covered reservoirs and pools with temporary covers were not considered structures.
Polygon creation had the following guidelines:
Outlines were usually made up of orthogonal segments (all segments parallel or at right angles) unless the building is octagonal, round, triangular, etc.
Outlines were traced at the elevation of the eaves or lowest part of the roof adjacent to the exterior vertical walls. If there are multiple roof levels for a single structure then internal boundaries created by joining the separate roofprints must be dissolved. Any part of the structure which is covered by a roof is included, so two buildings connected by a covered walkway would be represented as one polygon.
Any roof offset, jog or projection for which all sides are more than 3 feet in length should be captured.
In the creating of the outlines, building “lean” was not to be compensated for. (MassGIS addressed this issue. See the section "Roofprint Shifting" below)
No attributes were included in the creation of the polygons.
Criteria for Acceptance
The interpretation error rate must be less than 0.5%, determined as follows:
For each of the six delivery areas, MassGIS selected tiles randomly (using a ‘randomizing’ spreadsheet created within MassGIS) from the 2008/2009 ortho imagery. Additional tiles were selected until the total number of structures in the selected area exceeded 15,000. The roof outlines in the selected tiles were then reviewed against the DigitalGlobe imagery. Additional layers were used to supplement the review, including the LiDAR datasets, and the Level 3 Parcel dataset, especially where LiDAR data were not available.
The error rate was defined using two statistics from the review of the sample tiles for each delivery:
Eo = The number of errors of omission – structures that were missed
Ec = The number of errors of commission – structures that are not in fact structures (as defined above).
The combined error rate for interpretation was calculated to be Eo + Ec.
2011-2012
ROOFPRINT SHIFTING:
Elevated objects such as roof outlines in aerial imagery may appear displaced with respect to the base of the structure. In order to minimize or eliminate the effects of such displacement (often referred to as "building lean"), MassGIS undertook several automated processing steps to shift roofprint polygons as delivered by Rolta. Building lean effect may cause some buildings to cross over into adjacent parcels or overlap other features such as streets and water bodies. The shifting process was performed only in areas where MassGIS' LiDAR Terrain Data were available (Eastern Mass. inside of I-495; for details see the section "Adjustment Method" below). As a result, many of the shifted polygons better approximate building footprints.
Background on Building Lean
Ortho image data layers are really mosaics made up of portions of many overlapping aerial photo frames.
The principal point of an aerial photo is the intersection of the optical axis of the camera lens and the photo image. The nadir is the point directly beneath the camera at the time of exposure. On a vertical aerial photograph (looking downward) the nadir and the principal point will be at the same location.
If a building is close to the principal point, the roof and base will appear to coincide (the base and sides of the building will not be visible)
If a building is far from the principal point, toward the edge of the photo, the top of the building will appear to be farther away from the principal point than the bottom of the building. The building will appear to "lean" away from the principal point.
The buildings on either side of the seam line are from different photos, so the buildings seem to lean away from their respective principal points.
The magnitude of lean can be determined by:
H / (D+d) = h / d
Where H is the camera height, h is the building height, D is the distance of the building from the principal point.
Or, since d is usually much smaller than D, D+d ~ D, so
d ~ ( D / H ) x h
The average height of each building has been obtained from a LiDAR Normalized Difference Surface Model (NDSM). This raster is the difference between the LiDAR last-return elevations, and the LiDAR model of the ground.
It was assumed that the location of each principal point is at the mean center of each seam polygon, and that the aircraft altitude is 5,000 meters.
The lean also has a direction, so polygons representing "roofprints" have been moved a distance d in a direction opposite to the apparent displacement in the photo.
Adjustment Method
MassGIS used five Input Datasets:
1. DigitalGlobe 2011-2012 Orthoimagery (six blocks)
- Boston High Value Area
- Worcester High Value Area
- Cape Cod Refresh Area
- Standard Block 4171
- Standard Block 4172
- Standard Block 4272
2. NDSM Raster Images for LiDAR Project Areas (see Areas of LiDAR used in Shifting Index Map pdf format of Roofprint Shift Area at http://www.mass.gov/anf/docs/itd/services/massgis/roofprintshiftarea.pdf):
- FEMA 2010-2011:
Nashua
Concord River
Charles River
Blackstone
Quincy
- LiDAR for the Northeast
- 2004 SE Massachusetts Pilot
- Buzzards Bay (parts of Bristol and Plymouth Counties)
[Manageable processing areas were determined based on the intersections of these regions.]
