OVERVIEW

In spring 2008, the U.S. Geological Survey, as part of its Boston 133 Cities Urban Area mapping program, contracted for true-color imagery covering the metropolitan Boston area and beyond. Image type for the entire region (more than 1.7 million acres) is 24-bit, 3-band (red, green, blue) natural color. Each band has pixel values ranging 0-255. Pixel resolution is 30 cm., or approximately one foot.

Additionally, 30 municipalities participated in the Boston Upgrade of the USGS project; these cities and towns contributed funding for separate flights to produce 4-band (red, green, blue, near-infrared) imagery. Pixel resolution for these images is 15 centimeters (approximately 6 inches).

This digital orthoimagery can serve a variety of purposes, from general planning, to field reference for spatial analysis, to a tool for data development and revision of vector maps. It can also serve as a reference layer or basemap for myriad applications inside geographic information system (GIS) software.

MassGIS stores the 30cm imagery in ArcSDE as a single raster dataset named IMGCOQ2008_30CM.

Aerial Acquisition

Fugro Earthdata, Inc. acquired the aerial imagery from April 10-18, 2008, flying north-south to support the creation of digital orthophotography with a 15cm pixel (Boston Upgrade areas) and a 30cm pixel (entire Boston UA133 region). The 15cm imagery was acquired in seven flights, at approximately 4,800 feet above mean terrain (AMT). There were 120 flight lines. The 30cm imagery was acquired in six flights, at approximately 9,440 feet AMT. There were 52 flight lines. All imagery was collected using a Leica ADS40 digital pushbroom sensor. Source imagery was cloud-free. Photography was flown during leaf-off in deciduous vegetation regions. In downtown Boston a "true ortho" method was used to reduce the effects of tall building lean.

Orthorectification

Fugro EarthData used ISTAR to process a digital surface model (DSM) for orthorectification of the imagery. ISTAR digital surface modeling is based on an auto correlated pixel-matching system within the ISTAR software. The auto correlated DSM surface represents the initial surface model. The following is the process used:

1. Using the DSM, a technician performed a visual inspection of the data to verify that the flight lines met correctly. The technician also verified that there were no voids, and that the data covered the project limits. The technician then selected a series of areas from the dataset and inspected them where adjacent flight lines met.
2. This DSM surface is used in the rectification of the orthoimagery.

Ground Control

Dewberry and Davis LLC was contracted by the USGS to acquire 50 photo identifiable ground control points during the acquisition of aerial imagery. The ground control points were established using GPS for vertical (NAVD 88 Meters) and horizontal (NAD83(NSRS2007), UTM Zone 19N) coordinate values.

Spatial Data Quality - Horizontal Positional Accuracy

The 15cm orthophotos were created to be fully compliant with a horizontal positional accuracy not to exceed 0.6 meters Root Mean Squared Error (RMSE). Error XY does not exceed 0.424 meter RMSE in X or Y. The 30cm orthophotos were created to be fully compliant with a horizontal positional accuracy not to exceed 3 meters RMSE. Error XY does not exceed 2.12 meter RMSE in X or Y. Detailed accuracy reports were delivered to MassGIS.

Twenty-one points acquired by the MassHighway Survey Section were used to test the horizontal accuracy of the 15cm imagery. The surveyed coordinates were compared with coordinate values read for the corresponding image in the ortho product. The diagonal RMSE (XY) was 0.21m (NSSDA statistic was 0.36m). This fell within the 0.6m tolerance defined in the Statement of Work.

Forty-nine points also acquired by the MassHighway Survey Section were used to test the horizontal accuracy of the 30cm imagery. The surveyed coordinates for the points were compared with coordinate values read for the corresponding image in the ortho product. The diagonal RMSE (XY) was 0.44m (NSSDA statistic was 0.75m). This fell well within the 3.0m tolerance defined in the Statement of Work.

Processing

Processing by Fugro Earthdata included radiometric adjustments made using Earthdata proprietary tools and mosaicking the ADS40 imagery strips. QA/QC was performed to ensure that the mosaic lines were appropriately placed and that there was appropriate imagery coverage. Technicians also looked for anomalies, smears and other indications of problems within the digital orthophoto creation process. Where necessary, tonal balancing was performed over a group of images during the mosaicking process which served to lighten or darken adjacent images for better color tone matching. Two additional radiometric adjustments were applied to the completed orthos in Adobe Photoshop, including a sharpening mask filter, used to help increase sharpness of a digital image.