For each LiDAR project area, all LiDAR returns were filtered to create two ArcGIS Terrains:
Any return classified as "Ground" was used in a "Bare-Earth" Terrain
The last returns classified as "Ground" or as "Unclassified" went into a "Last Return" Terrain
These Terrains were then linearly interpolated to two 1.0 meter rasters. Finally, the Bare-Earth raster was subtracted from the Last Return raster, resulting in a Normalized Difference Surface Model (NDSM).
3. Orthoimage polygon tiles (irregularly-shaped "seam polygons") corresponding to each DigitalGlobe area
NDSMs were cut into smaller subimages (tiles) using the seam polygons for the corresponding area.
4. Seam center points were determined for each seam polygon.
5. Un-recified roofprint polygons (unshifted polygons as delivered by Rolta)
Processing
A model was developed in Trimble eCognition Developer 8.7.2 and run on eCognition Server that determined the distance and direction from each roofprint centroid to the tile's seam center, as well as the mean height the building. Output was a point shapefile.
An ArcGIS Toolbox script that prepared the output points and roofprints for rectification was run, followed by an ArcGIS Python script that created a dataset of shifted roofprint polygons.
The two sets of shifted roofprints in overlapping processing areas were examined, and where there were differences, the roofprints with the more accurate shift were kept.
Roofprints straddling seam lines usually contain two (or more) points with different values for angle and distance. These roofprints were generally not moved, but were coded TOUCH_SEAM = 1 so they could be tracked after processing.
Sources of possible error in the shifting process include:
The orthoimage used to determine the roofprint
The roofprint polygon as drawn
The estimated position of the principal point
The estimated camera altitude
The LiDAR NDSM raster
The estimate of the building height derived from the LiDAR NDSM raster
Situations which may cause the roofprint to shift more or less than it should:
The building represented by a roofprint was not built at the time of the LiDAR acquisition.
Trees may overhang a building, so that the elevation obtained may be higher than the building height.
Greenhouses may be represented in the roofprints layer, but not in the LiDAR.
A single roofprint representing a complex roof with different elevations may be shifted based on a single elevation value.
In a small number of cases, the shifting process caused some polygons to overlap others. These were found using ArcGIS topology and the polygons were moved manually so that no overlaps were present. Once the shifting process was complete, the shifted polygons replaced those in a copy of the original Rolta deliverable. The version of the Structures dataset distributed by MassGIS, therefore, is a hybrid of as-delivered polygons and those shifted by MassGIS. Finally, MassGIS took the hybrid layer and performed an Identity operation with the Survey-based Communities layer to populate the TOWN_ID fields.
Philip John
MassGIS
GIS Image Analyst
mailing and physical address
One Ashburton Place, Room 1601
Boston
MA
02108
USA
617-619-5637
philip.john@state.ma.us
Michael Trust
MassGIS
Sr. GIS Database Administrator
mailing and physical address
One Ashburton Place, Room 1601
Boston
MA
02108
USA
617-619-5615
michael.trust@state.ma.us
In April 2013 footprint data from the town of Dedham replaced the majority of polygons in that community.
20130412
Craig Austin
MassGIS
mailing and physical address
One Ashburton Place
Boston
MA
02108
USA
617-626-4592
craig.austin@state.ma.us
STRUCTURE UPDATES
As MassGIS receives newer imagery, new structures were identified and added to the data layer, and structures that have since been demolished were deleted. In the case of structures that have changed significantly, the original polygon was deleted (and saved in-house to an “archive” dataset) and a new one was added.
Ongoing from Jan. 8, 2016
None
ORIGINAL ROOFPRINT PRODUCTION
Rolta created the polygons based on the 2011 and 2012 DigitalGlobe Ortho images (see Year of Photography Index pdf format of DigitalGlobe Ortho Imagery Index Map 2011-2012 at http://www.mass.gov/anf/docs/itd/services/massgis/dg-2011-2012-index.pdf), the latest available orthos at the time, using LiDAR as a supplement to determine the shape of structures that were difficult to distinguish in the orthos. The data were saved in ESRI shapefile format and delivered to MassGIS for QA review, then processed to create the final deliverable.