The final digital ortho product delivered to MassGIS was a set of GeoTIFF files in the UTM Zone 19N meters coordinate system. Images were non-overlapping tiles covering a 1,500-meter by 1,500-meter block on the ground. As requested by the Mass. Executive Office of Public Safety, MassGIS "deresolved" the imagery in a few areas for security reasons. Using ArcGIS Desktop 9.2, MassGIS then merged the UTM files into large blocks and projected the data into the Massachusetts State Plane Mainland NAD83 Meters coordinate system. The projection process resulted in a slight offset of large bands of pixels, most noticeable in linear features (like tennis court lines) when viewed at close scales. Tiles were clipped out into new 1,500-meter by 1,500-meter blocks; each image name is based on the first four digits of the X and Y coordinates of the center of the tile in NAD83 Mass. State Plane meters. MassGIS' quality assurance included extensive visual examination for errors, anomalies, spatial accuracy and completeness of the imagery.

Because of the large size of the GeoTIFFs, MassGIS is making these images available in the compressed MrSID format. Options include images tiled by the 2008 state plane orthophoto index as well as large regional mosaics. Users may access the data by free download from the MassGIS ftp server or by ordering the data on DVD. Details are provided below. All the 2008 image data distributed by MassGIS are referenced to the Mass. State Plane Mainland NAD83 Meters coordinate system.

Free Download

Images are available for download as 1,500 meters × 1,500 meters tiles (based on the 2008 Ortho Index [PDF] tiling scheme):

• MrSID, lossy, at 15:1 compression ratio, 3 bands (RGB). Two sets:
• 30 cm imagery (approx. 1.5 to 5 MB each)  [ DOWNLOAD ]
• 15 cm imagery (approx. 4 to 20 MB each)  [ DOWNLOAD ]

Order (from the MassGIS Online Order Form - choose Digital Data products)

On DVD:

• Regional MrSID mosaics (RGB) created from both the 15cm and 30cm GeoTIFF imagery (compression ratio 20:1) See the 2008 Ortho Index [PDF] for details on the tiling schemes.
  

On user-provided hard drive:

• The uncompressed 1,500m x 1.500m GeoTIFF tiles (based on the 2008 Ortho Index [PDF] tiling scheme).
  
• 15 cm resolution (4-band RGBN, 385 MB each)
  
• 30 cm resolution (3-band RGB, 73 MB each)
  

For the data layers used in the creation of the PDF Index Map, see the USGS Color Ortho Indexes (2008) datalayer page.

Displaying in ArcView GIS 3.x
For MrSID images, place the newest AVMrSID.dll file in the BIN32 folder where ArcView is installed (e.g., C:\ESRI\AV_GIS30\ARCVIEW\BIN32).
The GeoTIFFs are not compatible with ArcView GIS 3.x.

Image Catalogs
A file-based image catalog is available for download for use with the MrSID files. Users can load the following image catalog file into some GIS software (including ESRI's ArcMap and ArcView GIS 3.x) instead of loading in each individual image; as you pan around and zoom in and and out, the catalog will automatically retrieve and display the image(s) for the current geographic extent. Simply place the image catalog in the same folder on disk with the image files.

• IMGCAT_COQ2008SID_15CM.dbf - 15cm MrSID tiles
• IMGCAT_COQ2008SID_30CM.dbf - 30cm MrSID tiles

Displaying with Feet-based data
Users may successfully display our meters-based imagery in ArcView 3.x with other data in NAD83 Mass. State Plane feet simply by changing the accompanying .sdw "world" files and installing the newest version of the file AVMrSID.dll.
The process is as follows: Open the .sdw file in a text editor and multiply all values by 3.2808333316 and save.
ArcMap will automatically project the imagery on the fly with all spatially referenced datasets. (ArcMap does not use the .sdw files; the .sid file contains the georeferencing and statistics information).

The datalayer is maintained by MassGIS. Additional information concerning the technical details of the project may be available upon request.

For other ortho image data available from MassGIS, see the following datalayers:

• 1:5,000 Color Ortho Imagery (2005)
  
• 1:5,000 Color Ortho Imagery (2001, 2003)
• 1:5,000 Black and White Digital Orthophoto Images (1992-1999)
• 1:12,000 USGS Black and White Digital Orthophoto Images (1992, 1995)
• 1:10,000 Coastal Color Orthophoto Images (1994)
  

Please note that MassGIS does not have any aerial photographs prior to 1992. For information on such products please see http://erg.usgs.gov/isb/pubs/factsheets/fs12796.html.

Last Updated 6/11/2009
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