Criteria Used for Creating a Roofprint
The following is a summary of the guidelines used in creating roofprints (as described in the Request for Response for this project):
A roofprint is a map polygon, with real world coordinates, representing the perimeter outline as it appears in aerial imagery of every structure or portion of a structure which has a roof. Roofprints shall be mapped for all structures equal to or larger than 150 square feet including the following:
Residential, commercial, and industrial structures (including roof over porches and decks)
Trailer homes and offices
Mobile homes
Garages, sheds, and other isolated structures
Additionally:
Features that do not have a roof covering usable areas, such as an open deck, the top surface of an electrical transmission or cell tower base, platforms for utility equipment, or other structures which do not have a usable “interior” or covered volume, shall not be interpreted for mapping. Also, vehicles, including truck trailers that are parked with or without a tractor attached, boats, airplanes, etc. should not be mapped. However, trailers with any kind of residential or business use such as temporary classrooms, construction site field offices and the like must be captured.
Greenhouses were generally considered not structures, unless attached to a roofed structure. Roofed dugouts of sufficient size were also included as structures. Tanks and covered reservoirs and pools with temporary covers were not considered structures.
Polygon creation had the following guidelines:
Outlines will usually be made up of orthogonal segments (all segments parallel or at right angles) unless the building is octagonal, round, triangular, etc.
Outlines are to be traced at the elevation of the eaves or lowest part of the roof adjacent to the exterior vertical walls. If there are multiple roof levels for a single structure then internal boundaries created by joining the separate roofprints must be dissolved. Any part of the structure which is covered by a roof is included, so two buildings connected by a covered walkway are to be represented as one polygon.
Any roof offset, jog or projection for which all sides are more than 3 feet in length should be captured.
In the creation of the outlines, building “lean” was not compensated for. (MassGIS addressed this issue for part of the state. See the section "Roofprint Shifting" below.) No attributes were included in the creation of the polygons.
Criteria for Acceptance
The interpretation error rate was less than 0.5%, conformance to this standard was determined as follows:
For each of the six delivery areas, MassGIS selected tiles randomly (using a ‘randomizing’ spreadsheet created within MassGIS) from the 2008/2009 ortho imagery. Tiles were selected until the total number of structures in the selected area exceeded 15,000. The roof outlines in the selected tiles were then reviewed against the DigitalGlobe imagery. Additional layers were used to supplement the review, including the LiDAR datasets, and the Level 3 Parcel dataset, especially where LiDAR data were not available.
The error rate was defined using two statistics from the review of the sample tiles for each delivery:
Eo = The number of errors of omission – structures that were missed
Ec = The number of errors of commission – structures that are not in fact structures (as defined above).
The combined error rate for interpretation was calculated to be Eo + Ec.
Rolta
2012-10-01T00:00:00
ROOFPRINT SHIFTING
Elevated objects such as roof outlines in aerial imagery may appear displaced with respect to the base of the structure. In order to minimize or eliminate the effects of such displacement (often referred to as "building lean"), MassGIS undertook several automated processing steps to shift roofprint polygons as delivered by Rolta. Building lean effect may cause some buildings to cross over into adjacent parcels or overlap other features such as streets and water bodies. The shifting process was performed only in areas where MassGIS' LiDAR Terrain Data were available (Eastern Mass. inside of I-495; for details see the section "Adjustment Method" below). As a result, many of the shifted polygons better approximate building footprints.
Background on Building Lean
Ortho image data layers are really mosaics made up of portions of many overlapping aerial photo frames.
The principal point of an aerial photo is the intersection of the optical axis of the camera lens and the photo image. The nadir is the point directly beneath the camera at the time of exposure. On a vertical aerial photograph (looking downward) the nadir and the principal point will be at the same location.
If a building is close to the principal point, the roof and base will appear to coincide (the base and sides of the building will not be visible)
If a building is far from the principal point, toward the edge of the photo, the top of the building will appear to be farther away from the principal point than the bottom of the building. The building will appear to "lean" away from the principal point.
The buildings on either side of the seam line are from different photos, so the buildings seem to lean away from their respective principal points.
The magnitude of lean can be determined by:
H / (D+d) = h / d
Where H is the camera height, h is the building height, D is the distance of the building from the principal point.
Or, since d is usually much smaller than D, D+d ~ D, so
d ~ ( D / H ) x h
The average height of each building has been obtained from a LiDAR Normalized Difference Surface Model (NDSM). This raster is the difference between the LiDAR last-return elevations, and the LiDAR model of the ground.
It was assumed that the location of each principal point is at the mean center of each seam polygon, and that the aircraft altitude is 5,000 meters.
The lean also has a direction, so polygons representing "roofprints" have been moved a distance d in a direction opposite to the apparent displacement in the photo.
Adjustment Method
MassGIS used five Input Datasets:
1. DigitalGlobe 2011-2012 Orthoimagery (six blocks)
- Boston High Value Area
- Worcester High Value Area
- Cape Cod Refresh Area
- Standard Block 4171
- Standard Block 4172
- Standard Block 4272
2. NDSM Raster Images for LiDAR Project Areas (see Areas of LiDAR used in Shifting Index Map pdf format of Roofprint Shift Area at http://www.mass.gov/anf/docs/itd/services/massgis/roofprintshiftarea.pdf):
- FEMA 2010-2011:
Nashua
Concord River
Charles River
Blackstone
Quincy
- LiDAR for the Northeast
- 2004 SE Massachusetts Pilot
- Buzzards Bay (parts of Bristol and Plymouth Counties)
[Manageable processing areas were determined based on the intersections of these regions.]
For each LiDAR project area, all LiDAR returns were filtered to create two ArcGIS Terrains:
Any return classified as "Ground" was used in a "Bare-Earth" Terrain
The last returns classified as "Ground" or as "Unclassified" went into a "Last Return" Terrain
These Terrains were then linearly interpolated to two 1.0 meter rasters. Finally, the Bare-Earth raster was subtracted from the Last Return raster, resulting in a Normalized Difference Surface Model (NDSM).
3. Orthoimage polygon tiles (irregularly-shaped "seam polygons") corresponding to each DigitalGlobe area
NDSMs were cut into smaller subimages (tiles) using the seam polygons for the corresponding area.
4. Seam center points were determined for each seam polygon.
5. Un-recified roofprint polygons (unshifted polygons as delivered by Rolta)
Processing
A model was developed in Trimble eCognition Developer 8.7.2 and run on eCognition Server that determined the distance and direction from each roofprint centroid to the tile's seam center, as well as the mean height the building. Output was a point shapefile.
An ArcGIS Toolbox script that prepared the output points and roofprints for rectification was run, followed by an ArcGIS Python script that created a dataset of shifted roofprint polygons.
The two sets of shifted roofprints in overlapping processing areas were examined, and where there were differences, the roofprints with the more accurate shift were kept.
Roofprints straddling seam lines usually contain two (or more) points with different values for angle and distance. These roofprints were generally not moved, but were coded TOUCH_SEAM = 1 so they could be tracked after processing.
Sources of possible error in the shifting process include:
The orthoimage used to determine the roofprint
The roofprint polygon as drawn
The estimated position of the principal point
The estimated camera altitude
The LiDAR NDSM raster
The estimate of the building height derived from the LiDAR NDSM raster
Situations which may cause the roofprint to shift more or less than it should:
The building represented by a roofprint was not built at the time of the LiDAR acquisition.
Trees may overhang a building, so that the elevation obtained may be higher than the building height.
Greenhouses may be represented in the roofprints layer, but not in the LiDAR.
A single roofprint representing a complex roof with different elevations may be shifted based on a single elevation value.
In a small number of cases, the shifting process caused some polygons to overlap others. These were found using ArcGIS topology and the polygons were moved manually so that no overlaps were present. Once the shifting process was complete, the shifted polygons replaced those in a copy of the original Rolta deliverable. The version of the Structures dataset distributed by MassGIS, therefore, is a hybrid of as-delivered polygons and those shifted by MassGIS. Finally, MassGIS took the hybrid layer and performed an Identity operation with the Survey-based Communities layer to populate the TOWN_ID fields.
Philip John
MassGIS
GIS Image Analyst
One Ashburton Place, Room 1601
Boston
MA
02108
philip.john@state.ma.us
USA
617-619-5637
2013-12-06T00:00:00
STRUCTURE UPDATES
As MassGIS receives newer imagery, new structures were identified and added to the data layer, and structures that have since been demolished were deleted. In the case of structures that have changed significantly, the original polygon was deleted (and saved in-house to an “archive” dataset) and a new one was added.
2016-01-08T00:00:00
Craig Austin
MassGIS
One Ashburton Place, Room 819
Boston
MA
02108
617-626-4592
NAD83 Massachusetts State Plane Mainland
North American Datum of 1983
Geodetic Reference System 80
Michael Trust
MassGIS
Sr. GIS Database Administrator
One Ashburton Place, Room 819
Boston
MA
02108
michael.trust@state.ma.us
USA
617-619-5615
Michael Trust